@c Copyright (C) 1988,89,92,93,94,96,98,99,2000 Free Software Foundation, Inc. @c This is part of the GCC manual. @c For copying conditions, see the file gcc.texi. @node C Extensions @chapter Extensions to the C Language Family @cindex extensions, C language @cindex C language extensions GNU C provides several language features not found in ANSI standard C. (The @samp{-pedantic} option directs GNU CC to print a warning message if any of these features is used.) To test for the availability of these features in conditional compilation, check for a predefined macro @code{__GNUC__}, which is always defined under GNU CC. These extensions are available in C and Objective C. Most of them are also available in C++. @xref{C++ Extensions,,Extensions to the C++ Language}, for extensions that apply @emph{only} to C++. @c The only difference between the two versions of this menu is that the @c version for clear INTERNALS has an extra node, "Constraints" (which @c appears in a separate chapter in the other version of the manual). @ifset INTERNALS @menu * Statement Exprs:: Putting statements and declarations inside expressions. * Local Labels:: Labels local to a statement-expression. * Labels as Values:: Getting pointers to labels, and computed gotos. * Nested Functions:: As in Algol and Pascal, lexical scoping of functions. * Constructing Calls:: Dispatching a call to another function. * Naming Types:: Giving a name to the type of some expression. * Typeof:: @code{typeof}: referring to the type of an expression. * Lvalues:: Using @samp{?:}, @samp{,} and casts in lvalues. * Conditionals:: Omitting the middle operand of a @samp{?:} expression. * Long Long:: Double-word integers---@code{long long int}. * Complex:: Data types for complex numbers. * Hex Floats:: Hexadecimal floating-point constants. * Zero Length:: Zero-length arrays. * Variable Length:: Arrays whose length is computed at run time. * Macro Varargs:: Macros with variable number of arguments. * Subscripting:: Any array can be subscripted, even if not an lvalue. * Pointer Arith:: Arithmetic on @code{void}-pointers and function pointers. * Initializers:: Non-constant initializers. * Constructors:: Constructor expressions give structures, unions or arrays as values. * Labeled Elements:: Labeling elements of initializers. * Cast to Union:: Casting to union type from any member of the union. * Case Ranges:: `case 1 ... 9' and such. * Function Attributes:: Declaring that functions have no side effects, or that they can never return. * Function Prototypes:: Prototype declarations and old-style definitions. * C++ Comments:: C++ comments are recognized. * Dollar Signs:: Dollar sign is allowed in identifiers. * Character Escapes:: @samp{\e} stands for the character @key{ESC}. * Variable Attributes:: Specifying attributes of variables. * Type Attributes:: Specifying attributes of types. * Alignment:: Inquiring about the alignment of a type or variable. * Inline:: Defining inline functions (as fast as macros). * Extended Asm:: Assembler instructions with C expressions as operands. (With them you can define ``built-in'' functions.) * Asm Labels:: Specifying the assembler name to use for a C symbol. * Explicit Reg Vars:: Defining variables residing in specified registers. * Alternate Keywords:: @code{__const__}, @code{__asm__}, etc., for header files. * Incomplete Enums:: @code{enum foo;}, with details to follow. * Function Names:: Printable strings which are the name of the current function. * Return Address:: Getting the return or frame address of a function. * Other Builtins:: Other built-in functions. * Deprecated Features:: Things might disappear from g++. * Backwards Compatibility:: Compatibilities with earlier definitions of C++. @end menu @end ifset @ifclear INTERNALS @menu * Statement Exprs:: Putting statements and declarations inside expressions. * Local Labels:: Labels local to a statement-expression. * Labels as Values:: Getting pointers to labels, and computed gotos. * Nested Functions:: As in Algol and Pascal, lexical scoping of functions. * Constructing Calls:: Dispatching a call to another function. * Naming Types:: Giving a name to the type of some expression. * Typeof:: @code{typeof}: referring to the type of an expression. * Lvalues:: Using @samp{?:}, @samp{,} and casts in lvalues. * Conditionals:: Omitting the middle operand of a @samp{?:} expression. * Long Long:: Double-word integers---@code{long long int}. * Complex:: Data types for complex numbers. * Hex Floats:: Hexadecimal floating-point constants. * Zero Length:: Zero-length arrays. * Variable Length:: Arrays whose length is computed at run time. * Macro Varargs:: Macros with variable number of arguments. * Subscripting:: Any array can be subscripted, even if not an lvalue. * Pointer Arith:: Arithmetic on @code{void}-pointers and function pointers. * Initializers:: Non-constant initializers. * Constructors:: Constructor expressions give structures, unions or arrays as values. * Labeled Elements:: Labeling elements of initializers. * Cast to Union:: Casting to union type from any member of the union. * Case Ranges:: `case 1 ... 9' and such. * Function Attributes:: Declaring that functions have no side effects, or that they can never return. * Function Prototypes:: Prototype declarations and old-style definitions. * C++ Comments:: C++ comments are recognized. * Dollar Signs:: Dollar sign is allowed in identifiers. * Character Escapes:: @samp{\e} stands for the character @key{ESC}. * Variable Attributes:: Specifying attributes of variables. * Type Attributes:: Specifying attributes of types. * Alignment:: Inquiring about the alignment of a type or variable. * Inline:: Defining inline functions (as fast as macros). * Extended Asm:: Assembler instructions with C expressions as operands. (With them you can define ``built-in'' functions.) * Constraints:: Constraints for asm operands * Asm Labels:: Specifying the assembler name to use for a C symbol. * Explicit Reg Vars:: Defining variables residing in specified registers. * Alternate Keywords:: @code{__const__}, @code{__asm__}, etc., for header files. * Incomplete Enums:: @code{enum foo;}, with details to follow. * Function Names:: Printable strings which are the name of the current function. * Return Address:: Getting the return or frame address of a function. * Deprecated Features:: Things might disappear from g++. * Backwards Compatibility:: Compatibilities with earlier definitions of C++. * Other Builtins:: Other built-in functions. @end menu @end ifclear @node Statement Exprs @section Statements and Declarations in Expressions @cindex statements inside expressions @cindex declarations inside expressions @cindex expressions containing statements @cindex macros, statements in expressions @c the above section title wrapped and causes an underfull hbox.. i @c changed it from "within" to "in". --mew 4feb93 A compound statement enclosed in parentheses may appear as an expression in GNU C. This allows you to use loops, switches, and local variables within an expression. Recall that a compound statement is a sequence of statements surrounded by braces; in this construct, parentheses go around the braces. For example: @example (@{ int y = foo (); int z; if (y > 0) z = y; else z = - y; z; @}) @end example @noindent is a valid (though slightly more complex than necessary) expression for the absolute value of @code{foo ()}. The last thing in the compound statement should be an expression followed by a semicolon; the value of this subexpression serves as the value of the entire construct. (If you use some other kind of statement last within the braces, the construct has type @code{void}, and thus effectively no value.) This feature is especially useful in making macro definitions ``safe'' (so that they evaluate each operand exactly once). For example, the ``maximum'' function is commonly defined as a macro in standard C as follows: @example #define max(a,b) ((a) > (b) ? (a) : (b)) @end example @noindent @cindex side effects, macro argument But this definition computes either @var{a} or @var{b} twice, with bad results if the operand has side effects. In GNU C, if you know the type of the operands (here let's assume @code{int}), you can define the macro safely as follows: @example #define maxint(a,b) \ (@{int _a = (a), _b = (b); _a > _b ? _a : _b; @}) @end example Embedded statements are not allowed in constant expressions, such as the value of an enumeration constant, the width of a bit field, or the initial value of a static variable. If you don't know the type of the operand, you can still do this, but you must use @code{typeof} (@pxref{Typeof}) or type naming (@pxref{Naming Types}). @node Local Labels @section Locally Declared Labels @cindex local labels @cindex macros, local labels Each statement expression is a scope in which @dfn{local labels} can be declared. A local label is simply an identifier; you can jump to it with an ordinary @code{goto} statement, but only from within the statement expression it belongs to. A local label declaration looks like this: @example __label__ @var{label}; @end example @noindent or @example __label__ @var{label1}, @var{label2}, @dots{}; @end example Local label declarations must come at the beginning of the statement expression, right after the @samp{(@{}, before any ordinary declarations. The label declaration defines the label @emph{name}, but does not define the label itself. You must do this in the usual way, with @code{@var{label}:}, within the statements of the statement expression. The local label feature is useful because statement expressions are often used in macros. If the macro contains nested loops, a @code{goto} can be useful for breaking out of them. However, an ordinary label whose scope is the whole function cannot be used: if the macro can be expanded several times in one function, the label will be multiply defined in that function. A local label avoids this problem. For example: @example #define SEARCH(array, target) \ (@{ \ __label__ found; \ typeof (target) _SEARCH_target = (target); \ typeof (*(array)) *_SEARCH_array = (array); \ int i, j; \ int value; \ for (i = 0; i < max; i++) \ for (j = 0; j < max; j++) \ if (_SEARCH_array[i][j] == _SEARCH_target) \ @{ value = i; goto found; @} \ value = -1; \ found: \ value; \ @}) @end example @node Labels as Values @section Labels as Values @cindex labels as values @cindex computed gotos @cindex goto with computed label @cindex address of a label You can get the address of a label defined in the current function (or a containing function) with the unary operator @samp{&&}. The value has type @code{void *}. This value is a constant and can be used wherever a constant of that type is valid. For example: @example void *ptr; @dots{} ptr = &&foo; @end example To use these values, you need to be able to jump to one. This is done with the computed goto statement@footnote{The analogous feature in Fortran is called an assigned goto, but that name seems inappropriate in C, where one can do more than simply store label addresses in label variables.}, @code{goto *@var{exp};}. For example, @example goto *ptr; @end example @noindent Any expression of type @code{void *} is allowed. One way of using these constants is in initializing a static array that will serve as a jump table: @example static void *array[] = @{ &&foo, &&bar, &&hack @}; @end example Then you can select a label with indexing, like this: @example goto *array[i]; @end example @noindent Note that this does not check whether the subscript is in bounds---array indexing in C never does that. Such an array of label values serves a purpose much like that of the @code{switch} statement. The @code{switch} statement is cleaner, so use that rather than an array unless the problem does not fit a @code{switch} statement very well. Another use of label values is in an interpreter for threaded code. The labels within the interpreter function can be stored in the threaded code for super-fast dispatching. You may not use this mechanism to jump to code in a different function. If you do that, totally unpredictable things will happen. The best way to avoid this is to store the label address only in automatic variables and never pass it as an argument. An alternate way to write the above example is @example static const int array[] = @{ &&foo - &&foo, &&bar - &&foo, &&hack - &&foo @}; goto *(&&foo + array[i]); @end example @noindent This is more friendly to code living in shared libraries, as it reduces the number of dynamic relocations that are needed, and by consequence, allows the data to be read-only. @node Nested Functions @section Nested Functions @cindex nested functions @cindex downward funargs @cindex thunks A @dfn{nested function} is a function defined inside another function. (Nested functions are not supported for GNU C++.) The nested function's name is local to the block where it is defined. For example, here we define a nested function named @code{square}, and call it twice: @example @group foo (double a, double b) @{ double square (double z) @{ return z * z; @} return square (a) + square (b); @} @end group @end example The nested function can access all the variables of the containing function that are visible at the point of its definition. This is called @dfn{lexical scoping}. For example, here we show a nested function which uses an inherited variable named @code{offset}: @example bar (int *array, int offset, int size) @{ int access (int *array, int index) @{ return array[index + offset]; @} int i; @dots{} for (i = 0; i < size; i++) @dots{} access (array, i) @dots{} @} @end example Nested function definitions are permitted within functions in the places where variable definitions are allowed; that is, in any block, before the first statement in the block. It is possible to call the nested function from outside the scope of its name by storing its address or passing the address to another function: @example hack (int *array, int size) @{ void store (int index, int value) @{ array[index] = value; @} intermediate (store, size); @} @end example Here, the function @code{intermediate} receives the address of @code{store} as an argument. If @code{intermediate} calls @code{store}, the arguments given to @code{store} are used to store into @code{array}. But this technique works only so long as the containing function (@code{hack}, in this example) does not exit. If you try to call the nested function through its address after the containing function has exited, all hell will break loose. If you try to call it after a containing scope level has exited, and if it refers to some of the variables that are no longer in scope, you may be lucky, but it's not wise to take the risk. If, however, the nested function does not refer to anything that has gone out of scope, you should be safe. GNU CC implements taking the address of a nested function using a technique called @dfn{trampolines}. A paper describing them is available as @samp{http://master.debian.org/~karlheg/Usenix88-lexic.pdf}. A nested function can jump to a label inherited from a containing function, provided the label was explicitly declared in the containing function (@pxref{Local Labels}). Such a jump returns instantly to the containing function, exiting the nested function which did the @code{goto} and any intermediate functions as well. Here is an example: @example @group bar (int *array, int offset, int size) @{ __label__ failure; int access (int *array, int index) @{ if (index > size) goto failure; return array[index + offset]; @} int i; @dots{} for (i = 0; i < size; i++) @dots{} access (array, i) @dots{} @dots{} return 0; /* @r{Control comes here from @code{access} if it detects an error.} */ failure: return -1; @} @end group @end example A nested function always has internal linkage. Declaring one with @code{extern} is erroneous. If you need to declare the nested function before its definition, use @code{auto} (which is otherwise meaningless for function declarations). @example bar (int *array, int offset, int size) @{ __label__ failure; auto int access (int *, int); @dots{} int access (int *array, int index) @{ if (index > size) goto failure; return array[index + offset]; @} @dots{} @} @end example @node Constructing Calls @section Constructing Function Calls @cindex constructing calls @cindex forwarding calls Using the built-in functions described below, you can record the arguments a function received, and call another function with the same arguments, without knowing the number or types of the arguments. You can also record the return value of that function call, and later return that value, without knowing what data type the function tried to return (as long as your caller expects that data type). @table @code @findex __builtin_apply_args @item __builtin_apply_args () This built-in function returns a pointer of type @code{void *} to data describing how to perform a call with the same arguments as were passed to the current function. The function saves the arg pointer register, structure value address, and all registers that might be used to pass arguments to a function into a block of memory allocated on the stack. Then it returns the address of that block. @findex __builtin_apply @item __builtin_apply (@var{function}, @var{arguments}, @var{size}) This built-in function invokes @var{function} (type @code{void (*)()}) with a copy of the parameters described by @var{arguments} (type @code{void *}) and @var{size} (type @code{int}). The value of @var{arguments} should be the value returned by @code{__builtin_apply_args}. The argument @var{size} specifies the size of the stack argument data, in bytes. This function returns a pointer of type @code{void *} to data describing how to return whatever value was returned by @var{function}. The data is saved in a block of memory allocated on the stack. It is not always simple to compute the proper value for @var{size}. The value is used by @code{__builtin_apply} to compute the amount of data that should be pushed on the stack and copied from the incoming argument area. @findex __builtin_return @item __builtin_return (@var{result}) This built-in function returns the value described by @var{result} from the containing function. You should specify, for @var{result}, a value returned by @code{__builtin_apply}. @end table @node Naming Types @section Naming an Expression's Type @cindex naming types You can give a name to the type of an expression using a @code{typedef} declaration with an initializer. Here is how to define @var{name} as a type name for the type of @var{exp}: @example typedef @var{name} = @var{exp}; @end example This is useful in conjunction with the statements-within-expressions feature. Here is how the two together can be used to define a safe ``maximum'' macro that operates on any arithmetic type: @example #define max(a,b) \ (@{typedef _ta = (a), _tb = (b); \ _ta _a = (a); _tb _b = (b); \ _a > _b ? _a : _b; @}) @end example @cindex underscores in variables in macros @cindex @samp{_} in variables in macros @cindex local variables in macros @cindex variables, local, in macros @cindex macros, local variables in The reason for using names that start with underscores for the local variables is to avoid conflicts with variable names that occur within the expressions that are substituted for @code{a} and @code{b}. Eventually we hope to design a new form of declaration syntax that allows you to declare variables whose scopes start only after their initializers; this will be a more reliable way to prevent such conflicts. @node Typeof @section Referring to a Type with @code{typeof} @findex typeof @findex sizeof @cindex macros, types of arguments Another way to refer to the type of an expression is with @code{typeof}. The syntax of using of this keyword looks like @code{sizeof}, but the construct acts semantically like a type name defined with @code{typedef}. There are two ways of writing the argument to @code{typeof}: with an expression or with a type. Here is an example with an expression: @example typeof (x[0](1)) @end example @noindent This assumes that @code{x} is an array of functions; the type described is that of the values of the functions. Here is an example with a typename as the argument: @example typeof (int *) @end example @noindent Here the type described is that of pointers to @code{int}. If you are writing a header file that must work when included in ANSI C programs, write @code{__typeof__} instead of @code{typeof}. @xref{Alternate Keywords}. A @code{typeof}-construct can be used anywhere a typedef name could be used. For example, you can use it in a declaration, in a cast, or inside of @code{sizeof} or @code{typeof}. @itemize @bullet @item This declares @code{y} with the type of what @code{x} points to. @example typeof (*x) y; @end example @item This declares @code{y} as an array of such values. @example typeof (*x) y[4]; @end example @item This declares @code{y} as an array of pointers to characters: @example typeof (typeof (char *)[4]) y; @end example @noindent It is equivalent to the following traditional C declaration: @example char *y[4]; @end example To see the meaning of the declaration using @code{typeof}, and why it might be a useful way to write, let's rewrite it with these macros: @example #define pointer(T) typeof(T *) #define array(T, N) typeof(T [N]) @end example @noindent Now the declaration can be rewritten this way: @example array (pointer (char), 4) y; @end example @noindent Thus, @code{array (pointer (char), 4)} is the type of arrays of 4 pointers to @code{char}. @end itemize @node Lvalues @section Generalized Lvalues @cindex compound expressions as lvalues @cindex expressions, compound, as lvalues @cindex conditional expressions as lvalues @cindex expressions, conditional, as lvalues @cindex casts as lvalues @cindex generalized lvalues @cindex lvalues, generalized @cindex extensions, @code{?:} @cindex @code{?:} extensions Compound expressions, conditional expressions and casts are allowed as lvalues provided their operands are lvalues. This means that you can take their addresses or store values into them. Standard C++ allows compound expressions and conditional expressions as lvalues, and permits casts to reference type, so use of this extension is deprecated for C++ code. For example, a compound expression can be assigned, provided the last expression in the sequence is an lvalue. These two expressions are equivalent: @example (a, b) += 5 a, (b += 5) @end example Similarly, the address of the compound expression can be taken. These two expressions are equivalent: @example &(a, b) a, &b @end example A conditional expression is a valid lvalue if its type is not void and the true and false branches are both valid lvalues. For example, these two expressions are equivalent: @example (a ? b : c) = 5 (a ? b = 5 : (c = 5)) @end example A cast is a valid lvalue if its operand is an lvalue. A simple assignment whose left-hand side is a cast works by converting the right-hand side first to the specified type, then to the type of the inner left-hand side expression. After this is stored, the value is converted back to the specified type to become the value of the assignment. Thus, if @code{a} has type @code{char *}, the following two expressions are equivalent: @example (int)a = 5 (int)(a = (char *)(int)5) @end example An assignment-with-arithmetic operation such as @samp{+=} applied to a cast performs the arithmetic using the type resulting from the cast, and then continues as in the previous case. Therefore, these two expressions are equivalent: @example (int)a += 5 (int)(a = (char *)(int) ((int)a + 5)) @end example You cannot take the address of an lvalue cast, because the use of its address would not work out coherently. Suppose that @code{&(int)f} were permitted, where @code{f} has type @code{float}. Then the following statement would try to store an integer bit-pattern where a floating point number belongs: @example *&(int)f = 1; @end example This is quite different from what @code{(int)f = 1} would do---that would convert 1 to floating point and store it. Rather than cause this inconsistency, we think it is better to prohibit use of @samp{&} on a cast. If you really do want an @code{int *} pointer with the address of @code{f}, you can simply write @code{(int *)&f}. @node Conditionals @section Conditionals with Omitted Operands @cindex conditional expressions, extensions @cindex omitted middle-operands @cindex middle-operands, omitted @cindex extensions, @code{?:} @cindex @code{?:} extensions The middle operand in a conditional expression may be omitted. Then if the first operand is nonzero, its value is the value of the conditional expression. Therefore, the expression @example x ? : y @end example @noindent has the value of @code{x} if that is nonzero; otherwise, the value of @code{y}. This example is perfectly equivalent to @example x ? x : y @end example @cindex side effect in ?: @cindex ?: side effect @noindent In this simple case, the ability to omit the middle operand is not especially useful. When it becomes useful is when the first operand does, or may (if it is a macro argument), contain a side effect. Then repeating the operand in the middle would perform the side effect twice. Omitting the middle operand uses the value already computed without the undesirable effects of recomputing it. @node Long Long @section Double-Word Integers @cindex @code{long long} data types @cindex double-word arithmetic @cindex multiprecision arithmetic GNU C supports data types for integers that are twice as long as @code{int}. Simply write @code{long long int} for a signed integer, or @code{unsigned long long int} for an unsigned integer. To make an integer constant of type @code{long long int}, add the suffix @code{LL} to the integer. To make an integer constant of type @code{unsigned long long int}, add the suffix @code{ULL} to the integer. You can use these types in arithmetic like any other integer types. Addition, subtraction, and bitwise boolean operations on these types are open-coded on all types of machines. Multiplication is open-coded if the machine supports fullword-to-doubleword a widening multiply instruction. Division and shifts are open-coded only on machines that provide special support. The operations that are not open-coded use special library routines that come with GNU CC. There may be pitfalls when you use @code{long long} types for function arguments, unless you declare function prototypes. If a function expects type @code{int} for its argument, and you pass a value of type @code{long long int}, confusion will result because the caller and the subroutine will disagree about the number of bytes for the argument. Likewise, if the function expects @code{long long int} and you pass @code{int}. The best way to avoid such problems is to use prototypes. @node Complex @section Complex Numbers @cindex complex numbers GNU C supports complex data types. You can declare both complex integer types and complex floating types, using the keyword @code{__complex__}. For example, @samp{__complex__ double x;} declares @code{x} as a variable whose real part and imaginary part are both of type @code{double}. @samp{__complex__ short int y;} declares @code{y} to have real and imaginary parts of type @code{short int}; this is not likely to be useful, but it shows that the set of complex types is complete. To write a constant with a complex data type, use the suffix @samp{i} or @samp{j} (either one; they are equivalent). For example, @code{2.5fi} has type @code{__complex__ float} and @code{3i} has type @code{__complex__ int}. Such a constant always has a pure imaginary value, but you can form any complex value you like by adding one to a real constant. To extract the real part of a complex-valued expression @var{exp}, write @code{__real__ @var{exp}}. Likewise, use @code{__imag__} to extract the imaginary part. The operator @samp{~} performs complex conjugation when used on a value with a complex type. GNU CC can allocate complex automatic variables in a noncontiguous fashion; it's even possible for the real part to be in a register while the imaginary part is on the stack (or vice-versa). None of the supported debugging info formats has a way to represent noncontiguous allocation like this, so GNU CC describes a noncontiguous complex variable as if it were two separate variables of noncomplex type. If the variable's actual name is @code{foo}, the two fictitious variables are named @code{foo$real} and @code{foo$imag}. You can examine and set these two fictitious variables with your debugger. A future version of GDB will know how to recognize such pairs and treat them as a single variable with a complex type. @node Hex Floats @section Hex Floats @cindex hex floats GNU CC recognizes floating-point numbers written not only in the usual decimal notation, such as @code{1.55e1}, but also numbers such as @code{0x1.fp3} written in hexadecimal format. In that format the @code{0x} hex introducer and the @code{p} or @code{P} exponent field are mandatory. The exponent is a decimal number that indicates the power of 2 by which the significand part will be multiplied. Thus @code{0x1.f} is 1 15/16, @code{p3} multiplies it by 8, and the value of @code{0x1.fp3} is the same as @code{1.55e1}. Unlike for floating-point numbers in the decimal notation the exponent is always required in the hexadecimal notation. Otherwise the compiler would not be able to resolve the ambiguity of, e.g., @code{0x1.f}. This could mean @code{1.0f} or @code{1.9375} since @code{f} is also the extension for floating-point constants of type @code{float}. @node Zero Length @section Arrays of Length Zero @cindex arrays of length zero @cindex zero-length arrays @cindex length-zero arrays Zero-length arrays are allowed in GNU C. They are very useful as the last element of a structure which is really a header for a variable-length object: @example struct line @{ int length; char contents[0]; @}; @{ struct line *thisline = (struct line *) malloc (sizeof (struct line) + this_length); thisline->length = this_length; @} @end example In standard C, you would have to give @code{contents} a length of 1, which means either you waste space or complicate the argument to @code{malloc}. @node Variable Length @section Arrays of Variable Length @cindex variable-length arrays @cindex arrays of variable length Variable-length automatic arrays are allowed in GNU C. These arrays are declared like any other automatic arrays, but with a length that is not a constant expression. The storage is allocated at the point of declaration and deallocated when the brace-level is exited. For example: @example FILE * concat_fopen (char *s1, char *s2, char *mode) @{ char str[strlen (s1) + strlen (s2) + 1]; strcpy (str, s1); strcat (str, s2); return fopen (str, mode); @} @end example @cindex scope of a variable length array @cindex variable-length array scope @cindex deallocating variable length arrays Jumping or breaking out of the scope of the array name deallocates the storage. Jumping into the scope is not allowed; you get an error message for it. @cindex @code{alloca} vs variable-length arrays You can use the function @code{alloca} to get an effect much like variable-length arrays. The function @code{alloca} is available in many other C implementations (but not in all). On the other hand, variable-length arrays are more elegant. There are other differences between these two methods. Space allocated with @code{alloca} exists until the containing @emph{function} returns. The space for a variable-length array is deallocated as soon as the array name's scope ends. (If you use both variable-length arrays and @code{alloca} in the same function, deallocation of a variable-length array will also deallocate anything more recently allocated with @code{alloca}.) You can also use variable-length arrays as arguments to functions: @example struct entry tester (int len, char data[len][len]) @{ @dots{} @} @end example The length of an array is computed once when the storage is allocated and is remembered for the scope of the array in case you access it with @code{sizeof}. If you want to pass the array first and the length afterward, you can use a forward declaration in the parameter list---another GNU extension. @example struct entry tester (int len; char data[len][len], int len) @{ @dots{} @} @end example @cindex parameter forward declaration The @samp{int len} before the semicolon is a @dfn{parameter forward declaration}, and it serves the purpose of making the name @code{len} known when the declaration of @code{data} is parsed. You can write any number of such parameter forward declarations in the parameter list. They can be separated by commas or semicolons, but the last one must end with a semicolon, which is followed by the ``real'' parameter declarations. Each forward declaration must match a ``real'' declaration in parameter name and data type. @node Macro Varargs @section Macros with Variable Numbers of Arguments @cindex variable number of arguments @cindex macro with variable arguments @cindex rest argument (in macro) In GNU C, a macro can accept a variable number of arguments, much as a function can. The syntax for defining the macro looks much like that used for a function. Here is an example: @example #define eprintf(format, args...) \ fprintf (stderr, format , ## args) @end example Here @code{args} is a @dfn{rest argument}: it takes in zero or more arguments, as many as the call contains. All of them plus the commas between them form the value of @code{args}, which is substituted into the macro body where @code{args} is used. Thus, we have this expansion: @example eprintf ("%s:%d: ", input_file_name, line_number) @expansion{} fprintf (stderr, "%s:%d: " , input_file_name, line_number) @end example @noindent Note that the comma after the string constant comes from the definition of @code{eprintf}, whereas the last comma comes from the value of @code{args}. The reason for using @samp{##} is to handle the case when @code{args} matches no arguments at all. In this case, @code{args} has an empty value. In this case, the second comma in the definition becomes an embarrassment: if it got through to the expansion of the macro, we would get something like this: @example fprintf (stderr, "success!\n" , ) @end example @noindent which is invalid C syntax. @samp{##} gets rid of the comma, so we get the following instead: @example fprintf (stderr, "success!\n") @end example This is a special feature of the GNU C preprocessor: @samp{##} before a rest argument that is empty discards the preceding sequence of non-whitespace characters from the macro definition. (If another macro argument precedes, none of it is discarded.) It might be better to discard the last preprocessor token instead of the last preceding sequence of non-whitespace characters; in fact, we may someday change this feature to do so. We advise you to write the macro definition so that the preceding sequence of non-whitespace characters is just a single token, so that the meaning will not change if we change the definition of this feature. @node Subscripting @section Non-Lvalue Arrays May Have Subscripts @cindex subscripting @cindex arrays, non-lvalue @cindex subscripting and function values Subscripting is allowed on arrays that are not lvalues, even though the unary @samp{&} operator is not. For example, this is valid in GNU C though not valid in other C dialects: @example @group struct foo @{int a[4];@}; struct foo f(); bar (int index) @{ return f().a[index]; @} @end group @end example @node Pointer Arith @section Arithmetic on @code{void}- and Function-Pointers @cindex void pointers, arithmetic @cindex void, size of pointer to @cindex function pointers, arithmetic @cindex function, size of pointer to In GNU C, addition and subtraction operations are supported on pointers to @code{void} and on pointers to functions. This is done by treating the size of a @code{void} or of a function as 1. A consequence of this is that @code{sizeof} is also allowed on @code{void} and on function types, and returns 1. The option @samp{-Wpointer-arith} requests a warning if these extensions are used. @node Initializers @section Non-Constant Initializers @cindex initializers, non-constant @cindex non-constant initializers As in standard C++, the elements of an aggregate initializer for an automatic variable are not required to be constant expressions in GNU C. Here is an example of an initializer with run-time varying elements: @example foo (float f, float g) @{ float beat_freqs[2] = @{ f-g, f+g @}; @dots{} @} @end example @node Constructors @section Constructor Expressions @cindex constructor expressions @cindex initializations in expressions @cindex structures, constructor expression @cindex expressions, constructor GNU C supports constructor expressions. A constructor looks like a cast containing an initializer. Its value is an object of the type specified in the cast, containing the elements specified in the initializer. Usually, the specified type is a structure. Assume that @code{struct foo} and @code{structure} are declared as shown: @example struct foo @{int a; char b[2];@} structure; @end example @noindent Here is an example of constructing a @code{struct foo} with a constructor: @example structure = ((struct foo) @{x + y, 'a', 0@}); @end example @noindent This is equivalent to writing the following: @example @{ struct foo temp = @{x + y, 'a', 0@}; structure = temp; @} @end example You can also construct an array. If all the elements of the constructor are (made up of) simple constant expressions, suitable for use in initializers, then the constructor is an lvalue and can be coerced to a pointer to its first element, as shown here: @example char **foo = (char *[]) @{ "x", "y", "z" @}; @end example Array constructors whose elements are not simple constants are not very useful, because the constructor is not an lvalue. There are only two valid ways to use it: to subscript it, or initialize an array variable with it. The former is probably slower than a @code{switch} statement, while the latter does the same thing an ordinary C initializer would do. Here is an example of subscripting an array constructor: @example output = ((int[]) @{ 2, x, 28 @}) [input]; @end example Constructor expressions for scalar types and union types are is also allowed, but then the constructor expression is equivalent to a cast. @node Labeled Elements @section Labeled Elements in Initializers @cindex initializers with labeled elements @cindex labeled elements in initializers @cindex case labels in initializers Standard C requires the elements of an initializer to appear in a fixed order, the same as the order of the elements in the array or structure being initialized. In GNU C you can give the elements in any order, specifying the array indices or structure field names they apply to. This extension is not implemented in GNU C++. To specify an array index, write @samp{[@var{index}]} or @samp{[@var{index}] =} before the element value. For example, @example int a[6] = @{ [4] 29, [2] = 15 @}; @end example @noindent is equivalent to @example int a[6] = @{ 0, 0, 15, 0, 29, 0 @}; @end example @noindent The index values must be constant expressions, even if the array being initialized is automatic. To initialize a range of elements to the same value, write @samp{[@var{first} ... @var{last}] = @var{value}}. For example, @example int widths[] = @{ [0 ... 9] = 1, [10 ... 99] = 2, [100] = 3 @}; @end example @noindent Note that the length of the array is the highest value specified plus one. In a structure initializer, specify the name of a field to initialize with @samp{@var{fieldname}:} before the element value. For example, given the following structure, @example struct point @{ int x, y; @}; @end example @noindent the following initialization @example struct point p = @{ y: yvalue, x: xvalue @}; @end example @noindent is equivalent to @example struct point p = @{ xvalue, yvalue @}; @end example Another syntax which has the same meaning is @samp{.@var{fieldname} =}., as shown here: @example struct point p = @{ .y = yvalue, .x = xvalue @}; @end example You can also use an element label (with either the colon syntax or the period-equal syntax) when initializing a union, to specify which element of the union should be used. For example, @example union foo @{ int i; double d; @}; union foo f = @{ d: 4 @}; @end example @noindent will convert 4 to a @code{double} to store it in the union using the second element. By contrast, casting 4 to type @code{union foo} would store it into the union as the integer @code{i}, since it is an integer. (@xref{Cast to Union}.) You can combine this technique of naming elements with ordinary C initialization of successive elements. Each initializer element that does not have a label applies to the next consecutive element of the array or structure. For example, @example int a[6] = @{ [1] = v1, v2, [4] = v4 @}; @end example @noindent is equivalent to @example int a[6] = @{ 0, v1, v2, 0, v4, 0 @}; @end example Labeling the elements of an array initializer is especially useful when the indices are characters or belong to an @code{enum} type. For example: @example int whitespace[256] = @{ [' '] = 1, ['\t'] = 1, ['\h'] = 1, ['\f'] = 1, ['\n'] = 1, ['\r'] = 1 @}; @end example @node Case Ranges @section Case Ranges @cindex case ranges @cindex ranges in case statements You can specify a range of consecutive values in a single @code{case} label, like this: @example case @var{low} ... @var{high}: @end example @noindent This has the same effect as the proper number of individual @code{case} labels, one for each integer value from @var{low} to @var{high}, inclusive. This feature is especially useful for ranges of ASCII character codes: @example case 'A' ... 'Z': @end example @strong{Be careful:} Write spaces around the @code{...}, for otherwise it may be parsed wrong when you use it with integer values. For example, write this: @example case 1 ... 5: @end example @noindent rather than this: @example case 1...5: @end example @node Cast to Union @section Cast to a Union Type @cindex cast to a union @cindex union, casting to a A cast to union type is similar to other casts, except that the type specified is a union type. You can specify the type either with @code{union @var{tag}} or with a typedef name. A cast to union is actually a constructor though, not a cast, and hence does not yield an lvalue like normal casts. (@xref{Constructors}.) The types that may be cast to the union type are those of the members of the union. Thus, given the following union and variables: @example union foo @{ int i; double d; @}; int x; double y; @end example @noindent both @code{x} and @code{y} can be cast to type @code{union} foo. Using the cast as the right-hand side of an assignment to a variable of union type is equivalent to storing in a member of the union: @example union foo u; @dots{} u = (union foo) x @equiv{} u.i = x u = (union foo) y @equiv{} u.d = y @end example You can also use the union cast as a function argument: @example void hack (union foo); @dots{} hack ((union foo) x); @end example @node Function Attributes @section Declaring Attributes of Functions @cindex function attributes @cindex declaring attributes of functions @cindex functions that never return @cindex functions that have no side effects @cindex functions in arbitrary sections @cindex functions that behave like malloc @cindex @code{volatile} applied to function @cindex @code{const} applied to function @cindex functions with @code{printf}, @code{scanf} or @code{strftime} style arguments @cindex functions that are passed arguments in registers on the 386 @cindex functions that pop the argument stack on the 386 @cindex functions that do not pop the argument stack on the 386 In GNU C, you declare certain things about functions called in your program which help the compiler optimize function calls and check your code more carefully. The keyword @code{__attribute__} allows you to specify special attributes when making a declaration. This keyword is followed by an attribute specification inside double parentheses. Ten attributes, @code{noreturn}, @code{const}, @code{format}, @code{no_instrument_function}, @code{section}, @code{constructor}, @code{destructor}, @code{unused}, @code{weak} and @code{malloc} are currently defined for functions. Other attributes, including @code{section} are supported for variables declarations (@pxref{Variable Attributes}) and for types (@pxref{Type Attributes}). You may also specify attributes with @samp{__} preceding and following each keyword. This allows you to use them in header files without being concerned about a possible macro of the same name. For example, you may use @code{__noreturn__} instead of @code{noreturn}. @table @code @cindex @code{noreturn} function attribute @item noreturn A few standard library functions, such as @code{abort} and @code{exit}, cannot return. GNU CC knows this automatically. Some programs define their own functions that never return. You can declare them @code{noreturn} to tell the compiler this fact. For example, @smallexample void fatal () __attribute__ ((noreturn)); void fatal (@dots{}) @{ @dots{} /* @r{Print error message.} */ @dots{} exit (1); @} @end smallexample The @code{noreturn} keyword tells the compiler to assume that @code{fatal} cannot return. It can then optimize without regard to what would happen if @code{fatal} ever did return. This makes slightly better code. More importantly, it helps avoid spurious warnings of uninitialized variables. Do not assume that registers saved by the calling function are restored before calling the @code{noreturn} function. It does not make sense for a @code{noreturn} function to have a return type other than @code{void}. The attribute @code{noreturn} is not implemented in GNU C versions earlier than 2.5. An alternative way to declare that a function does not return, which works in the current version and in some older versions, is as follows: @smallexample typedef void voidfn (); volatile voidfn fatal; @end smallexample @cindex @code{pure} function attribute @item pure Many functions have no effects except the return value and their return value depends only on the parameters and/or global variables. Such a function can be subject to common subexpression elimination and loop optimization just as an arithmetic operator would be. These functions should be declared with the attribute @code{pure}. For example, @smallexample int square (int) __attribute__ ((pure)); @end smallexample @noindent says that the hypothetical function @code{square} is safe to call fewer times than the program says. Some of common examples of pure functions are @code{strlen} or @code{memcmp}. Interesting non-pure functions are functions with infinite loops or those depending on volatile memory or other system resource, that may change between two consecutive calls (such as @code{feof} in a multithreading environment). The attribute @code{pure} is not implemented in GNU C versions earlier than 2.96. @cindex @code{const} function attribute @item const Many functions do not examine any values except their arguments, and have no effects except the return value. Basically this is just slightly more strict class than the "pure" attribute above, since function is not allowed to read global memory. @cindex pointer arguments Note that a function that has pointer arguments and examines the data pointed to must @emph{not} be declared @code{const}. Likewise, a function that calls a non-@code{const} function usually must not be @code{const}. It does not make sense for a @code{const} function to return @code{void}. The attribute @code{const} is not implemented in GNU C versions earlier than 2.5. An alternative way to declare that a function has no side effects, which works in the current version and in some older versions, is as follows: @smallexample typedef int intfn (); extern const intfn square; @end smallexample This approach does not work in GNU C++ from 2.6.0 on, since the language specifies that the @samp{const} must be attached to the return value. @item format (@var{archetype}, @var{string-index}, @var{first-to-check}) @cindex @code{format} function attribute The @code{format} attribute specifies that a function takes @code{printf}, @code{scanf}, or @code{strftime} style arguments which should be type-checked against a format string. For example, the declaration: @smallexample extern int my_printf (void *my_object, const char *my_format, ...) __attribute__ ((format (printf, 2, 3))); @end smallexample @noindent causes the compiler to check the arguments in calls to @code{my_printf} for consistency with the @code{printf} style format string argument @code{my_format}. The parameter @var{archetype} determines how the format string is interpreted, and should be either @code{printf}, @code{scanf}, or @code{strftime}. The parameter @var{string-index} specifies which argument is the format string argument (starting from 1), while @var{first-to-check} is the number of the first argument to check against the format string. For functions where the arguments are not available to be checked (such as @code{vprintf}), specify the third parameter as zero. In this case the compiler only checks the format string for consistency. In the example above, the format string (@code{my_format}) is the second argument of the function @code{my_print}, and the arguments to check start with the third argument, so the correct parameters for the format attribute are 2 and 3. The @code{format} attribute allows you to identify your own functions which take format strings as arguments, so that GNU CC can check the calls to these functions for errors. The compiler always checks formats for the ANSI library functions @code{printf}, @code{fprintf}, @code{sprintf}, @code{scanf}, @code{fscanf}, @code{sscanf}, @code{strftime}, @code{vprintf}, @code{vfprintf} and @code{vsprintf} whenever such warnings are requested (using @samp{-Wformat}), so there is no need to modify the header file @file{stdio.h}. @item format_arg (@var{string-index}) @cindex @code{format_arg} function attribute The @code{format_arg} attribute specifies that a function takes @code{printf} or @code{scanf} style arguments, modifies it (for example, to translate it into another language), and passes it to a @code{printf} or @code{scanf} style function. For example, the declaration: @smallexample extern char * my_dgettext (char *my_domain, const char *my_format) __attribute__ ((format_arg (2))); @end smallexample @noindent causes the compiler to check the arguments in calls to @code{my_dgettext} whose result is passed to a @code{printf}, @code{scanf}, or @code{strftime} type function for consistency with the @code{printf} style format string argument @code{my_format}. The parameter @var{string-index} specifies which argument is the format string argument (starting from 1). The @code{format-arg} attribute allows you to identify your own functions which modify format strings, so that GNU CC can check the calls to @code{printf}, @code{scanf}, or @code{strftime} function whose operands are a call to one of your own function. The compiler always treats @code{gettext}, @code{dgettext}, and @code{dcgettext} in this manner. @item no_instrument_function @cindex @code{no_instrument_function} function attribute If @samp{-finstrument-functions} is given, profiling function calls will be generated at entry and exit of most user-compiled functions. Functions with this attribute will not be so instrumented. @item section ("section-name") @cindex @code{section} function attribute Normally, the compiler places the code it generates in the @code{text} section. Sometimes, however, you need additional sections, or you need certain particular functions to appear in special sections. The @code{section} attribute specifies that a function lives in a particular section. For example, the declaration: @smallexample extern void foobar (void) __attribute__ ((section ("bar"))); @end smallexample @noindent puts the function @code{foobar} in the @code{bar} section. Some file formats do not support arbitrary sections so the @code{section} attribute is not available on all platforms. If you need to map the entire contents of a module to a particular section, consider using the facilities of the linker instead. @item constructor @itemx destructor @cindex @code{constructor} function attribute @cindex @code{destructor} function attribute The @code{constructor} attribute causes the function to be called automatically before execution enters @code{main ()}. Similarly, the @code{destructor} attribute causes the function to be called automatically after @code{main ()} has completed or @code{exit ()} has been called. Functions with these attributes are useful for initializing data that will be used implicitly during the execution of the program. These attributes are not currently implemented for Objective C. @item unused This attribute, attached to a function, means that the function is meant to be possibly unused. GNU CC will not produce a warning for this function. GNU C++ does not currently support this attribute as definitions without parameters are valid in C++. @item weak @cindex @code{weak} attribute The @code{weak} attribute causes the declaration to be emitted as a weak symbol rather than a global. This is primarily useful in defining library functions which can be overridden in user code, though it can also be used with non-function declarations. Weak symbols are supported for ELF targets, and also for a.out targets when using the GNU assembler and linker. @item malloc @cindex @code{malloc} attribute The @code{malloc} attribute is used to tell the compiler that a function may be treated as if it were the malloc function. The compiler assumes that calls to malloc result in a pointers that cannot alias anything. This will often improve optimization. @item alias ("target") @cindex @code{alias} attribute The @code{alias} attribute causes the declaration to be emitted as an alias for another symbol, which must be specified. For instance, @smallexample void __f () @{ /* do something */; @} void f () __attribute__ ((weak, alias ("__f"))); @end smallexample declares @samp{f} to be a weak alias for @samp{__f}. In C++, the mangled name for the target must be used. Not all target machines support this attribute. @item no_check_memory_usage @cindex @code{no_check_memory_usage} function attribute The @code{no_check_memory_usage} attribute causes GNU CC to omit checks of memory references when it generates code for that function. Normally if you specify @samp{-fcheck-memory-usage} (see @pxref{Code Gen Options}), GNU CC generates calls to support routines before most memory accesses to permit support code to record usage and detect uses of uninitialized or unallocated storage. Since GNU CC cannot handle @code{asm} statements properly they are not allowed in such functions. If you declare a function with this attribute, GNU CC will not generate memory checking code for that function, permitting the use of @code{asm} statements without having to compile that function with different options. This also allows you to write support routines of your own if you wish, without getting infinite recursion if they get compiled with @code{-fcheck-memory-usage}. @item regparm (@var{number}) @cindex functions that are passed arguments in registers on the 386 On the Intel 386, the @code{regparm} attribute causes the compiler to pass up to @var{number} integer arguments in registers @var{EAX}, @var{EDX}, and @var{ECX} instead of on the stack. Functions that take a variable number of arguments will continue to be passed all of their arguments on the stack. @item stdcall @cindex functions that pop the argument stack on the 386 On the Intel 386, the @code{stdcall} attribute causes the compiler to assume that the called function will pop off the stack space used to pass arguments, unless it takes a variable number of arguments. The PowerPC compiler for Windows NT currently ignores the @code{stdcall} attribute. @item cdecl @cindex functions that do pop the argument stack on the 386 On the Intel 386, the @code{cdecl} attribute causes the compiler to assume that the calling function will pop off the stack space used to pass arguments. This is useful to override the effects of the @samp{-mrtd} switch. The PowerPC compiler for Windows NT currently ignores the @code{cdecl} attribute. @item longcall @cindex functions called via pointer on the RS/6000 and PowerPC On the RS/6000 and PowerPC, the @code{longcall} attribute causes the compiler to always call the function via a pointer, so that functions which reside further than 64 megabytes (67,108,864 bytes) from the current location can be called. @item long_call/short_call @cindex indirect calls on ARM This attribute allows to specify how to call a particular function on ARM. Both attributes override the @code{-mlong-calls} (@pxref{ARM Options}) command line switch and @code{#pragma long_calls} settings. The @code{long_call} attribute causes the compiler to always call the function by first loading its address into a register and then using the contents of that register. The @code{short_call} attribute always places the offset to the function from the call site into the @samp{BL} instruction directly. @item dllimport @cindex functions which are imported from a dll on PowerPC Windows NT On the PowerPC running Windows NT, the @code{dllimport} attribute causes the compiler to call the function via a global pointer to the function pointer that is set up by the Windows NT dll library. The pointer name is formed by combining @code{__imp_} and the function name. @item dllexport @cindex functions which are exported from a dll on PowerPC Windows NT On the PowerPC running Windows NT, the @code{dllexport} attribute causes the compiler to provide a global pointer to the function pointer, so that it can be called with the @code{dllimport} attribute. The pointer name is formed by combining @code{__imp_} and the function name. @item exception (@var{except-func} [, @var{except-arg}]) @cindex functions which specify exception handling on PowerPC Windows NT On the PowerPC running Windows NT, the @code{exception} attribute causes the compiler to modify the structured exception table entry it emits for the declared function. The string or identifier @var{except-func} is placed in the third entry of the structured exception table. It represents a function, which is called by the exception handling mechanism if an exception occurs. If it was specified, the string or identifier @var{except-arg} is placed in the fourth entry of the structured exception table. @item function_vector @cindex calling functions through the function vector on the H8/300 processors Use this option on the H8/300 and H8/300H to indicate that the specified function should be called through the function vector. Calling a function through the function vector will reduce code size, however; the function vector has a limited size (maximum 128 entries on the H8/300 and 64 entries on the H8/300H) and shares space with the interrupt vector. You must use GAS and GLD from GNU binutils version 2.7 or later for this option to work correctly. @item interrupt_handler @cindex interrupt handler functions on the H8/300 processors Use this option on the H8/300 and H8/300H to indicate that the specified function is an interrupt handler. The compiler will generate function entry and exit sequences suitable for use in an interrupt handler when this attribute is present. @item eightbit_data @cindex eight bit data on the H8/300 and H8/300H Use this option on the H8/300 and H8/300H to indicate that the specified variable should be placed into the eight bit data section. The compiler will generate more efficient code for certain operations on data in the eight bit data area. Note the eight bit data area is limited to 256 bytes of data. You must use GAS and GLD from GNU binutils version 2.7 or later for this option to work correctly. @item tiny_data @cindex tiny data section on the H8/300H Use this option on the H8/300H to indicate that the specified variable should be placed into the tiny data section. The compiler will generate more efficient code for loads and stores on data in the tiny data section. Note the tiny data area is limited to slightly under 32kbytes of data. @item interrupt @cindex interrupt handlers on the M32R/D Use this option on the M32R/D to indicate that the specified function is an interrupt handler. The compiler will generate function entry and exit sequences suitable for use in an interrupt handler when this attribute is present. Interrupt handler functions on the AVR processors Use this option on the AVR to indicate that the specified function is an interrupt handler. The compiler will generate function entry and exit sequences suitable for use in an interrupt handler when this attribute is present. Interrupts will be enabled inside function. @item signal @cindex signal handler functions on the AVR processors Use this option on the AVR to indicate that the specified function is an signal handler. The compiler will generate function entry and exit sequences suitable for use in an signal handler when this attribute is present. Interrupts will be disabled inside function. @item naked @cindex function without a prologue/epilogue code on the AVR processors Use this option on the AVR to indicate that the specified function don't have a prologue/epilogue. The compiler don't generate function entry and exit sequences. @item model (@var{model-name}) @cindex function addressability on the M32R/D Use this attribute on the M32R/D to set the addressability of an object, and the code generated for a function. The identifier @var{model-name} is one of @code{small}, @code{medium}, or @code{large}, representing each of the code models. Small model objects live in the lower 16MB of memory (so that their addresses can be loaded with the @code{ld24} instruction), and are callable with the @code{bl} instruction. Medium model objects may live anywhere in the 32 bit address space (the compiler will generate @code{seth/add3} instructions to load their addresses), and are callable with the @code{bl} instruction. Large model objects may live anywhere in the 32 bit address space (the compiler will generate @code{seth/add3} instructions to load their addresses), and may not be reachable with the @code{bl} instruction (the compiler will generate the much slower @code{seth/add3/jl} instruction sequence). @end table You can specify multiple attributes in a declaration by separating them by commas within the double parentheses or by immediately following an attribute declaration with another attribute declaration. @cindex @code{#pragma}, reason for not using @cindex pragma, reason for not using Some people object to the @code{__attribute__} feature, suggesting that ANSI C's @code{#pragma} should be used instead. There are two reasons for not doing this. @enumerate @item It is impossible to generate @code{#pragma} commands from a macro. @item There is no telling what the same @code{#pragma} might mean in another compiler. @end enumerate These two reasons apply to almost any application that might be proposed for @code{#pragma}. It is basically a mistake to use @code{#pragma} for @emph{anything}. @node Function Prototypes @section Prototypes and Old-Style Function Definitions @cindex function prototype declarations @cindex old-style function definitions @cindex promotion of formal parameters GNU C extends ANSI C to allow a function prototype to override a later old-style non-prototype definition. Consider the following example: @example /* @r{Use prototypes unless the compiler is old-fashioned.} */ #ifdef __STDC__ #define P(x) x #else #define P(x) () #endif /* @r{Prototype function declaration.} */ int isroot P((uid_t)); /* @r{Old-style function definition.} */ int isroot (x) /* ??? lossage here ??? */ uid_t x; @{ return x == 0; @} @end example Suppose the type @code{uid_t} happens to be @code{short}. ANSI C does not allow this example, because subword arguments in old-style non-prototype definitions are promoted. Therefore in this example the function definition's argument is really an @code{int}, which does not match the prototype argument type of @code{short}. This restriction of ANSI C makes it hard to write code that is portable to traditional C compilers, because the programmer does not know whether the @code{uid_t} type is @code{short}, @code{int}, or @code{long}. Therefore, in cases like these GNU C allows a prototype to override a later old-style definition. More precisely, in GNU C, a function prototype argument type overrides the argument type specified by a later old-style definition if the former type is the same as the latter type before promotion. Thus in GNU C the above example is equivalent to the following: @example int isroot (uid_t); int isroot (uid_t x) @{ return x == 0; @} @end example GNU C++ does not support old-style function definitions, so this extension is irrelevant. @node C++ Comments @section C++ Style Comments @cindex // @cindex C++ comments @cindex comments, C++ style In GNU C, you may use C++ style comments, which start with @samp{//} and continue until the end of the line. Many other C implementations allow such comments, and they are likely to be in a future C standard. However, C++ style comments are not recognized if you specify @w{@samp{-ansi}} or @w{@samp{-traditional}}, since they are incompatible with traditional constructs like @code{dividend//*comment*/divisor}. @node Dollar Signs @section Dollar Signs in Identifier Names @cindex $ @cindex dollar signs in identifier names @cindex identifier names, dollar signs in In GNU C, you may normally use dollar signs in identifier names. This is because many traditional C implementations allow such identifiers. However, dollar signs in identifiers are not supported on a few target machines, typically because the target assembler does not allow them. @node Character Escapes @section The Character @key{ESC} in Constants You can use the sequence @samp{\e} in a string or character constant to stand for the ASCII character @key{ESC}. @node Alignment @section Inquiring on Alignment of Types or Variables @cindex alignment @cindex type alignment @cindex variable alignment The keyword @code{__alignof__} allows you to inquire about how an object is aligned, or the minimum alignment usually required by a type. Its syntax is just like @code{sizeof}. For example, if the target machine requires a @code{double} value to be aligned on an 8-byte boundary, then @code{__alignof__ (double)} is 8. This is true on many RISC machines. On more traditional machine designs, @code{__alignof__ (double)} is 4 or even 2. Some machines never actually require alignment; they allow reference to any data type even at an odd addresses. For these machines, @code{__alignof__} reports the @emph{recommended} alignment of a type. When the operand of @code{__alignof__} is an lvalue rather than a type, the value is the largest alignment that the lvalue is known to have. It may have this alignment as a result of its data type, or because it is part of a structure and inherits alignment from that structure. For example, after this declaration: @example struct foo @{ int x; char y; @} foo1; @end example @noindent the value of @code{__alignof__ (foo1.y)} is probably 2 or 4, the same as @code{__alignof__ (int)}, even though the data type of @code{foo1.y} does not itself demand any alignment.@refill It is an error to ask for the alignment of an incomplete type. A related feature which lets you specify the alignment of an object is @code{__attribute__ ((aligned (@var{alignment})))}; see the following section. @node Variable Attributes @section Specifying Attributes of Variables @cindex attribute of variables @cindex variable attributes The keyword @code{__attribute__} allows you to specify special attributes of variables or structure fields. This keyword is followed by an attribute specification inside double parentheses. Eight attributes are currently defined for variables: @code{aligned}, @code{mode}, @code{nocommon}, @code{packed}, @code{section}, @code{transparent_union}, @code{unused}, and @code{weak}. Other attributes are available for functions (@pxref{Function Attributes}) and for types (@pxref{Type Attributes}). You may also specify attributes with @samp{__} preceding and following each keyword. This allows you to use them in header files without being concerned about a possible macro of the same name. For example, you may use @code{__aligned__} instead of @code{aligned}. @table @code @cindex @code{aligned} attribute @item aligned (@var{alignment}) This attribute specifies a minimum alignment for the variable or structure field, measured in bytes. For example, the declaration: @smallexample int x __attribute__ ((aligned (16))) = 0; @end smallexample @noindent causes the compiler to allocate the global variable @code{x} on a 16-byte boundary. On a 68040, this could be used in conjunction with an @code{asm} expression to access the @code{move16} instruction which requires 16-byte aligned operands. You can also specify the alignment of structure fields. For example, to create a double-word aligned @code{int} pair, you could write: @smallexample struct foo @{ int x[2] __attribute__ ((aligned (8))); @}; @end smallexample @noindent This is an alternative to creating a union with a @code{double} member that forces the union to be double-word aligned. It is not possible to specify the alignment of functions; the alignment of functions is determined by the machine's requirements and cannot be changed. You cannot specify alignment for a typedef name because such a name is just an alias, not a distinct type. As in the preceding examples, you can explicitly specify the alignment (in bytes) that you wish the compiler to use for a given variable or structure field. Alternatively, you can leave out the alignment factor and just ask the compiler to align a variable or field to the maximum useful alignment for the target machine you are compiling for. For example, you could write: @smallexample short array[3] __attribute__ ((aligned)); @end smallexample Whenever you leave out the alignment factor in an @code{aligned} attribute specification, the compiler automatically sets the alignment for the declared variable or field to the largest alignment which is ever used for any data type on the target machine you are compiling for. Doing this can often make copy operations more efficient, because the compiler can use whatever instructions copy the biggest chunks of memory when performing copies to or from the variables or fields that you have aligned this way. The @code{aligned} attribute can only increase the alignment; but you can decrease it by specifying @code{packed} as well. See below. Note that the effectiveness of @code{aligned} attributes may be limited by inherent limitations in your linker. On many systems, the linker is only able to arrange for variables to be aligned up to a certain maximum alignment. (For some linkers, the maximum supported alignment may be very very small.) If your linker is only able to align variables up to a maximum of 8 byte alignment, then specifying @code{aligned(16)} in an @code{__attribute__} will still only provide you with 8 byte alignment. See your linker documentation for further information. @item mode (@var{mode}) @cindex @code{mode} attribute This attribute specifies the data type for the declaration---whichever type corresponds to the mode @var{mode}. This in effect lets you request an integer or floating point type according to its width. You may also specify a mode of @samp{byte} or @samp{__byte__} to indicate the mode corresponding to a one-byte integer, @samp{word} or @samp{__word__} for the mode of a one-word integer, and @samp{pointer} or @samp{__pointer__} for the mode used to represent pointers. @item nocommon @cindex @code{nocommon} attribute This attribute specifies requests GNU CC not to place a variable ``common'' but instead to allocate space for it directly. If you specify the @samp{-fno-common} flag, GNU CC will do this for all variables. Specifying the @code{nocommon} attribute for a variable provides an initialization of zeros. A variable may only be initialized in one source file. @item packed @cindex @code{packed} attribute The @code{packed} attribute specifies that a variable or structure field should have the smallest possible alignment---one byte for a variable, and one bit for a field, unless you specify a larger value with the @code{aligned} attribute. Here is a structure in which the field @code{x} is packed, so that it immediately follows @code{a}: @example struct foo @{ char a; int x[2] __attribute__ ((packed)); @}; @end example @item section ("section-name") @cindex @code{section} variable attribute Normally, the compiler places the objects it generates in sections like @code{data} and @code{bss}. Sometimes, however, you need additional sections, or you need certain particular variables to appear in special sections, for example to map to special hardware. The @code{section} attribute specifies that a variable (or function) lives in a particular section. For example, this small program uses several specific section names: @smallexample struct duart a __attribute__ ((section ("DUART_A"))) = @{ 0 @}; struct duart b __attribute__ ((section ("DUART_B"))) = @{ 0 @}; char stack[10000] __attribute__ ((section ("STACK"))) = @{ 0 @}; int init_data __attribute__ ((section ("INITDATA"))) = 0; main() @{ /* Initialize stack pointer */ init_sp (stack + sizeof (stack)); /* Initialize initialized data */ memcpy (&init_data, &data, &edata - &data); /* Turn on the serial ports */ init_duart (&a); init_duart (&b); @} @end smallexample @noindent Use the @code{section} attribute with an @emph{initialized} definition of a @emph{global} variable, as shown in the example. GNU CC issues a warning and otherwise ignores the @code{section} attribute in uninitialized variable declarations. You may only use the @code{section} attribute with a fully initialized global definition because of the way linkers work. The linker requires each object be defined once, with the exception that uninitialized variables tentatively go in the @code{common} (or @code{bss}) section and can be multiply "defined". You can force a variable to be initialized with the @samp{-fno-common} flag or the @code{nocommon} attribute. Some file formats do not support arbitrary sections so the @code{section} attribute is not available on all platforms. If you need to map the entire contents of a module to a particular section, consider using the facilities of the linker instead. @item shared @cindex @code{shared} variable attribute On Windows NT, in addition to putting variable definitions in a named section, the section can also be shared among all running copies of an executable or DLL. For example, this small program defines shared data by putting it in a named section "shared" and marking the section shareable: @smallexample int foo __attribute__((section ("shared"), shared)) = 0; int main() @{ /* Read and write foo. All running copies see the same value. */ return 0; @} @end smallexample @noindent You may only use the @code{shared} attribute along with @code{section} attribute with a fully initialized global definition because of the way linkers work. See @code{section} attribute for more information. The @code{shared} attribute is only available on Windows NT. @item transparent_union This attribute, attached to a function parameter which is a union, means that the corresponding argument may have the type of any union member, but the argument is passed as if its type were that of the first union member. For more details see @xref{Type Attributes}. You can also use this attribute on a @code{typedef} for a union data type; then it applies to all function parameters with that type. @item unused This attribute, attached to a variable, means that the variable is meant to be possibly unused. GNU CC will not produce a warning for this variable. @item weak The @code{weak} attribute is described in @xref{Function Attributes}. @item model (@var{model-name}) @cindex variable addressability on the M32R/D Use this attribute on the M32R/D to set the addressability of an object. The identifier @var{model-name} is one of @code{small}, @code{medium}, or @code{large}, representing each of the code models. Small model objects live in the lower 16MB of memory (so that their addresses can be loaded with the @code{ld24} instruction). Medium and large model objects may live anywhere in the 32 bit address space (the compiler will generate @code{seth/add3} instructions to load their addresses). @end table To specify multiple attributes, separate them by commas within the double parentheses: for example, @samp{__attribute__ ((aligned (16), packed))}. @node Type Attributes @section Specifying Attributes of Types @cindex attribute of types @cindex type attributes The keyword @code{__attribute__} allows you to specify special attributes of @code{struct} and @code{union} types when you define such types. This keyword is followed by an attribute specification inside double parentheses. Three attributes are currently defined for types: @code{aligned}, @code{packed}, and @code{transparent_union}. Other attributes are defined for functions (@pxref{Function Attributes}) and for variables (@pxref{Variable Attributes}). You may also specify any one of these attributes with @samp{__} preceding and following its keyword. This allows you to use these attributes in header files without being concerned about a possible macro of the same name. For example, you may use @code{__aligned__} instead of @code{aligned}. You may specify the @code{aligned} and @code{transparent_union} attributes either in a @code{typedef} declaration or just past the closing curly brace of a complete enum, struct or union type @emph{definition} and the @code{packed} attribute only past the closing brace of a definition. You may also specify attributes between the enum, struct or union tag and the name of the type rather than after the closing brace. @table @code @cindex @code{aligned} attribute @item aligned (@var{alignment}) This attribute specifies a minimum alignment (in bytes) for variables of the specified type. For example, the declarations: @smallexample struct S @{ short f[3]; @} __attribute__ ((aligned (8))); typedef int more_aligned_int __attribute__ ((aligned (8))); @end smallexample @noindent force the compiler to insure (as far as it can) that each variable whose type is @code{struct S} or @code{more_aligned_int} will be allocated and aligned @emph{at least} on a 8-byte boundary. On a Sparc, having all variables of type @code{struct S} aligned to 8-byte boundaries allows the compiler to use the @code{ldd} and @code{std} (doubleword load and store) instructions when copying one variable of type @code{struct S} to another, thus improving run-time efficiency. Note that the alignment of any given @code{struct} or @code{union} type is required by the ANSI C standard to be at least a perfect multiple of the lowest common multiple of the alignments of all of the members of the @code{struct} or @code{union} in question. This means that you @emph{can} effectively adjust the alignment of a @code{struct} or @code{union} type by attaching an @code{aligned} attribute to any one of the members of such a type, but the notation illustrated in the example above is a more obvious, intuitive, and readable way to request the compiler to adjust the alignment of an entire @code{struct} or @code{union} type. As in the preceding example, you can explicitly specify the alignment (in bytes) that you wish the compiler to use for a given @code{struct} or @code{union} type. Alternatively, you can leave out the alignment factor and just ask the compiler to align a type to the maximum useful alignment for the target machine you are compiling for. For example, you could write: @smallexample struct S @{ short f[3]; @} __attribute__ ((aligned)); @end smallexample Whenever you leave out the alignment factor in an @code{aligned} attribute specification, the compiler automatically sets the alignment for the type to the largest alignment which is ever used for any data type on the target machine you are compiling for. Doing this can often make copy operations more efficient, because the compiler can use whatever instructions copy the biggest chunks of memory when performing copies to or from the variables which have types that you have aligned this way. In the example above, if the size of each @code{short} is 2 bytes, then the size of the entire @code{struct S} type is 6 bytes. The smallest power of two which is greater than or equal to that is 8, so the compiler sets the alignment for the entire @code{struct S} type to 8 bytes. Note that although you can ask the compiler to select a time-efficient alignment for a given type and then declare only individual stand-alone objects of that type, the compiler's ability to select a time-efficient alignment is primarily useful only when you plan to create arrays of variables having the relevant (efficiently aligned) type. If you declare or use arrays of variables of an efficiently-aligned type, then it is likely that your program will also be doing pointer arithmetic (or subscripting, which amounts to the same thing) on pointers to the relevant type, and the code that the compiler generates for these pointer arithmetic operations will often be more efficient for efficiently-aligned types than for other types. The @code{aligned} attribute can only increase the alignment; but you can decrease it by specifying @code{packed} as well. See below. Note that the effectiveness of @code{aligned} attributes may be limited by inherent limitations in your linker. On many systems, the linker is only able to arrange for variables to be aligned up to a certain maximum alignment. (For some linkers, the maximum supported alignment may be very very small.) If your linker is only able to align variables up to a maximum of 8 byte alignment, then specifying @code{aligned(16)} in an @code{__attribute__} will still only provide you with 8 byte alignment. See your linker documentation for further information. @item packed This attribute, attached to an @code{enum}, @code{struct}, or @code{union} type definition, specified that the minimum required memory be used to represent the type. Specifying this attribute for @code{struct} and @code{union} types is equivalent to specifying the @code{packed} attribute on each of the structure or union members. Specifying the @samp{-fshort-enums} flag on the line is equivalent to specifying the @code{packed} attribute on all @code{enum} definitions. You may only specify this attribute after a closing curly brace on an @code{enum} definition, not in a @code{typedef} declaration, unless that declaration also contains the definition of the @code{enum}. @item transparent_union This attribute, attached to a @code{union} type definition, indicates that any function parameter having that union type causes calls to that function to be treated in a special way. First, the argument corresponding to a transparent union type can be of any type in the union; no cast is required. Also, if the union contains a pointer type, the corresponding argument can be a null pointer constant or a void pointer expression; and if the union contains a void pointer type, the corresponding argument can be any pointer expression. If the union member type is a pointer, qualifiers like @code{const} on the referenced type must be respected, just as with normal pointer conversions. Second, the argument is passed to the function using the calling conventions of first member of the transparent union, not the calling conventions of the union itself. All members of the union must have the same machine representation; this is necessary for this argument passing to work properly. Transparent unions are designed for library functions that have multiple interfaces for compatibility reasons. For example, suppose the @code{wait} function must accept either a value of type @code{int *} to comply with Posix, or a value of type @code{union wait *} to comply with the 4.1BSD interface. If @code{wait}'s parameter were @code{void *}, @code{wait} would accept both kinds of arguments, but it would also accept any other pointer type and this would make argument type checking less useful. Instead, @code{} might define the interface as follows: @smallexample typedef union @{ int *__ip; union wait *__up; @} wait_status_ptr_t __attribute__ ((__transparent_union__)); pid_t wait (wait_status_ptr_t); @end smallexample This interface allows either @code{int *} or @code{union wait *} arguments to be passed, using the @code{int *} calling convention. The program can call @code{wait} with arguments of either type: @example int w1 () @{ int w; return wait (&w); @} int w2 () @{ union wait w; return wait (&w); @} @end example With this interface, @code{wait}'s implementation might look like this: @example pid_t wait (wait_status_ptr_t p) @{ return waitpid (-1, p.__ip, 0); @} @end example @item unused When attached to a type (including a @code{union} or a @code{struct}), this attribute means that variables of that type are meant to appear possibly unused. GNU CC will not produce a warning for any variables of that type, even if the variable appears to do nothing. This is often the case with lock or thread classes, which are usually defined and then not referenced, but contain constructors and destructors that have nontrivial bookkeeping functions. @end table To specify multiple attributes, separate them by commas within the double parentheses: for example, @samp{__attribute__ ((aligned (16), packed))}. @node Inline @section An Inline Function is As Fast As a Macro @cindex inline functions @cindex integrating function code @cindex open coding @cindex macros, inline alternative By declaring a function @code{inline}, you can direct GNU CC to integrate that function's code into the code for its callers. This makes execution faster by eliminating the function-call overhead; in addition, if any of the actual argument values are constant, their known values may permit simplifications at compile time so that not all of the inline function's code needs to be included. The effect on code size is less predictable; object code may be larger or smaller with function inlining, depending on the particular case. Inlining of functions is an optimization and it really ``works'' only in optimizing compilation. If you don't use @samp{-O}, no function is really inline. To declare a function inline, use the @code{inline} keyword in its declaration, like this: @example inline int inc (int *a) @{ (*a)++; @} @end example (If you are writing a header file to be included in ANSI C programs, write @code{__inline__} instead of @code{inline}. @xref{Alternate Keywords}.) You can also make all ``simple enough'' functions inline with the option @samp{-finline-functions}. Note that certain usages in a function definition can make it unsuitable for inline substitution. Among these usages are: use of varargs, use of alloca, use of variable sized data types (@pxref{Variable Length}), use of computed goto (@pxref{Labels as Values}), use of nonlocal goto, and nested functions (@pxref{Nested Functions}). Using @samp{-Winline} will warn when a function marked @code{inline} could not be substituted, and will give the reason for the failure. Note that in C and Objective C, unlike C++, the @code{inline} keyword does not affect the linkage of the function. @cindex automatic @code{inline} for C++ member fns @cindex @code{inline} automatic for C++ member fns @cindex member fns, automatically @code{inline} @cindex C++ member fns, automatically @code{inline} GNU CC automatically inlines member functions defined within the class body of C++ programs even if they are not explicitly declared @code{inline}. (You can override this with @samp{-fno-default-inline}; @pxref{C++ Dialect Options,,Options Controlling C++ Dialect}.) @cindex inline functions, omission of When a function is both inline and @code{static}, if all calls to the function are integrated into the caller, and the function's address is never used, then the function's own assembler code is never referenced. In this case, GNU CC does not actually output assembler code for the function, unless you specify the option @samp{-fkeep-inline-functions}. Some calls cannot be integrated for various reasons (in particular, calls that precede the function's definition cannot be integrated, and neither can recursive calls within the definition). If there is a nonintegrated call, then the function is compiled to assembler code as usual. The function must also be compiled as usual if the program refers to its address, because that can't be inlined. @cindex non-static inline function When an inline function is not @code{static}, then the compiler must assume that there may be calls from other source files; since a global symbol can be defined only once in any program, the function must not be defined in the other source files, so the calls therein cannot be integrated. Therefore, a non-@code{static} inline function is always compiled on its own in the usual fashion. If you specify both @code{inline} and @code{extern} in the function definition, then the definition is used only for inlining. In no case is the function compiled on its own, not even if you refer to its address explicitly. Such an address becomes an external reference, as if you had only declared the function, and had not defined it. This combination of @code{inline} and @code{extern} has almost the effect of a macro. The way to use it is to put a function definition in a header file with these keywords, and put another copy of the definition (lacking @code{inline} and @code{extern}) in a library file. The definition in the header file will cause most calls to the function to be inlined. If any uses of the function remain, they will refer to the single copy in the library. GNU C does not inline any functions when not optimizing. It is not clear whether it is better to inline or not, in this case, but we found that a correct implementation when not optimizing was difficult. So we did the easy thing, and turned it off. @node Extended Asm @section Assembler Instructions with C Expression Operands @cindex extended @code{asm} @cindex @code{asm} expressions @cindex assembler instructions @cindex registers In an assembler instruction using @code{asm}, you can specify the operands of the instruction using C expressions. This means you need not guess which registers or memory locations will contain the data you want to use. You must specify an assembler instruction template much like what appears in a machine description, plus an operand constraint string for each operand. For example, here is how to use the 68881's @code{fsinx} instruction: @example asm ("fsinx %1,%0" : "=f" (result) : "f" (angle)); @end example @noindent Here @code{angle} is the C expression for the input operand while @code{result} is that of the output operand. Each has @samp{"f"} as its operand constraint, saying that a floating point register is required. The @samp{=} in @samp{=f} indicates that the operand is an output; all output operands' constraints must use @samp{=}. The constraints use the same language used in the machine description (@pxref{Constraints}). Each operand is described by an operand-constraint string followed by the C expression in parentheses. A colon separates the assembler template from the first output operand and another separates the last output operand from the first input, if any. Commas separate the operands within each group. The total number of operands is limited to ten or to the maximum number of operands in any instruction pattern in the machine description, whichever is greater. If there are no output operands but there are input operands, you must place two consecutive colons surrounding the place where the output operands would go. Output operand expressions must be lvalues; the compiler can check this. The input operands need not be lvalues. The compiler cannot check whether the operands have data types that are reasonable for the instruction being executed. It does not parse the assembler instruction template and does not know what it means or even whether it is valid assembler input. The extended @code{asm} feature is most often used for machine instructions the compiler itself does not know exist. If the output expression cannot be directly addressed (for example, it is a bit field), your constraint must allow a register. In that case, GNU CC will use the register as the output of the @code{asm}, and then store that register into the output. The ordinary output operands must be write-only; GNU CC will assume that the values in these operands before the instruction are dead and need not be generated. Extended asm supports input-output or read-write operands. Use the constraint character @samp{+} to indicate such an operand and list it with the output operands. When the constraints for the read-write operand (or the operand in which only some of the bits are to be changed) allows a register, you may, as an alternative, logically split its function into two separate operands, one input operand and one write-only output operand. The connection between them is expressed by constraints which say they need to be in the same location when the instruction executes. You can use the same C expression for both operands, or different expressions. For example, here we write the (fictitious) @samp{combine} instruction with @code{bar} as its read-only source operand and @code{foo} as its read-write destination: @example asm ("combine %2,%0" : "=r" (foo) : "0" (foo), "g" (bar)); @end example @noindent The constraint @samp{"0"} for operand 1 says that it must occupy the same location as operand 0. A digit in constraint is allowed only in an input operand and it must refer to an output operand. Only a digit in the constraint can guarantee that one operand will be in the same place as another. The mere fact that @code{foo} is the value of both operands is not enough to guarantee that they will be in the same place in the generated assembler code. The following would not work reliably: @example asm ("combine %2,%0" : "=r" (foo) : "r" (foo), "g" (bar)); @end example Various optimizations or reloading could cause operands 0 and 1 to be in different registers; GNU CC knows no reason not to do so. For example, the compiler might find a copy of the value of @code{foo} in one register and use it for operand 1, but generate the output operand 0 in a different register (copying it afterward to @code{foo}'s own address). Of course, since the register for operand 1 is not even mentioned in the assembler code, the result will not work, but GNU CC can't tell that. Some instructions clobber specific hard registers. To describe this, write a third colon after the input operands, followed by the names of the clobbered hard registers (given as strings). Here is a realistic example for the VAX: @example asm volatile ("movc3 %0,%1,%2" : /* no outputs */ : "g" (from), "g" (to), "g" (count) : "r0", "r1", "r2", "r3", "r4", "r5"); @end example You may not write a clobber description in a way that overlaps with an input or output operand. For example, you may not have an operand describing a register class with one member if you mention that register in the clobber list. There is no way for you to specify that an input operand is modified without also specifying it as an output operand. Note that if all the output operands you specify are for this purpose (and hence unused), you will then also need to specify @code{volatile} for the @code{asm} construct, as described below, to prevent GNU CC from deleting the @code{asm} statement as unused. If you refer to a particular hardware register from the assembler code, you will probably have to list the register after the third colon to tell the compiler the register's value is modified. In some assemblers, the register names begin with @samp{%}; to produce one @samp{%} in the assembler code, you must write @samp{%%} in the input. If your assembler instruction can alter the condition code register, add @samp{cc} to the list of clobbered registers. GNU CC on some machines represents the condition codes as a specific hardware register; @samp{cc} serves to name this register. On other machines, the condition code is handled differently, and specifying @samp{cc} has no effect. But it is valid no matter what the machine. If your assembler instruction modifies memory in an unpredictable fashion, add @samp{memory} to the list of clobbered registers. This will cause GNU CC to not keep memory values cached in registers across the assembler instruction. You will also want to add the @code{volatile} keyword if the memory affected is not listed in the inputs or outputs of the @code{asm}, as the @samp{memory} clobber does not count as a side-effect of the @code{asm}. You can put multiple assembler instructions together in a single @code{asm} template, separated either with newlines (written as @samp{\n}) or with semicolons if the assembler allows such semicolons. The GNU assembler allows semicolons and most Unix assemblers seem to do so. The input operands are guaranteed not to use any of the clobbered registers, and neither will the output operands' addresses, so you can read and write the clobbered registers as many times as you like. Here is an example of multiple instructions in a template; it assumes the subroutine @code{_foo} accepts arguments in registers 9 and 10: @example asm ("movl %0,r9;movl %1,r10;call _foo" : /* no outputs */ : "g" (from), "g" (to) : "r9", "r10"); @end example Unless an output operand has the @samp{&} constraint modifier, GNU CC may allocate it in the same register as an unrelated input operand, on the assumption the inputs are consumed before the outputs are produced. This assumption may be false if the assembler code actually consists of more than one instruction. In such a case, use @samp{&} for each output operand that may not overlap an input. @xref{Modifiers}. If you want to test the condition code produced by an assembler instruction, you must include a branch and a label in the @code{asm} construct, as follows: @example asm ("clr %0;frob %1;beq 0f;mov #1,%0;0:" : "g" (result) : "g" (input)); @end example @noindent This assumes your assembler supports local labels, as the GNU assembler and most Unix assemblers do. Speaking of labels, jumps from one @code{asm} to another are not supported. The compiler's optimizers do not know about these jumps, and therefore they cannot take account of them when deciding how to optimize. @cindex macros containing @code{asm} Usually the most convenient way to use these @code{asm} instructions is to encapsulate them in macros that look like functions. For example, @example #define sin(x) \ (@{ double __value, __arg = (x); \ asm ("fsinx %1,%0": "=f" (__value): "f" (__arg)); \ __value; @}) @end example @noindent Here the variable @code{__arg} is used to make sure that the instruction operates on a proper @code{double} value, and to accept only those arguments @code{x} which can convert automatically to a @code{double}. Another way to make sure the instruction operates on the correct data type is to use a cast in the @code{asm}. This is different from using a variable @code{__arg} in that it converts more different types. For example, if the desired type were @code{int}, casting the argument to @code{int} would accept a pointer with no complaint, while assigning the argument to an @code{int} variable named @code{__arg} would warn about using a pointer unless the caller explicitly casts it. If an @code{asm} has output operands, GNU CC assumes for optimization purposes the instruction has no side effects except to change the output operands. This does not mean instructions with a side effect cannot be used, but you must be careful, because the compiler may eliminate them if the output operands aren't used, or move them out of loops, or replace two with one if they constitute a common subexpression. Also, if your instruction does have a side effect on a variable that otherwise appears not to change, the old value of the variable may be reused later if it happens to be found in a register. You can prevent an @code{asm} instruction from being deleted, moved significantly, or combined, by writing the keyword @code{volatile} after the @code{asm}. For example: @example #define get_and_set_priority(new) \ (@{ int __old; \ asm volatile ("get_and_set_priority %0, %1": "=g" (__old) : "g" (new)); \ __old; @}) @end example @noindent If you write an @code{asm} instruction with no outputs, GNU CC will know the instruction has side-effects and will not delete the instruction or move it outside of loops. If the side-effects of your instruction are not purely external, but will affect variables in your program in ways other than reading the inputs and clobbering the specified registers or memory, you should write the @code{volatile} keyword to prevent future versions of GNU CC from moving the instruction around within a core region. An @code{asm} instruction without any operands or clobbers (and ``old style'' @code{asm}) will not be deleted or moved significantly, regardless, unless it is unreachable, the same wasy as if you had written a @code{volatile} keyword. Note that even a volatile @code{asm} instruction can be moved in ways that appear insignificant to the compiler, such as across jump instructions. You can't expect a sequence of volatile @code{asm} instructions to remain perfectly consecutive. If you want consecutive output, use a single @code{asm}. It is a natural idea to look for a way to give access to the condition code left by the assembler instruction. However, when we attempted to implement this, we found no way to make it work reliably. The problem is that output operands might need reloading, which would result in additional following ``store'' instructions. On most machines, these instructions would alter the condition code before there was time to test it. This problem doesn't arise for ordinary ``test'' and ``compare'' instructions because they don't have any output operands. For reasons similar to those described above, it is not possible to give an assembler instruction access to the condition code left by previous instructions. If you are writing a header file that should be includable in ANSI C programs, write @code{__asm__} instead of @code{asm}. @xref{Alternate Keywords}. @subsection i386 floating point asm operands There are several rules on the usage of stack-like regs in asm_operands insns. These rules apply only to the operands that are stack-like regs: @enumerate @item Given a set of input regs that die in an asm_operands, it is necessary to know which are implicitly popped by the asm, and which must be explicitly popped by gcc. An input reg that is implicitly popped by the asm must be explicitly clobbered, unless it is constrained to match an output operand. @item For any input reg that is implicitly popped by an asm, it is necessary to know how to adjust the stack to compensate for the pop. If any non-popped input is closer to the top of the reg-stack than the implicitly popped reg, it would not be possible to know what the stack looked like --- it's not clear how the rest of the stack ``slides up''. All implicitly popped input regs must be closer to the top of the reg-stack than any input that is not implicitly popped. It is possible that if an input dies in an insn, reload might use the input reg for an output reload. Consider this example: @example asm ("foo" : "=t" (a) : "f" (b)); @end example This asm says that input B is not popped by the asm, and that the asm pushes a result onto the reg-stack, ie, the stack is one deeper after the asm than it was before. But, it is possible that reload will think that it can use the same reg for both the input and the output, if input B dies in this insn. If any input operand uses the @code{f} constraint, all output reg constraints must use the @code{&} earlyclobber. The asm above would be written as @example asm ("foo" : "=&t" (a) : "f" (b)); @end example @item Some operands need to be in particular places on the stack. All output operands fall in this category --- there is no other way to know which regs the outputs appear in unless the user indicates this in the constraints. Output operands must specifically indicate which reg an output appears in after an asm. @code{=f} is not allowed: the operand constraints must select a class with a single reg. @item Output operands may not be ``inserted'' between existing stack regs. Since no 387 opcode uses a read/write operand, all output operands are dead before the asm_operands, and are pushed by the asm_operands. It makes no sense to push anywhere but the top of the reg-stack. Output operands must start at the top of the reg-stack: output operands may not ``skip'' a reg. @item Some asm statements may need extra stack space for internal calculations. This can be guaranteed by clobbering stack registers unrelated to the inputs and outputs. @end enumerate Here are a couple of reasonable asms to want to write. This asm takes one input, which is internally popped, and produces two outputs. @example asm ("fsincos" : "=t" (cos), "=u" (sin) : "0" (inp)); @end example This asm takes two inputs, which are popped by the @code{fyl2xp1} opcode, and replaces them with one output. The user must code the @code{st(1)} clobber for reg-stack.c to know that @code{fyl2xp1} pops both inputs. @example asm ("fyl2xp1" : "=t" (result) : "0" (x), "u" (y) : "st(1)"); @end example @ifclear INTERNALS @c Show the details on constraints if they do not appear elsewhere in @c the manual @include md.texi @end ifclear @node Asm Labels @section Controlling Names Used in Assembler Code @cindex assembler names for identifiers @cindex names used in assembler code @cindex identifiers, names in assembler code You can specify the name to be used in the assembler code for a C function or variable by writing the @code{asm} (or @code{__asm__}) keyword after the declarator as follows: @example int foo asm ("myfoo") = 2; @end example @noindent This specifies that the name to be used for the variable @code{foo} in the assembler code should be @samp{myfoo} rather than the usual @samp{_foo}. On systems where an underscore is normally prepended to the name of a C function or variable, this feature allows you to define names for the linker that do not start with an underscore. You cannot use @code{asm} in this way in a function @emph{definition}; but you can get the same effect by writing a declaration for the function before its definition and putting @code{asm} there, like this: @example extern func () asm ("FUNC"); func (x, y) int x, y; @dots{} @end example It is up to you to make sure that the assembler names you choose do not conflict with any other assembler symbols. Also, you must not use a register name; that would produce completely invalid assembler code. GNU CC does not as yet have the ability to store static variables in registers. Perhaps that will be added. @node Explicit Reg Vars @section Variables in Specified Registers @cindex explicit register variables @cindex variables in specified registers @cindex specified registers @cindex registers, global allocation GNU C allows you to put a few global variables into specified hardware registers. You can also specify the register in which an ordinary register variable should be allocated. @itemize @bullet @item Global register variables reserve registers throughout the program. This may be useful in programs such as programming language interpreters which have a couple of global variables that are accessed very often. @item Local register variables in specific registers do not reserve the registers. The compiler's data flow analysis is capable of determining where the specified registers contain live values, and where they are available for other uses. Stores into local register variables may be deleted when they appear to be dead according to dataflow analysis. References to local register variables may be deleted or moved or simplified. These local variables are sometimes convenient for use with the extended @code{asm} feature (@pxref{Extended Asm}), if you want to write one output of the assembler instruction directly into a particular register. (This will work provided the register you specify fits the constraints specified for that operand in the @code{asm}.) @end itemize @menu * Global Reg Vars:: * Local Reg Vars:: @end menu @node Global Reg Vars @subsection Defining Global Register Variables @cindex global register variables @cindex registers, global variables in You can define a global register variable in GNU C like this: @example register int *foo asm ("a5"); @end example @noindent Here @code{a5} is the name of the register which should be used. Choose a register which is normally saved and restored by function calls on your machine, so that library routines will not clobber it. Naturally the register name is cpu-dependent, so you would need to conditionalize your program according to cpu type. The register @code{a5} would be a good choice on a 68000 for a variable of pointer type. On machines with register windows, be sure to choose a ``global'' register that is not affected magically by the function call mechanism. In addition, operating systems on one type of cpu may differ in how they name the registers; then you would need additional conditionals. For example, some 68000 operating systems call this register @code{%a5}. Eventually there may be a way of asking the compiler to choose a register automatically, but first we need to figure out how it should choose and how to enable you to guide the choice. No solution is evident. Defining a global register variable in a certain register reserves that register entirely for this use, at least within the current compilation. The register will not be allocated for any other purpose in the functions in the current compilation. The register will not be saved and restored by these functions. Stores into this register are never deleted even if they would appear to be dead, but references may be deleted or moved or simplified. It is not safe to access the global register variables from signal handlers, or from more than one thread of control, because the system library routines may temporarily use the register for other things (unless you recompile them specially for the task at hand). @cindex @code{qsort}, and global register variables It is not safe for one function that uses a global register variable to call another such function @code{foo} by way of a third function @code{lose} that was compiled without knowledge of this variable (i.e. in a different source file in which the variable wasn't declared). This is because @code{lose} might save the register and put some other value there. For example, you can't expect a global register variable to be available in the comparison-function that you pass to @code{qsort}, since @code{qsort} might have put something else in that register. (If you are prepared to recompile @code{qsort} with the same global register variable, you can solve this problem.) If you want to recompile @code{qsort} or other source files which do not actually use your global register variable, so that they will not use that register for any other purpose, then it suffices to specify the compiler option @samp{-ffixed-@var{reg}}. You need not actually add a global register declaration to their source code. A function which can alter the value of a global register variable cannot safely be called from a function compiled without this variable, because it could clobber the value the caller expects to find there on return. Therefore, the function which is the entry point into the part of the program that uses the global register variable must explicitly save and restore the value which belongs to its caller. @cindex register variable after @code{longjmp} @cindex global register after @code{longjmp} @cindex value after @code{longjmp} @findex longjmp @findex setjmp On most machines, @code{longjmp} will restore to each global register variable the value it had at the time of the @code{setjmp}. On some machines, however, @code{longjmp} will not change the value of global register variables. To be portable, the function that called @code{setjmp} should make other arrangements to save the values of the global register variables, and to restore them in a @code{longjmp}. This way, the same thing will happen regardless of what @code{longjmp} does. All global register variable declarations must precede all function definitions. If such a declaration could appear after function definitions, the declaration would be too late to prevent the register from being used for other purposes in the preceding functions. Global register variables may not have initial values, because an executable file has no means to supply initial contents for a register. On the Sparc, there are reports that g3 @dots{} g7 are suitable registers, but certain library functions, such as @code{getwd}, as well as the subroutines for division and remainder, modify g3 and g4. g1 and g2 are local temporaries. On the 68000, a2 @dots{} a5 should be suitable, as should d2 @dots{} d7. Of course, it will not do to use more than a few of those. @node Local Reg Vars @subsection Specifying Registers for Local Variables @cindex local variables, specifying registers @cindex specifying registers for local variables @cindex registers for local variables You can define a local register variable with a specified register like this: @example register int *foo asm ("a5"); @end example @noindent Here @code{a5} is the name of the register which should be used. Note that this is the same syntax used for defining global register variables, but for a local variable it would appear within a function. Naturally the register name is cpu-dependent, but this is not a problem, since specific registers are most often useful with explicit assembler instructions (@pxref{Extended Asm}). Both of these things generally require that you conditionalize your program according to cpu type. In addition, operating systems on one type of cpu may differ in how they name the registers; then you would need additional conditionals. For example, some 68000 operating systems call this register @code{%a5}. Defining such a register variable does not reserve the register; it remains available for other uses in places where flow control determines the variable's value is not live. However, these registers are made unavailable for use in the reload pass; excessive use of this feature leaves the compiler too few available registers to compile certain functions. This option does not guarantee that GNU CC will generate code that has this variable in the register you specify at all times. You may not code an explicit reference to this register in an @code{asm} statement and assume it will always refer to this variable. Stores into local register variables may be deleted when they appear to be dead according to dataflow analysis. References to local register variables may be deleted or moved or simplified. @node Alternate Keywords @section Alternate Keywords @cindex alternate keywords @cindex keywords, alternate The option @samp{-traditional} disables certain keywords; @samp{-ansi} disables certain others. This causes trouble when you want to use GNU C extensions, or ANSI C features, in a general-purpose header file that should be usable by all programs, including ANSI C programs and traditional ones. The keywords @code{asm}, @code{typeof} and @code{inline} cannot be used since they won't work in a program compiled with @samp{-ansi}, while the keywords @code{const}, @code{volatile}, @code{signed}, @code{typeof} and @code{inline} won't work in a program compiled with @samp{-traditional}.@refill The way to solve these problems is to put @samp{__} at the beginning and end of each problematical keyword. For example, use @code{__asm__} instead of @code{asm}, @code{__const__} instead of @code{const}, and @code{__inline__} instead of @code{inline}. Other C compilers won't accept these alternative keywords; if you want to compile with another compiler, you can define the alternate keywords as macros to replace them with the customary keywords. It looks like this: @example #ifndef __GNUC__ #define __asm__ asm #endif @end example @findex __extension__ @samp{-pedantic} and other options cause warnings for many GNU C extensions. You can prevent such warnings within one expression by writing @code{__extension__} before the expression. @code{__extension__} has no effect aside from this. @node Incomplete Enums @section Incomplete @code{enum} Types You can define an @code{enum} tag without specifying its possible values. This results in an incomplete type, much like what you get if you write @code{struct foo} without describing the elements. A later declaration which does specify the possible values completes the type. You can't allocate variables or storage using the type while it is incomplete. However, you can work with pointers to that type. This extension may not be very useful, but it makes the handling of @code{enum} more consistent with the way @code{struct} and @code{union} are handled. This extension is not supported by GNU C++. @node Function Names @section Function Names as Strings GNU CC predefines two magic identifiers to hold the name of the current function. The identifier @code{__FUNCTION__} holds the name of the function as it appears in the source. The identifier @code{__PRETTY_FUNCTION__} holds the name of the function pretty printed in a language specific fashion. These names are always the same in a C function, but in a C++ function they may be different. For example, this program: @smallexample extern "C" @{ extern int printf (char *, ...); @} class a @{ public: sub (int i) @{ printf ("__FUNCTION__ = %s\n", __FUNCTION__); printf ("__PRETTY_FUNCTION__ = %s\n", __PRETTY_FUNCTION__); @} @}; int main (void) @{ a ax; ax.sub (0); return 0; @} @end smallexample @noindent gives this output: @smallexample __FUNCTION__ = sub __PRETTY_FUNCTION__ = int a::sub (int) @end smallexample The compiler automagically replaces the identifiers with a string literal containing the appropriate name. Thus, they are neither preprocessor macros, like @code{__FILE__} and @code{__LINE__}, nor variables. This means that they catenate with other string literals, and that they can be used to initialize char arrays. For example @smallexample char here[] = "Function " __FUNCTION__ " in " __FILE__; @end smallexample On the other hand, @samp{#ifdef __FUNCTION__} does not have any special meaning inside a function, since the preprocessor does not do anything special with the identifier @code{__FUNCTION__}. GNU CC also supports the magic word @code{__func__}, defined by the ISO standard C-99: @display The identifier @code{__func__} is implicitly declared by the translator as if, immediately following the opening brace of each function definition, the declaration @smallexample static const char __func__[] = "function-name"; @end smallexample appeared, where function-name is the name of the lexically-enclosing function. This name is the unadorned name of the function. @end display By this definition, @code{__func__} is a variable, not a string literal. In particular, @code{__func__} does not catenate with other string literals. In @code{C++}, @code{__FUNCTION__} and @code{__PRETTY_FUNCTION__} are variables, declared in the same way as @code{__func__}. @node Return Address @section Getting the Return or Frame Address of a Function These functions may be used to get information about the callers of a function. @table @code @findex __builtin_return_address @item __builtin_return_address (@var{level}) This function returns the return address of the current function, or of one of its callers. The @var{level} argument is number of frames to scan up the call stack. A value of @code{0} yields the return address of the current function, a value of @code{1} yields the return address of the caller of the current function, and so forth. The @var{level} argument must be a constant integer. On some machines it may be impossible to determine the return address of any function other than the current one; in such cases, or when the top of the stack has been reached, this function will return @code{0}. This function should only be used with a non-zero argument for debugging purposes. @findex __builtin_frame_address @item __builtin_frame_address (@var{level}) This function is similar to @code{__builtin_return_address}, but it returns the address of the function frame rather than the return address of the function. Calling @code{__builtin_frame_address} with a value of @code{0} yields the frame address of the current function, a value of @code{1} yields the frame address of the caller of the current function, and so forth. The frame is the area on the stack which holds local variables and saved registers. The frame address is normally the address of the first word pushed on to the stack by the function. However, the exact definition depends upon the processor and the calling convention. If the processor has a dedicated frame pointer register, and the function has a frame, then @code{__builtin_frame_address} will return the value of the frame pointer register. The caveats that apply to @code{__builtin_return_address} apply to this function as well. @end table @node Other Builtins @section Other built-in functions provided by GNU CC GNU CC provides a large number of built-in functions other than the ones mentioned above. Some of these are for internal use in the processing of exceptions or variable-length argument lists and will not be documented here because they may change from time to time; we do not recommend general use of these functions. The remaining functions are provided for optimization purposes. GNU CC includes builtin versions of many of the functions in the standard C library. These will always be treated as having the same meaning as the C library function even if you specify the @samp{-fno-builtin} (@pxref{C Dialect Options}) option. These functions correspond to the C library functions @code{abort}, @code{abs}, @code{alloca}, @code{cos}, @code{cosf}, @code{cosl}, @code{exit}, @code{_exit}, @code{fabs}, @code{fabsf}, @code{fabsl}, @code{ffs}, @code{labs}, @code{memcmp}, @code{memcpy}, @code{memset}, @code{sin}, @code{sinf}, @code{sinl}, @code{sqrt}, @code{sqrtf}, @code{sqrtl}, @code{strcmp}, @code{strcpy}, and @code{strlen}. @table @code @findex __builtin_constant_p @item __builtin_constant_p (@var{exp}) You can use the builtin function @code{__builtin_constant_p} to determine if a value is known to be constant at compile-time and hence that GNU CC can perform constant-folding on expressions involving that value. The argument of the function is the value to test. The function returns the integer 1 if the argument is known to be a compile-time constant and 0 if it is not known to be a compile-time constant. A return of 0 does not indicate that the value is @emph{not} a constant, but merely that GNU CC cannot prove it is a constant with the specified value of the @samp{-O} option. You would typically use this function in an embedded application where memory was a critical resource. If you have some complex calculation, you may want it to be folded if it involves constants, but need to call a function if it does not. For example: @smallexample #define Scale_Value(X) \ (__builtin_constant_p (X) ? ((X) * SCALE + OFFSET) : Scale (X)) @end smallexample You may use this builtin function in either a macro or an inline function. However, if you use it in an inlined function and pass an argument of the function as the argument to the builtin, GNU CC will never return 1 when you call the inline function with a string constant or constructor expression (@pxref{Constructors}) and will not return 1 when you pass a constant numeric value to the inline function unless you specify the @samp{-O} option. @findex __builtin_expect @item __builtin_expect(@var{exp}, @var{c}) You may use @code{__builtin_expect} to provide the compiler with branch prediction information. In general, you should prefer to use actual profile feedback for this (@samp{-fprofile-arcs}), as programmers are notoriously bad at predicting how their programs actually perform. However, there are applications in which this data is hard to collect. The return value is the value of @var{exp}, which should be an integral expression. The value of @var{c} must be a compile-time constant. The semantics of the builtin are that it is expected that @var{exp} == @var{c}. For example: @smallexample if (__builtin_expect (x, 0)) foo (); @end smallexample @noindent would indicate that we do not expect to call @code{foo}, since we expect @code{x} to be zero. Since you are limited to integral expressions for @var{exp}, you should use constructions such as @smallexample if (__builtin_expect (ptr != NULL, 1)) error (); @end smallexample @noindent when testing pointer or floating-point values. @end table @node Deprecated Features @section Deprecated Features In the past, the GNU C++ compiler was extended to experiment with new features, at a time when the C++ language was still evolving. Now that the C++ standard is complete, some of those features are superseded by superior alternatives. Using the old features might cause a warning in some cases that the feature will be dropped in the future. In other cases, the feature might be gone already. While the list below is not exhaustive, it documents some of the options that are now deprecated: @table @code @item -fexternal-templates @itemx -falt-external-templates These are two of the many ways for g++ to implement template instantiation. @xref{Template Instantiation}. The C++ standard clearly defines how template definitions have to be organized across implementation units. g++ has an implicit instantiation mechanism that should work just fine for standard-conforming code. @item -fstrict-prototype @itemx -fno-strict-prototype Previously it was possible to use an empty prototype parameter list to indicate an unspecified number of parameters (like C), rather than no parameters, as C++ demands. This feature has been removed, except where it is required for backwards compatibility @xref{Backwards Compatibility} @end table @node Backwards Compatibility @section Backwards Compatibility @cindex Backwards Compatibility @cindex ARM Now that there is a definitive ISO standard C++, g++ has a specification to adhere to. The C++ language evolved over time, and features that used to be acceptable in previous drafts of the standard, such as the ARM, are no longer accepted. In order to allow compilation of C++ written to such drafts, g++ contains some backwards compatibilities. @emph{All such backwards compatibility features are liable to disappear in future versions of g++.} They should be considered deprecated @xref{Deprecated Features}. @table @code @item For scope If a variable is declared at for scope, it used to remain in scope until the end of the scope which contained the for statement (rather than just within the for scope). g++ retains this, but issues a warning, if such a variable is accessed outside the for scope. @item implicit C language Old C system header files did not contain an @code{extern "C" @{...@}} scope to set the language. On such systems, all header files are implicitly scoped inside a C language scope. Also, an empty prototype @code{()} will be treated as an unspecified number of arguments, rather than no arguments, as C++ demands. @end table @node C++ Extensions @chapter Extensions to the C++ Language @cindex extensions, C++ language @cindex C++ language extensions The GNU compiler provides these extensions to the C++ language (and you can also use most of the C language extensions in your C++ programs). If you want to write code that checks whether these features are available, you can test for the GNU compiler the same way as for C programs: check for a predefined macro @code{__GNUC__}. You can also use @code{__GNUG__} to test specifically for GNU C++ (@pxref{Standard Predefined,,Standard Predefined Macros,cpp.info,The C Preprocessor}). @menu * Naming Results:: Giving a name to C++ function return values. * Min and Max:: C++ Minimum and maximum operators. * Volatiles:: What constitutes an access to a volatile object. * Restricted Pointers:: C99 restricted pointers and references. * C++ Interface:: You can use a single C++ header file for both declarations and definitions. * Template Instantiation:: Methods for ensuring that exactly one copy of each needed template instantiation is emitted. * Bound member functions:: You can extract a function pointer to the method denoted by a @samp{->*} or @samp{.*} expression. @end menu @node Naming Results @section Named Return Values in C++ @cindex @code{return}, in C++ function header @cindex return value, named, in C++ @cindex named return value in C++ @cindex C++ named return value GNU C++ extends the function-definition syntax to allow you to specify a name for the result of a function outside the body of the definition, in C++ programs: @example @group @var{type} @var{functionname} (@var{args}) return @var{resultname}; @{ @dots{} @var{body} @dots{} @} @end group @end example You can use this feature to avoid an extra constructor call when a function result has a class type. For example, consider a function @code{m}, declared as @w{@samp{X v = m ();}}, whose result is of class @code{X}: @example X m () @{ X b; b.a = 23; return b; @} @end example @cindex implicit argument: return value Although @code{m} appears to have no arguments, in fact it has one implicit argument: the address of the return value. At invocation, the address of enough space to hold @code{v} is sent in as the implicit argument. Then @code{b} is constructed and its @code{a} field is set to the value 23. Finally, a copy constructor (a constructor of the form @samp{X(X&)}) is applied to @code{b}, with the (implicit) return value location as the target, so that @code{v} is now bound to the return value. But this is wasteful. The local @code{b} is declared just to hold something that will be copied right out. While a compiler that combined an ``elision'' algorithm with interprocedural data flow analysis could conceivably eliminate all of this, it is much more practical to allow you to assist the compiler in generating efficient code by manipulating the return value explicitly, thus avoiding the local variable and copy constructor altogether. Using the extended GNU C++ function-definition syntax, you can avoid the temporary allocation and copying by naming @code{r} as your return value at the outset, and assigning to its @code{a} field directly: @example X m () return r; @{ r.a = 23; @} @end example @noindent The declaration of @code{r} is a standard, proper declaration, whose effects are executed @strong{before} any of the body of @code{m}. Functions of this type impose no additional restrictions; in particular, you can execute @code{return} statements, or return implicitly by reaching the end of the function body (``falling off the edge''). Cases like @example X m () return r (23); @{ return; @} @end example @noindent (or even @w{@samp{X m () return r (23); @{ @}}}) are unambiguous, since the return value @code{r} has been initialized in either case. The following code may be hard to read, but also works predictably: @example X m () return r; @{ X b; return b; @} @end example The return value slot denoted by @code{r} is initialized at the outset, but the statement @samp{return b;} overrides this value. The compiler deals with this by destroying @code{r} (calling the destructor if there is one, or doing nothing if there is not), and then reinitializing @code{r} with @code{b}. This extension is provided primarily to help people who use overloaded operators, where there is a great need to control not just the arguments, but the return values of functions. For classes where the copy constructor incurs a heavy performance penalty (especially in the common case where there is a quick default constructor), this is a major savings. The disadvantage of this extension is that you do not control when the default constructor for the return value is called: it is always called at the beginning. @node Min and Max @section Minimum and Maximum Operators in C++ It is very convenient to have operators which return the ``minimum'' or the ``maximum'' of two arguments. In GNU C++ (but not in GNU C), @table @code @item @var{a} ? @var{b} @findex >? @cindex maximum operator is the @dfn{maximum}, returning the larger of the numeric values @var{a} and @var{b}. @end table These operations are not primitive in ordinary C++, since you can use a macro to return the minimum of two things in C++, as in the following example. @example #define MIN(X,Y) ((X) < (Y) ? : (X) : (Y)) @end example @noindent You might then use @w{@samp{int min = MIN (i, j);}} to set @var{min} to the minimum value of variables @var{i} and @var{j}. However, side effects in @code{X} or @code{Y} may cause unintended behavior. For example, @code{MIN (i++, j++)} will fail, incrementing the smaller counter twice. A GNU C extension allows you to write safe macros that avoid this kind of problem (@pxref{Naming Types,,Naming an Expression's Type}). However, writing @code{MIN} and @code{MAX} as macros also forces you to use function-call notation for a fundamental arithmetic operation. Using GNU C++ extensions, you can write @w{@samp{int min = i ?} are built into the compiler, they properly handle expressions with side-effects; @w{@samp{int min = i++ ; volatile int *src = ; *dst = *src; @end example @noindent will cause a read of the volatile object pointed to by @var{src} and stores the value into the volatile object pointed to by @var{dst}. There is no guarantee that these reads and writes are atomic, especially for objects larger than @code{int}. Less obvious expressions are where something which looks like an access is used in a void context. An example would be, @example volatile int *src = ; *src; @end example With C, such expressions are rvalues, and as rvalues cause a read of the object, gcc interprets this as a read of the volatile being pointed to. The C++ standard specifies that such expressions do not undergo lvalue to rvalue conversion, and that the type of the dereferenced object may be incomplete. The C++ standard does not specify explicitly that it is this lvalue to rvalue conversion which is responsible for causing an access. However, there is reason to believe that it is, because otherwise certain simple expressions become undefined. However, because it would surprise most programmers, g++ treats dereferencing a pointer to volatile object of complete type in a void context as a read of the object. When the object has incomplete type, g++ issues a warning. @example struct S; struct T @{int m;@}; volatile S *ptr1 = ; volatile T *ptr2 = ; *ptr1; *ptr2; @end example In this example, a warning is issued for @code{*ptr1}, and @code{*ptr2} causes a read of the object pointed to. If you wish to force an error on the first case, you must force a conversion to rvalue with, for instance a static cast, @code{static_cast(*ptr1)}. When using a reference to volatile, g++ does not treat equivalent expressions as accesses to volatiles, but instead issues a warning that no volatile is accessed. The rationale for this is that otherwise it becomes difficult to determine where volatile access occur, and not possible to ignore the return value from functions returning volatile references. Again, if you wish to force a read, cast the reference to an rvalue. @node Restricted Pointers @section Restricting Pointer Aliasing @cindex restricted pointers @cindex restricted references @cindex restricted this pointer As with gcc, g++ understands the C99 feature of restricted pointers, specified with the @code{__restrict__}, or @code{__restrict} type qualifier. Because you cannot compile C++ by specifying the -std=c99 language flag, @code{restrict} is not a keyword in C++. In addition to allowing restricted pointers, you can specify restricted references, which indicate that the reference is not aliased in the local context. @example void fn (int *__restrict__ rptr, int &__restrict__ rref) @{ @dots{} @} @end example @noindent In the body of @code{fn}, @var{rptr} points to an unaliased integer and @var{rref} refers to a (different) unaliased integer. You may also specify whether a member function's @var{this} pointer is unaliased by using @code{__restrict__} as a member function qualifier. @example void T::fn () __restrict__ @{ @dots{} @} @end example @noindent Within the body of @code{T::fn}, @var{this} will have the effective definition @code{T *__restrict__ const this}. Notice that the interpretation of a @code{__restrict__} member function qualifier is different to that of @code{const} or @code{volatile} qualifier, in that it is applied to the pointer rather than the object. This is consistent with other compilers which implement restricted pointers. As with all outermost parameter qualifiers, @code{__restrict__} is ignored in function definition matching. This means you only need to specify @code{__restrict__} in a function definition, rather than in a function prototype as well. @node C++ Interface @section Declarations and Definitions in One Header @cindex interface and implementation headers, C++ @cindex C++ interface and implementation headers C++ object definitions can be quite complex. In principle, your source code will need two kinds of things for each object that you use across more than one source file. First, you need an @dfn{interface} specification, describing its structure with type declarations and function prototypes. Second, you need the @dfn{implementation} itself. It can be tedious to maintain a separate interface description in a header file, in parallel to the actual implementation. It is also dangerous, since separate interface and implementation definitions may not remain parallel. @cindex pragmas, interface and implementation With GNU C++, you can use a single header file for both purposes. @quotation @emph{Warning:} The mechanism to specify this is in transition. For the nonce, you must use one of two @code{#pragma} commands; in a future release of GNU C++, an alternative mechanism will make these @code{#pragma} commands unnecessary. @end quotation The header file contains the full definitions, but is marked with @samp{#pragma interface} in the source code. This allows the compiler to use the header file only as an interface specification when ordinary source files incorporate it with @code{#include}. In the single source file where the full implementation belongs, you can use either a naming convention or @samp{#pragma implementation} to indicate this alternate use of the header file. @table @code @item #pragma interface @itemx #pragma interface "@var{subdir}/@var{objects}.h" @kindex #pragma interface Use this directive in @emph{header files} that define object classes, to save space in most of the object files that use those classes. Normally, local copies of certain information (backup copies of inline member functions, debugging information, and the internal tables that implement virtual functions) must be kept in each object file that includes class definitions. You can use this pragma to avoid such duplication. When a header file containing @samp{#pragma interface} is included in a compilation, this auxiliary information will not be generated (unless the main input source file itself uses @samp{#pragma implementation}). Instead, the object files will contain references to be resolved at link time. The second form of this directive is useful for the case where you have multiple headers with the same name in different directories. If you use this form, you must specify the same string to @samp{#pragma implementation}. @item #pragma implementation @itemx #pragma implementation "@var{objects}.h" @kindex #pragma implementation Use this pragma in a @emph{main input file}, when you want full output from included header files to be generated (and made globally visible). The included header file, in turn, should use @samp{#pragma interface}. Backup copies of inline member functions, debugging information, and the internal tables used to implement virtual functions are all generated in implementation files. @cindex implied @code{#pragma implementation} @cindex @code{#pragma implementation}, implied @cindex naming convention, implementation headers If you use @samp{#pragma implementation} with no argument, it applies to an include file with the same basename@footnote{A file's @dfn{basename} was the name stripped of all leading path information and of trailing suffixes, such as @samp{.h} or @samp{.C} or @samp{.cc}.} as your source file. For example, in @file{allclass.cc}, giving just @samp{#pragma implementation} by itself is equivalent to @samp{#pragma implementation "allclass.h"}. In versions of GNU C++ prior to 2.6.0 @file{allclass.h} was treated as an implementation file whenever you would include it from @file{allclass.cc} even if you never specified @samp{#pragma implementation}. This was deemed to be more trouble than it was worth, however, and disabled. If you use an explicit @samp{#pragma implementation}, it must appear in your source file @emph{before} you include the affected header files. Use the string argument if you want a single implementation file to include code from multiple header files. (You must also use @samp{#include} to include the header file; @samp{#pragma implementation} only specifies how to use the file---it doesn't actually include it.) There is no way to split up the contents of a single header file into multiple implementation files. @end table @cindex inlining and C++ pragmas @cindex C++ pragmas, effect on inlining @cindex pragmas in C++, effect on inlining @samp{#pragma implementation} and @samp{#pragma interface} also have an effect on function inlining. If you define a class in a header file marked with @samp{#pragma interface}, the effect on a function defined in that class is similar to an explicit @code{extern} declaration---the compiler emits no code at all to define an independent version of the function. Its definition is used only for inlining with its callers. Conversely, when you include the same header file in a main source file that declares it as @samp{#pragma implementation}, the compiler emits code for the function itself; this defines a version of the function that can be found via pointers (or by callers compiled without inlining). If all calls to the function can be inlined, you can avoid emitting the function by compiling with @samp{-fno-implement-inlines}. If any calls were not inlined, you will get linker errors. @node Template Instantiation @section Where's the Template? @cindex template instantiation C++ templates are the first language feature to require more intelligence from the environment than one usually finds on a UNIX system. Somehow the compiler and linker have to make sure that each template instance occurs exactly once in the executable if it is needed, and not at all otherwise. There are two basic approaches to this problem, which I will refer to as the Borland model and the Cfront model. @table @asis @item Borland model Borland C++ solved the template instantiation problem by adding the code equivalent of common blocks to their linker; the compiler emits template instances in each translation unit that uses them, and the linker collapses them together. The advantage of this model is that the linker only has to consider the object files themselves; there is no external complexity to worry about. This disadvantage is that compilation time is increased because the template code is being compiled repeatedly. Code written for this model tends to include definitions of all templates in the header file, since they must be seen to be instantiated. @item Cfront model The AT&T C++ translator, Cfront, solved the template instantiation problem by creating the notion of a template repository, an automatically maintained place where template instances are stored. A more modern version of the repository works as follows: As individual object files are built, the compiler places any template definitions and instantiations encountered in the repository. At link time, the link wrapper adds in the objects in the repository and compiles any needed instances that were not previously emitted. The advantages of this model are more optimal compilation speed and the ability to use the system linker; to implement the Borland model a compiler vendor also needs to replace the linker. The disadvantages are vastly increased complexity, and thus potential for error; for some code this can be just as transparent, but in practice it can been very difficult to build multiple programs in one directory and one program in multiple directories. Code written for this model tends to separate definitions of non-inline member templates into a separate file, which should be compiled separately. @end table When used with GNU ld version 2.8 or later on an ELF system such as Linux/GNU or Solaris 2, or on Microsoft Windows, g++ supports the Borland model. On other systems, g++ implements neither automatic model. A future version of g++ will support a hybrid model whereby the compiler will emit any instantiations for which the template definition is included in the compile, and store template definitions and instantiation context information into the object file for the rest. The link wrapper will extract that information as necessary and invoke the compiler to produce the remaining instantiations. The linker will then combine duplicate instantiations. In the mean time, you have the following options for dealing with template instantiations: @enumerate @item Compile your template-using code with @samp{-frepo}. The compiler will generate files with the extension @samp{.rpo} listing all of the template instantiations used in the corresponding object files which could be instantiated there; the link wrapper, @samp{collect2}, will then update the @samp{.rpo} files to tell the compiler where to place those instantiations and rebuild any affected object files. The link-time overhead is negligible after the first pass, as the compiler will continue to place the instantiations in the same files. This is your best option for application code written for the Borland model, as it will just work. Code written for the Cfront model will need to be modified so that the template definitions are available at one or more points of instantiation; usually this is as simple as adding @code{#include } to the end of each template header. For library code, if you want the library to provide all of the template instantiations it needs, just try to link all of its object files together; the link will fail, but cause the instantiations to be generated as a side effect. Be warned, however, that this may cause conflicts if multiple libraries try to provide the same instantiations. For greater control, use explicit instantiation as described in the next option. @item Compile your code with @samp{-fno-implicit-templates} to disable the implicit generation of template instances, and explicitly instantiate all the ones you use. This approach requires more knowledge of exactly which instances you need than do the others, but it's less mysterious and allows greater control. You can scatter the explicit instantiations throughout your program, perhaps putting them in the translation units where the instances are used or the translation units that define the templates themselves; you can put all of the explicit instantiations you need into one big file; or you can create small files like @example #include "Foo.h" #include "Foo.cc" template class Foo; template ostream& operator << (ostream&, const Foo&); @end example for each of the instances you need, and create a template instantiation library from those. If you are using Cfront-model code, you can probably get away with not using @samp{-fno-implicit-templates} when compiling files that don't @samp{#include} the member template definitions. If you use one big file to do the instantiations, you may want to compile it without @samp{-fno-implicit-templates} so you get all of the instances required by your explicit instantiations (but not by any other files) without having to specify them as well. g++ has extended the template instantiation syntax outlined in the Working Paper to allow forward declaration of explicit instantiations (with @code{extern}), instantiation of the compiler support data for a template class (i.e. the vtable) without instantiating any of its members (with @code{inline}), and instantiation of only the static data members of a template class, without the support data or member functions (with (@code{static}): @example extern template int max (int, int); inline template class Foo; static template class Foo; @end example @item Do nothing. Pretend g++ does implement automatic instantiation management. Code written for the Borland model will work fine, but each translation unit will contain instances of each of the templates it uses. In a large program, this can lead to an unacceptable amount of code duplication. @item Add @samp{#pragma interface} to all files containing template definitions. For each of these files, add @samp{#pragma implementation "@var{filename}"} to the top of some @samp{.C} file which @samp{#include}s it. Then compile everything with @samp{-fexternal-templates}. The templates will then only be expanded in the translation unit which implements them (i.e. has a @samp{#pragma implementation} line for the file where they live); all other files will use external references. If you're lucky, everything should work properly. If you get undefined symbol errors, you need to make sure that each template instance which is used in the program is used in the file which implements that template. If you don't have any use for a particular instance in that file, you can just instantiate it explicitly, using the syntax from the latest C++ working paper: @example template class A; template ostream& operator << (ostream&, const A&); @end example This strategy will work with code written for either model. If you are using code written for the Cfront model, the file containing a class template and the file containing its member templates should be implemented in the same translation unit. A slight variation on this approach is to instead use the flag @samp{-falt-external-templates}; this flag causes template instances to be emitted in the translation unit that implements the header where they are first instantiated, rather than the one which implements the file where the templates are defined. This header must be the same in all translation units, or things are likely to break. @xref{C++ Interface,,Declarations and Definitions in One Header}, for more discussion of these pragmas. @end enumerate @node Bound member functions @section Extracting the function pointer from a bound pointer to member function @cindex pmf @cindex pointer to member function @cindex bound pointer to member function In C++, pointer to member functions (PMFs) are implemented using a wide pointer of sorts to handle all the possible call mechanisms; the PMF needs to store information about how to adjust the @samp{this} pointer, and if the function pointed to is virtual, where to find the vtable, and where in the vtable to look for the member function. If you are using PMFs in an inner loop, you should really reconsider that decision. If that is not an option, you can extract the pointer to the function that would be called for a given object/PMF pair and call it directly inside the inner loop, to save a bit of time. Note that you will still be paying the penalty for the call through a function pointer; on most modern architectures, such a call defeats the branch prediction features of the CPU. This is also true of normal virtual function calls. The syntax for this extension is @example extern A a; extern int (A::*fp)(); typedef int (*fptr)(A *); fptr p = (fptr)(a.*fp); @end example For PMF constants (i.e. expressions of the form @samp{&Klasse::Member}), no object is needed to obtain the address of the function. They can be converted to function pointers directly: @example fptr p1 = (fptr)(&A::foo); @end example You must specify @samp{-Wno-pmf-conversions} to use this extension.