From d31d3e069e7f0811a17942414e5eb4f28c04bf5e Mon Sep 17 00:00:00 2001 From: simonmar Date: Mon, 5 Aug 2002 11:11:46 +0000 Subject: [project @ 2002-08-05 11:11:44 by simonmar] - Update the old coding style document - Move it to the commentary under the "RTS & libs" section --- docs/coding-style.html | 333 +++++++++++++++++++++++-------------------------- 1 file changed, 156 insertions(+), 177 deletions(-) (limited to 'docs/coding-style.html') diff --git a/docs/coding-style.html b/docs/coding-style.html index 4f39cbfc53..250ab88c3d 100644 --- a/docs/coding-style.html +++ b/docs/coding-style.html @@ -5,11 +5,14 @@ -

Coding suggestions for GHC/Hugs related code

GHC Style guidelines for C code

Comments

-NB These are just suggestions. They're not set in stone. Some of +

These coding style guidelines are mainly intended for use in +ghc/rts and ghc/includes. + +

NB These are just suggestions. They're not set in stone. Some of them are probably misguided. If you disagree with them, feel free to modify this document (and make your commit message reasonably informative) or mail someone (eg. +

+The C99 standard. One reasonable reference is here. + +

Writing Solid Code, Microsoft Press. (Highly recommended. Possibly -the only Microsoft Press book that's worth reading. SimonPJ has a -copy.) +the only Microsoft Press book that's worth reading.)

-Autoconf documentation (which doesn't seem to be on the web). +Autoconf documentation. See also The autoconf macro archive and Cyclic Software's description @@ -50,57 +57,57 @@ Hill C Style and Coding Standards.

Portability issues

-We use ANSI C with some extensions. In particular, we use: +
We try to stick to C99 where possible. We use the following +C99 features relative to C89, some of which were previously GCC +extensions (possibly with different syntax): +
- enum -
- ANSI style prototypes -
- #elsif, #error, ## and other cpp features -
+
Variable length arrays as the last field of a struct. GCC has +a similar extension, but the syntax is slightly different: in GCC you +would declare the array as arr[0], whereas in C99 it is +declared as arr[]. -
-We use the following gcc extensions (see gcc documentation): +
Inline annotations on functions (see later) -
- #warning +
- Labeled elements in initialisers. Again, GCC has a slightly +different syntax from C99 here, and we stick with the GCC syntax until +GCC implements the C99 proposal. -
- zero length arrays (useful as the last field of a struct) +
- C++-style comments. These are part of the C99 standard, and we +prefer to use them whenever possible. +
-
inline annotations on functions (see later) +
In addition we use ANSI-C-style function declarations and +prototypes exclusively. Every function should have a prototype; +static function prototypes may be placed near the top of the file in +which they are declared, and external prototypes are usually placed in +a header file with the same basename as the source file (although there +are exceptions to this rule, particularly when several source files +together implement a subsystem which is described by a single external +header file). -
Labeled elements in initialisers +
We use the following GCC extensions, but surround them with +#ifdef __GNUC__: +
- Function attributes (mostly just no_return and unused) +
- Inline assembly.
-
Some of these gcc/ANSI features could be avoided (for example, we -could use __inline__ instead of inline or use the usual PROTO((...)) -trick in function prototypes) - but some of them can't be avoided -so we don't try with the others.
- -
Most of these features are actually part of the new C9X standard, -so we hope to have mostly conformant code at some point in the future. - -
-Please don't use C++ style comments, they aren't standard C. -
char can be signed or unsigned - always say which you mean -
Our POSIX policy: try to write code that only uses POSIX (IEEE -Std 1003.1) interfaces and APIs. When you include Rts.h, -POSIX_SOURCE is automatically defined for you before any -system headers are slurped in, unless you define -NON_POSIX_SOURCE prior to including Rts.h. -A good C library will use the POSIX_SOURCE define to -eliminate non-posix types and function prototypes, so the compiler -should complain if you venture outside the POSIX spec.

Our POSIX policy: try to write code that only uses POSIX (IEEE +Std 1003.1) interfaces and APIs. We used to define +POSIX_SOURCE by default, but found that this caused more +problems than it solved, so now we require any code that is +POSIX-compliant to explicitly say so by having #include +"PosixSource.h" at the top. Try to do this whenever possible. -
-Some architectures have memory alignment constraints. -Others don't have any constraints but go faster if you align things. -These macros tell you which alignment to use +
Some architectures have memory alignment constraints. Others +don't have any constraints but go faster if you align things. These +macros (from config.h) tell you which alignment to use
```
   /* minimum alignment of unsigned int */
@@ -116,47 +123,37 @@ These macros tell you which alignment to use
   #define ALIGNMENT_DOUBLE 4
 
```
-
-Use StgInt, StgWord and StgPtr when reading/writing ints and ptrs to -the stack or heap. Note that, by definition, StgInt, StgWord and -StgPtr are the same size and have the same alignment constraints -even if sizeof(int) != sizeof(ptr) on that platform. - -
-Use StgInt8, StgInt16, etc when you need a certain minimum number of -bits in a type. Use int and nat when there's no particular -constraint. ANSI C only guarantees that ints are at least 16 bits but -within GHC we assume they are 32 bits (do we? --SDM). - -
-Use StgFloat and StgDouble for floating point values which will go -on/have come from the stack or heap. Note that StgFloat may be the -same as StgDouble on some architectures (eg Alphas) and that it might -occupy many StgWords. +
Use StgInt, StgWord and StgPtr when +reading/writing ints and ptrs to the stack or heap. Note that, by +definition, StgInt, StgWord and StgPtr are +the same size and have the same alignment constraints even if +sizeof(int) != sizeof(ptr) on that platform. -
-Use PK_FLT(addr), PK_DBL(addr) to read StgFloat and -StgDouble values from the stack/heap, and ASSIGN_FLT(val,addr)/ -ASSIGN_DBL(val,addr) to assign StgFloat/StgDouble values to heap/stack -locations. These macros take care of alignment restrictions. +
Use StgInt8, StgInt16, etc when you need a +certain minimum number of bits in a type. Use int and +nat when there's no particular constraint. ANSI C only +guarantees that ints are at least 16 bits but within GHC we assume +they are 32 bits. -
-Heap/Stack locations are StgWord aligned; the alignment requirements -of an StgDouble may be more than that of StgWord, but we don't pad -misaligned StgDoubles (doing so would be too much hassle). +
Use StgFloat and StgDouble for floating +point values which will go on/have come from the stack or heap. Note +that StgDouble may occupy more than one StgWord, but +it will always be a whole number multiple.
-Doing a direct assignment/read of an StgDouble to/from a mis-aligned -location may not work, so we use the ASSIGN_DBL(,)/PK_DBL() macro, -which goes via a temporary. +Use PK_FLT(addr), PK_DBL(addr) to read +StgFloat and StgDouble values from the stack/heap, +and ASSIGN_FLT(val,addr) / +ASSIGN_DBL(val,addr) to assign StgFloat/StgDouble values +to heap/stack locations. These macros take care of alignment +restrictions.
-Problem: if the architecture allows mis-aligned accesses, but prefers -aligned accesses, these macros just add an extra level of indirection. -We need to distinguish between an architecture that allows mis-aligned -accesses and one that just imposes a performance penalty (this is most -of them). Perhaps have PREFERRED_ALIGNMENT and REQUIRED_ALIGMENT -configurations? +Heap/Stack locations are always StgWord aligned; the +alignment requirements of an StgDouble may be more than that +of StgWord, but we don't pad misaligned StgDoubles +because doing so would be too much hassle (see PK_DBL & +co above).
Avoid conditional code like this: @@ -176,7 +173,7 @@ the result of that test instead. #endif -The problem is that things change from one version of an OS to another +
The problem is that things change from one version of an OS to another - things get added, things get deleted, things get broken, some things are optional extras. Using "feature tests" instead of "system tests" makes things a lot less brittle. Things also tend to get documented @@ -194,19 +191,48 @@ than trying to find a bug which only shows up when running GHC on itself and doesn't manifest itself until 10 seconds after the actual cause of the problem. +
We put all our debugging code inside #ifdef DEBUG. The +general policy is we don't ship code with debugging checks and +assertions in it, but we do run with those checks in place when +developing and testing. Anything inside #ifdef DEBUG should +not slow down the code by more than a factor of 2. + +
We also have more expensive "sanity checking" code for hardcore +debugging - this can slow down the code by a large factor, but is only +enabled on demand by a command-line flag. General sanity checking in +the RTS is currently enabled with the -DS RTS flag. + +
There are a number of RTS flags which control debugging output and +sanity checking in various parts of the system when DEBUG is +defined. For example, to get the scheduler to be verbose about what +it is doing, you would say +RTS -Ds -RTS. See +includes/RtsFlags.h and rts/RtsFlags.c for the full +set of debugging flags. To check one of these flags in the code, +write: + +
```
+  IF_DEBUG(gc, fprintf(stderr, "..."));
+
```
+ +would check the gc flag before generating the output (and the +code is removed altogether if DEBUG is not defined). + +
All debugging output should go to stderr. +
-The ideas in this section are mostly aimed at this issue: +Particular guidelines for writing robust code:
- Use assertions. Use lots of assertions. If you write a comment that says "takes a +ve number" add an assertion. If you're casting an int to a nat, add an assertion. If you're casting an int to a char, -add an assertion. +add an assertion. We use the ASSERT macro for writing +assertions; it goes away when DEBUG is not defined.
- Write special debugging code to check the integrity of your data structures. -(Most of the runtime checking code is in src/Sanity.c) +(Most of the runtime checking code is in rts/Sanity.c) Add extra assertions which call this code at the start and end of any code that operates on your data structures. @@ -229,7 +255,8 @@ typedef enum ... -
- Use #warning or #error whenever you write a piece of incomplete/broken code. +
- Use #warning or #error whenever you write a +piece of incomplete/broken code.
- When testing, try to make infrequent things happen often. For example, make a context switch/gc/etc happen every time a @@ -253,6 +280,7 @@ In particular: Put braces round the body of for loops, while loops, if statements, etc. even if they "aren't needed" because it's really hard to find the resulting bug if you mess up. Indent them any way you like but put them in there! +
When defining a macro, always put parens round args - just in case. @@ -265,73 +293,9 @@ instead of #define add(x,y) x+y -

-Don't define macros that expand to a list of statements. -You could just use braces as in: - -

-  #define ASSIGN_CC_ID(ccID)              \
-        {                                 \
-        ccID = CC_ID;                     \
-        CC_ID++;                          \
-        }
-

- -but it's better to use the "do { ... } while (0)" trick instead: - -

-  #define ASSIGN_CC_ID(ccID)              \
-        do {                              \
-        ccID = CC_ID;                     \
-        CC_ID++;                          \
-        } while(0)
-

- -The following explanation comes from -The Usenet C programming FAQ -

-10.4:   What's the best way to write a multi-statement macro?
-
-A:      The usual goal is to write a macro that can be invoked as if it
-        were a statement consisting of a single function call.  This
-        means that the "caller" will be supplying the final semicolon,
-        so the macro body should not.  The macro body cannot therefore
-        be a simple brace-enclosed compound statement, because syntax
-        errors would result if it were invoked (apparently as a single
-        statement, but with a resultant extra semicolon) as the if
-        branch of an if/else statement with an explicit else clause.
-
-        The traditional solution, therefore, is to use
-
-                #define MACRO(arg1, arg2) do {  \
-                        /* declarations */      \
-                        stmt1;                  \
-                        stmt2;                  \
-                        /* ... */               \
-                        } while(0)      /* (no trailing ; ) */
-
-        When the caller appends a semicolon, this expansion becomes a
-        single statement regardless of context.  (An optimizing compiler
-        will remove any "dead" tests or branches on the constant
-        condition 0, although lint may complain.)
-
-        If all of the statements in the intended macro are simple
-        expressions, with no declarations or loops, another technique is
-        to write a single, parenthesized expression using one or more
-        comma operators.  (For an example, see the first DEBUG() macro
-        in question 10.26.)  This technique also allows a value to be
-        "returned."
-
-        References: H&S Sec. 3.3.2 p. 45; CT&P Sec. 6.3 pp. 82-3.
-

- -

-Don't even write macros that expand to 0 statements - they can mess you -up as well. Use the doNothing macro instead. -
```
-  #define doNothing() do { } while (0)
-
```
-

Don't declare and initialize variables at the same time. +Separating the declaration and initialization takes more lines, but +make the code clearer.

Use inline functions instead of macros if possible - they're a lot @@ -413,6 +377,28 @@ If any uses of the function remain, they will refer to the single copy in the library. +

+Don't define macros that expand to a list of statements. +You could just use braces as in: + +

+  #define ASSIGN_CC_ID(ccID)              \
+        {                                 \
+        ccID = CC_ID;                     \
+        CC_ID++;                          \
+        }
+

+ +(but it's usually better to use an inline function instead - see above). + +

+Don't even write macros that expand to 0 statements - they can mess you +up as well. Use the doNothing macro instead. +

+  #define doNothing() do { } while (0)
+

+ +

This code

@@ -486,36 +472,30 @@ and by keeping it to 80 columns we can ensure that code looks OK on
 everyone's screen.  Long lines are hard to read, and a sign that the
 code needs to be restructured anyway.
 
+
 When commenting out large chunks of code, use #ifdef 0
+... #endif rather than /* ... */ because C doesn't
+have nested comments.
+
+
When declaring a typedef for a struct, give the struct a name
+as well, so that other headers can forward-reference the struct name
+and it becomes possible to have opaque pointers to the struct.  Our
+convention is to name the struct the same as the typedef, but add a
+leading underscore.  For example:
+
++  typedef struct _Foo {
+    ...
+  } Foo;
+
+
+
Do not use ! instead of explicit comparison against
+NULL or '\0';  the latter is much clearer.
+
 
 We don't care too much about your indentation style but, if
 you're modifying a function, please try to use the same style as the
 rest of the function (or file).  If you're writing new code, a
 tab width of 4 is preferred.
 
-
-On which note:
-Hugs related pieces of code often start with the line:
-
-  /* -*- mode: hugs-c; -*- */
-
-which helps Emacs mimic the indentation style used by Mark P Jones
-within Hugs.  Add this to your .emacs file.
--  (defun hugs-c-mode ()
-    "C mode with adjusted defaults for use with Hugs (based on linux-c-mode)"
-    (interactive)
-    (c-mode)
-    (setq c-basic-offset 4)
-    (setq indent-tabs-mode nil) ; don't use tabs to indent
-    (setq c-recognize-knr-r nil)  ; no K&R here - don't pay the price
-    ; no: (setq tab-width 4)
-
-    (c-set-offset 'knr-argdecl-intro    0)
-    (c-set-offset 'case-label           0)
-    (c-set-offset 'statement-case-intro '++)
-    (c-set-offset 'statement-case-open  '+)
-    )
-
-
 
 
 CVS issues
@@ -526,12 +506,11 @@ Don't be tempted to reindent or reorganise large chunks of code - it
 generates large diffs in which it's hard to see whether anything else
 was changed.
 
-If you must reindent or reorganise, don't do anything else in that commit
-and give advance warning that you're about to do it in case anyone else
-is changing that file.
+If you must reindent or reorganise, don't include any functional
+changes that commit and give advance warning that you're about to do
+it in case anyone else is changing that file.
 
 
 
-
 
 
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