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Diffstat (limited to 'doc/autoconf.texi')
-rw-r--r-- | doc/autoconf.texi | 110 |
1 files changed, 64 insertions, 46 deletions
diff --git a/doc/autoconf.texi b/doc/autoconf.texi index cdd056c5..264ec29a 100644 --- a/doc/autoconf.texi +++ b/doc/autoconf.texi @@ -7466,51 +7466,67 @@ makes this invalid. That is why Autoconf stopped issuing @ovindex CC @ovindex CFLAGS @caindex prog_cc_stdc -Determine a C compiler to use. If @code{CC} is not already set in the -environment, check for @code{gcc} and @code{cc}, then for other C -compilers. Set output variable @code{CC} to the name of the compiler -found. +Determine a C compiler to use. -This macro may, however, be invoked with an optional first argument -which, if specified, must be a blank-separated list of C compilers to -search for. This just gives the user an opportunity to specify an -alternative search list for the C compiler. For example, if you didn't -like the default order, then you could invoke @code{AC_PROG_CC} like -this: +If the environment variable @code{CC} is set, its value will be taken as +the name of the C compiler to use. Otherwise, search for a C compiler +under a series of likely names, trying @code{gcc} and @code{cc} first. +Regardless, the output variable @code{CC} is set to the chosen compiler. + +If the optional first argument to the macro is used, it must be a +whitespace-separated list of potential names for a C compiler, +which overrides the built-in list. + +If no C compiler can be found, @command{configure} will error out. + +If the selected C compiler is found to be GNU C (regardless of +its name), the shell variable @code{GCC} will be set to @samp{yes}. +If the shell variable @code{CFLAGS} was not already set, it is set +to @option{-g -O2} for the GNU C compiler (@option{-O2} on systems +where GCC does not accept @option{-g}), or @option{-g} for other +compilers. @code{CFLAGS} is then made an output variable. +You can override the default for @code{CFLAGS} by inserting a shell +default assignment between @code{AC_INIT} and @code{AC_PROG_CC}: @example -AC_PROG_CC([gcc cl cc]) -@end example - -If necessary, add an option to output variable @code{CC} to enable -support for ISO Standard C features with extensions. Prefer the newest -C standard that is supported. Currently the newest standard is ISO C11, -with ISO C99 and ANSI C89 (ISO C90) being the older versions. -After calling this macro you can check whether the C compiler has been -set to accept Standard C; if not, the shell variable -@code{ac_cv_prog_cc_stdc} is set to @samp{no}. - -When attempting to add compiler options, prefer extended functionality -to strict conformance: the goal is to enable whatever standard features -that are available, not to check for full conformance to the standard or -to prohibit incompatible extensions. Test for C11 support by checking -for @code{_Alignas}, @code{_Alignof}, @code{_Noreturn}, -@code{_Static_assert}, UTF-8 string literals, duplicate @code{typedef}s, -and anonymous structures and unions. Test for C99 support by checking -for @code{_Bool}, @code{//} comments, flexible array members, -@code{inline}, signed and unsigned @code{long long int}, mixed code and -declarations, named initialization of structs, @code{restrict}, -@code{va_copy}, varargs macros, variable declarations in @code{for} -loops, and variable length arrays. Test for C89 support by checking for -function prototypes. - -If using a compiler that supports GNU C, set shell variable @code{GCC} to -@samp{yes}. If output variable @code{CFLAGS} was not already set, set -it to @option{-g -O2} for a GNU C compiler (@option{-O2} on systems -where the compiler does not accept @option{-g}), or @option{-g} for -other compilers. If your package does not like this default, then it is -acceptable to insert the line @samp{: $@{CFLAGS=""@}} after @code{AC_INIT} -and before @code{AC_PROG_CC} to select an empty default instead. +: $@{CFLAGS="@var{options}"@} +@end example + +where @var{options} are the appropriate set of options to use by +default. (It is important to use this construct rather than a normal +assignment, so that @code{CFLAGS} can still be overridden by the +person building the package. @xref{Preset Output Variables}.) + +If necessary, options are added to @code{CC} to enable support for ISO +Standard C features with extensions, preferring the newest C standard +that is supported. Currently the newest standard Autoconf knows how to +detect support for is ISO C 2011. After calling this macro you can +check whether the C compiler has been set to accept Standard C by +inspecting cache variables. If @code{ac_cv_prog_cc_c11} is set to any +value other than @samp{no} (including the empty string), then @code{CC} +can compile code as standard C 2011, and this mode has been enabled. +Otherwise, if @code{ac_cv_prog_cc_c99} is set to any value other than +@samp{no} (including the empty string), then @code{CC} can compile code +as standard C 1999, and this mode has been enabled. Otherwise, if +@code{ac_cv_prog_cc_c89} is set to any value other than @samp{no} +(including the empty string), then @code{CC} can compile code as +standard C 1989, and this mode has been enabled. Finally, if all +three variables are set to @samp{no}, then @code{CC} cannot compile +standard C at all. + +The tests for standard conformance are not comprehensive. They test the +values of @code{__STDC__} and @code{__STDC_VERSION__}, and a +representative sample of the language features added in each version of +the C standard. They do not test the C standard library, because the C +compiler might be generating code for a ``freestanding environment'' +(in which most of the standard library is optional). If you need to know +whether a particular C standard header exists, use @code{AC_CHECK_HEADER}. + +None of the options that may be added to @code{CC} by this macro +enable @emph{strict} conformance to the C standard. In particular, +system-specific extensions are not disabled. (For example, for GNU C, +the @option{-std=gnu@var{nn}} options may be used, but not the +@option{-std=c@var{nn}} options.) Many Autoconf macros use a compiler, and thus call @samp{AC_REQUIRE([AC_PROG_CC])} to ensure that the compiler has been @@ -7900,10 +7916,12 @@ been enabled. Finally, if both variables are set to @samp{no}, then The tests for standard conformance are not comprehensive. They test the value of @code{__cplusplus} and a representative sample of the -language features added in each version of the C++ standard. They do -not exercise the C++ standard library, because this can be extremely -slow. If you need to know whether a particular C++ standard header -exists, use @code{AC_CHECK_HEADER}. +language features added in each version of the C++ standard. They +do not test the C++ standard library, because this can be extremely +slow, and because the C++ compiler might be generating code for a +``freestanding environment'' (in which most of the C++ standard library +is optional). If you need to know whether a particular C++ standard +header exists, use @code{AC_CHECK_HEADER}. None of the options that may be added to @code{CXX} by this macro enable @emph{strict} conformance to the C++ standard. In particular, |