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5.10.3 C Compiler Characteristics

The following macros provide ways to find and exercise a C Compiler. There are a few constructs that ought to be avoided, but do not deserve being checked for, since they can easily be worked around.

Don't use lines containing solitary backslashes
They tickle a bug in the HP-UX C compiler (checked on HP-UX 10.20, 11.00, and 11i). When given the following source: 
          #ifdef __STDC__
          /\
          * A comment with backslash-newlines in it.  %{ %} *\
          \
          /
          char str[] = "\\
          " A string with backslash-newlines in it %{ %} \\
          "";
          char apostrophe = '\\
          \
          '\
          ';
          #endif

the compiler incorrectly fails with the diagnostics “Non-terminating comment at end of file” and “Missing ‘#endif’ at end of file.” Removing the lines with solitary backslashes solves the problem.

Don't compile several files at once if output matters to you
Some compilers, such as the HP's, reports the name of the file it is compiling when they are several. For instance: 
          $ cc a.c b.c
          a.c:
          b.c:

This can cause problems if you observe the output of the compiler to detect failures. Invoking ‘cc -c a.c && cc -c b.c && cc -o c a.o b.o’ solves the issue.

Don't rely on #error failing
The irix C compiler does not fail when #error is preprocessed; it simply emits a diagnostic and continues, exiting successfully. So, instead of an error directive like #error "Unsupported word size" it is more portable to use an invalid directive like #Unsupported word size in Autoconf tests. In ordinary source code, #error is OK, since installers with inadequate compilers like irix can simply examine these compilers' diagnostic output.
Don't rely on correct #line support
On Solaris, c89 (at least Sun C 5.3 through 5.8) diagnoses #line directives whose line numbers are greater than 32767. Nothing in Posix makes this invalid. That is why Autoconf stopped issuing #line directives.
— Macro: AC_PROG_CC ([compiler-search-list])

Determine a C compiler to use. If CC is not already set in the environment, check for gcc and cc, then for other C compilers. Set output variable CC to the name of the compiler found.

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 AC_PROG_CC like this: 

          AC_PROG_CC([gcc cl cc])

If the C compiler does not handle function prototypes correctly by default, try to add an option to output variable CC to make it so. This macro tries various options that select standard-conformance modes on various systems.

After calling this macro you can check whether the C compiler has been set to accept ANSI C89 (ISO C90); if not, the shell variable ac_cv_prog_cc_c89 is set to ‘no’. See also AC_C_PROTOTYPES below.

If using the GNU C compiler, set shell variable GCC to ‘yes’. If output variable CFLAGS was not already set, set it to -g -O2 for the GNU C compiler (-O2 on systems where GCC does not accept -g), or -g for other compilers.

— Macro: AC_PROG_CC_C_O

If the C compiler does not accept the -c and -o options simultaneously, define NO_MINUS_C_MINUS_O. This macro actually tests both the compiler found by AC_PROG_CC, and, if different, the first cc in the path. The test fails if one fails. This macro was created for GNU Make to choose the default C compilation rule.

— Macro: AC_PROG_CPP

Set output variable CPP to a command that runs the C preprocessor. If ‘$CC -E’ doesn't work, /lib/cpp is used. It is only portable to run CPP on files with a .c extension.

Some preprocessors don't indicate missing include files by the error status. For such preprocessors an internal variable is set that causes other macros to check the standard error from the preprocessor and consider the test failed if any warnings have been reported. For most preprocessors, though, warnings do not cause include-file tests to fail unless AC_PROG_CPP_WERROR is also specified.

— Macro: AC_PROG_CPP_WERROR

This acts like AC_PROG_CPP, except it treats warnings from the preprocessor as errors even if the preprocessor exit status indicates success. This is useful for avoiding headers that generate mandatory warnings, such as deprecation notices.

The following macros check for C compiler or machine architecture features. To check for characteristics not listed here, use AC_COMPILE_IFELSE (see Running the Compiler) or AC_RUN_IFELSE (see Runtime).

— Macro: AC_PROG_CC_STDC

If the C compiler cannot compile ISO Standard C (currently C99), try to add an option to output variable CC to make it work. If the compiler does not support C99, fall back to supporting ANSI C89 (ISO C90).

After calling this macro you can check whether the C compiler has been set to accept Standard C; if not, the shell variable ac_cv_prog_cc_stdc is set to ‘no’.

— Macro: AC_PROG_CC_C89

If the C compiler is not in ANSI C89 (ISO C90) mode by default, try to add an option to output variable CC to make it so. This macro tries various options that select ANSI C89 on some system or another. It considers the compiler to be in ANSI C89 mode if it handles function prototypes correctly.

After calling this macro you can check whether the C compiler has been set to accept ANSI C89; if not, the shell variable ac_cv_prog_cc_c89 is set to ‘no’.

This macro is called automatically by AC_PROG_CC.

— Macro: AC_PROG_CC_C99

If the C compiler is not in C99 mode by default, try to add an option to output variable CC to make it so. This macro tries various options that select C99 on some system or another. It considers the compiler to be in C99 mode if it handles _Bool, flexible arrays, inline, long long int, mixed code and declarations, named initialization of structs, restrict, varargs macros, variable declarations in for loops and variable length arrays.

After calling this macro you can check whether the C compiler has been set to accept C99; if not, the shell variable ac_cv_prog_cc_c99 is set to ‘no’.

— Macro: AC_C_BACKSLASH_A

Define ‘HAVE_C_BACKSLASH_A’ to 1 if the C compiler understands ‘\a’.

This macro is obsolescent, as current C compilers understand ‘\a’. New programs need not use this macro.

— Macro: AC_C_BIGENDIAN ([action-if-true], [action-if-false], [action-if-unknown])

If words are stored with the most significant byte first (like Motorola and SPARC CPUs), execute action-if-true. If words are stored with the least significant byte first (like Intel and VAX CPUs), execute action-if-false.

This macro runs a test-case if endianness cannot be determined from the system header files. When cross-compiling, the test-case is not run but grep'ed for some magic values. action-if-unknown is executed if the latter case fails to determine the byte sex of the host system.

The default for action-if-true is to define ‘WORDS_BIGENDIAN’. The default for action-if-false is to do nothing. And finally, the default for action-if-unknown is to abort configure and tell the installer which variable he should preset to bypass this test.

— Macro: AC_C_CONST

If the C compiler does not fully support the const keyword, define const to be empty. Some C compilers that do not define __STDC__ do support const; some compilers that define __STDC__ do not completely support const. Programs can simply use const as if every C compiler supported it; for those that don't, the makefile or configuration header file defines it as empty.

Occasionally installers use a C++ compiler to compile C code, typically because they lack a C compiler. This causes problems with const, because C and C++ treat const differently. For example: 

          const int foo;

is valid in C but not in C++. These differences unfortunately cannot be papered over by defining const to be empty.

If autoconf detects this situation, it leaves const alone, as this generally yields better results in practice. However, using a C++ compiler to compile C code is not recommended or supported, and installers who run into trouble in this area should get a C compiler like GCC to compile their C code.

This macro is obsolescent, as current C compilers support const. New programs need not use this macro.

— Macro: AC_C_RESTRICT

If the C compiler recognizes the restrict keyword, don't do anything. If it recognizes only a variant spelling (__restrict, __restrict__, or _Restrict), then define restrict to that. Otherwise, define restrict to be empty. Thus, programs may simply use restrict as if every C compiler supported it; for those that do not, the makefile or configuration header defines it away.

Although support in C++ for the restrict keyword is not required, several C++ compilers do accept the keyword. This macro works for them, too.

— Macro: AC_C_VOLATILE

If the C compiler does not understand the keyword volatile, define volatile to be empty. Programs can simply use volatile as if every C compiler supported it; for those that do not, the makefile or configuration header defines it as empty.

If the correctness of your program depends on the semantics of volatile, simply defining it to be empty does, in a sense, break your code. However, given that the compiler does not support volatile, you are at its mercy anyway. At least your program compiles, when it wouldn't before.

In general, the volatile keyword is a standard C feature, so you might expect that volatile is available only when __STDC__ is defined. However, Ultrix 4.3's native compiler does support volatile, but does not define __STDC__.

This macro is obsolescent, as current C compilers support volatile. New programs need not use this macro.

— Macro: AC_C_INLINE

If the C compiler supports the keyword inline, do nothing. Otherwise define inline to __inline__ or __inline if it accepts one of those, otherwise define inline to be empty.

— Macro: AC_C_CHAR_UNSIGNED

If the C type char is unsigned, define __CHAR_UNSIGNED__, unless the C compiler predefines it.

— Macro: AC_C_STRINGIZE

If the C preprocessor supports the stringizing operator, define HAVE_STRINGIZE. The stringizing operator is ‘#’ and is found in macros such as this: 

          #define x(y) #y

This macro is obsolescent, as current C compilers support the stringizing operator. New programs need not use this macro.

— Macro: AC_C_TYPEOF

If the C compiler supports GCC's typeof syntax either directly or through a different spelling of the keyword (e.g., __typeof__), define HAVE_TYPEOF. If the support is available only through a different spelling, define typeof to that spelling.

— Macro: AC_C_PROTOTYPES

If function prototypes are understood by the compiler (as determined by AC_PROG_CC), define PROTOTYPES and __PROTOTYPES. Defining __PROTOTYPES is for the benefit of header files that cannot use macros that infringe on user name space.

This macro is obsolescent, as current C compilers support prototypes. New programs need not use this macro.

— Macro: AC_PROG_GCC_TRADITIONAL

Add -traditional to output variable CC if using the GNU C compiler and ioctl does not work properly without -traditional. That usually happens when the fixed header files have not been installed on an old system.

This macro is obsolescent, since current versions of the GNU C compiler fix the header files automatically when installed.