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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.
#ifdef __STDC__
/\
* A comment with backslash-newlines in it. %{ %} *\
\
/
char str[] = "\\
" A string with backslash-newlines in it %{ %} \\
"";
char apostrophe = '\\
\
'\
';
#endif
|
yields
error-->cpp: "foo.c", line 13: error 4048: Non-terminating comment at end of file. error-->cpp: "foo.c", line 13: error 4033: Missing #endif at end of file. |
Removing the lines with solitary backslashes solves the problem.
$ 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 -o a.o; cc -c b.c -o b.o; cc a.o b.o -o c' solves the issue.
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 space 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(cl egcs gcc cc) |
If the C compiler is not in ANSI C mode by default, try to add an
option to output variable CC to make it so. This macro tries
various options that select ANSI C on some system or another. It
considers the compiler to be in ANSI C mode if it handles function
prototypes correctly.
After calling this macro you can check whether the C compiler has been
set to accept ANSI C; if not, the shell variable
ac_cv_prog_cc_stdc is set to `no'. If you wrote your source
code in ANSI C, you can make an un-ANSIfied copy of it by
using the program ansi2knr, which comes with Automake. See also
under 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.
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.
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.
The following macros check for C compiler or machine architecture
features. To check for characteristics not listed here, use
AC_COMPILE_IFELSE (see section 6.4 Running the Compiler) or
AC_RUN_IFELSE (see section 6.6 Checking Run Time Behavior).
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.
const, 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 will
define 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.
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 will define 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 will compile, when it wouldn't before.
In general, the volatile keyword is a feature of ANSI C, 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 defined __STDC__.
inline, do nothing.
Otherwise define inline to __inline__ or __inline
if it accepts one of those, otherwise define inline to be empty.
char is unsigned, define __CHAR_UNSIGNED__,
unless the C compiler predefines it.
long double type with more
range or precision than the double type, define
HAVE_LONG_DOUBLE.
HAVE_STRINGIZE. The stringizing operator is `#' and is
found in macros such as this:
#define x(y) #y |
AC_PROG_CC), define PROTOTYPES and __PROTOTYPES.
In the case the compiler does not handle
prototypes, you should use ansi2knr, which comes with the
Automake distribution, to unprotoize function definitions. For
function prototypes, you should first define PARAMS:
#ifndef PARAMS # if PROTOTYPES # define PARAMS(protos) protos # else /* no PROTOTYPES */ # define PARAMS(protos) () # endif /* no PROTOTYPES */ #endif |
then use it this way:
size_t my_strlen PARAMS ((const char *)); |
This macro also defines __PROTOTYPES; this is for the benefit of
header files that cannot use macros that infringe on user name space.
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. Since recent versions of the
GNU C compiler fix the header files automatically when installed,
this is becoming a less prevalent problem.
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