In the C11 standard is written that compilers should provide some macros to test for optional feature presence. In what headers do I find them?
For example where is located __STDC_NO_VLA__?
With GCC, i.e., if I try to find __STDC_NO_COMPLEX__ into complex.h I don't find it there...
2 Answers
They are not defined in any header, a compiler will define them itself.
You can dump all of the preprocessor defines. For example for gcc write:
gcc -dM -E - < /dev/null
For example for me:
bob@bob-fedora:~/trunk/software$ gcc --version
gcc (GCC) 4.7.2 20121109 (Red Hat 4.7.2-8)
Copyright (C) 2012 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
bob@bob-fedora:~/trunk/software$ gcc -std=gnu11 -dM -E - < /dev/null | grep STDC
#define __STDC_HOSTED__ 1
#define __STDC_UTF_16__ 1
#define __STDC_VERSION__ 201112L
#define __GNUC_STDC_INLINE__ 1
#define __STDC_UTF_32__ 1
#define __STDC__ 1
In the example you gave, __STDC_NO_VLA__ the presence of this means that the compiler does not supported variable length arrays. You could write:
#ifdef __STDC_NO_VLA__
#error Your compiler does not support VLAs! Please use a supported compiler.
#endif
Or
#ifndef __STDC_NO_VLA__
// code using variable length arrays
#else
// fallback code for when they are not supported
#endif
if STDC_NO_COMPLEX is defined, it indicates that there is is no complex.h
STDC_NO_COMPLEX is defined by the compiler, not by header file
source:
