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
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