c - M_PI not available with gcc --std=c11 but with --std=gnu11?

Question

I noticed M_PI is unavailable on c11. By looking at /usr/include/math.h I can see M_PI is defined if:

#if !defined(__STRICT_ANSI__) || ((_XOPEN_SOURCE - 0) >= 500)
...
#define M_PI 3.1415...
#endif 

Moreover in the math.h from glibc __STRICT_ANSI__ is replaced with __USE_MISC. I am completely lost with this.

What is the story in between --std=c11 and the constants defined in math.h?

Which libc should I consider on a debian distribution ?

By the way, M_PI is defined in c99 and gnu11...

Answer 1

It's simple: M_PI is not defined in standard C. Provide your own definition if you want to be standard-compliant.

C compilers cannot introduce such constants without breaking legal C programs (the name is not reserved, and could be used as an identifier), and as such, they are only defined as an extension.

GCC 4.9 when used with -std=c99 doesn't define M_PI, but does when used with -std=gnu99