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

c gcc c11
nowox picture nowox · Mar 25, 2015 · Viewed 13.8k times · Source

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

milleniumbug picture milleniumbug · Mar 25, 2015

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

Related questions

How to enable c11 on later versions of gcc?

I currently use gcc 4.6.3. My understanding is that gcc by default uses the gnu89 standard and I would like to enable C11, the latest C standard. I tried: [pauldb@pauldb-laptop test ]$ gcc -std=c11 -o test test.c cc1: error: …

Read More
What is the default C -std standard version for the current GCC (especially on Ubuntu)?

When I ask to see the current version of cc I get this. $ cc --version cc (Ubuntu/Linaro 4.7.2-2ubuntu1) 4.7.2 Copyright (C) 2012 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; …

Read More
C11 <thread.h> in GCC?

I’m trying to compile some C11 code using thread.h, but I can’t. I've recompiled GCC (running 4.6.2 now), and I’m trying to compile with gcc -std=c1x file.c -o file. I can do this in …

Read More