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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...

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  • Why do you have the c++11 tag if what you ask is clearly about C11 and not C++11? Commented Mar 25, 2015 at 19:15
  • 2
    Yes there is, and your post was already edited to have the c11 tag instead of c++11. Commented Mar 25, 2015 at 19:22

3 Answers 3

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26

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

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7 Comments

It was defined in 'c99' though.
@coin It shouldn't be - C99 also doesn't define M_PI.
The default of GCC is gnu90. 5.1.0 (not released yet) will default to gnu11.
@coin: It uses the same libc as with different or without -std options (which usually is glibc, but doesn't have to be. E.g. on Windows). Also read man7.org/linux/man-pages/man7/feature_test_macros.7.html and gnu.org/software/libc/manual/html_node/Feature-Test-Macros.html for more information.
@coin Why bother? Either you want more stuff, then you use -std=gnu99 with extensions, or you don't want extensions because you want portability and don't want to accidentally limit yourself to a single compiler, in that case you use -std=c99.
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If you just want M_PI while looking for a more comprehensive answer with POSIX / XOPEN feature tests macros, etc., an interim solution is:

#ifndef M_PI
#define M_PI (3.14159265358979323846)
#endif

That's "1.20" format, which is also sufficient for 'round-trip' representation for an 80 bit extended type. double precision is "1.16". For 128-bit quad precision:

#define M_PI (3.14159265358979323846264338327950288)

The "1.35" format for round-trip precision. This means if you want to print out a floating point double, and recover the same value when you read it back, you should use "%+1.16" for printf functions, as so on. You might say that a double doesn't have 17 significant digits, but those digits are not 'junk' if you want to recover a value.

Anyway - there are better resources than this available.

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2 Comments

For correct 1.16 --> +1. BTW: See any downside to having lots of PI digits like 80 digits?
@chux - not that I can think of. gcc at least includes the gmp / mpfr / mpc libraries to take care of all multiple-precision issues, with correct (and selectable) rounding modes, etc. If an 80 digit PI value was assigned to a single precision float, I would expect it to do the right thing. Don't know how up to date this page is. I don't know clang's approach.
3

The M_PI macro isn't defined by the C11 standard: http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1548.pdf

Therefore, the #if guards are protecting you from problems in case you want to define your own M_PI macro. gcc is doing exactly the right thing. The standard headers shouldn't arbitrarily define macros that are not in the standard.

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