The header math.h defines several useful mathematical constants.
All values are defined as preprocessor macros starting with M_.
The values provided are:

M_E

The base of natural logarithms.

M_LOG2E

The logarithm to base 2 of M_E.

M_LOG10E

The logarithm to base 10 of M_E.

M_LN2

The natural logarithm of 2.

M_LN10

The natural logarithm of 10.

M_PI

Pi, the ratio of a circle's circumference to its diameter.

M_PI_2

Pi divided by two.

M_PI_4

Pi divided by four.

M_1_PI

The reciprocal of pi (1/pi)

M_2_PI

Two times the reciprocal of pi.

M_2_SQRTPI

Two times the reciprocal of the square root of pi.

M_SQRT2

The square root of two.

M_SQRT1_2

The reciprocal of the square root of two (also the square root of 1/2).

These constants come from the Unix98 standard and were also available in
4.4BSD; therefore they are only defined if _BSD_SOURCE or
_XOPEN_SOURCE=500, or a more general feature select macro, is
defined. The default set of features includes these constants.
See Feature Test Macros.

All values are of type double. As an extension, the GNU C
library also defines these constants with type long double. The
long double macros have a lowercase `l' appended to their
names: M_El, M_PIl, and so forth. These are only
available if _GNU_SOURCE is defined.

Note: Some programs use a constant named PI which has the
same value as M_PI. This constant is not standard; it may have
appeared in some old AT&T headers, and is mentioned in Stroustrup's book
on C++. It infringes on the user's name space, so the GNU C library
does not define it. Fixing programs written to expect it is simple:
replace PI with M_PI throughout, or put `-DPI=M_PI'
on the compiler command line.

Published under the terms of the GNU General Public License