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.