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(libc.info)Mathematical Constants

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Predefined Mathematical Constants
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   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.  Note:
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.