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
_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 and
float. The long double macros have a lowercase ‘l’
while the float macros have a lowercase ‘f’ appended to
their names: M_El, M_PIl, and so forth. These are only
available if _GNU_SOURCE is defined.
Likewise, the GNU C Library also defines these constants with the types
_FloatN and _FloatNx for the machines that
have support for such types enabled (see Mathematics) and if
_GNU_SOURCE is defined. When available, the macros names are
appended with ‘fN’ or ‘fNx’, such as ‘f128’
for the type _Float128.
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.