The header math.h defines several useful mathematical constants.
All values are defined as preprocessor macros starting with
The values provided are:
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. The
long double macros have a lowercase `l' appended to their
M_PIl, and so forth. These are only
_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:
M_PI throughout, or put `-DPI=M_PI'
on the compiler command line.