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Node:Mathematical Constants,
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Predefined Mathematical Constants
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.
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