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19.1 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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