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The GNU C Library

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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 section 1.3.4 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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