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# Files Reference

## math.h File

### Purpose

Defines math subroutines and constants.

### Description

The /usr/include/math.h header file contains declarations
of all the subroutines in the Math library (libm.a)
and of various subroutines in the Standard C Library (libc.a) that return floating-point values.

Among other things, the math.h file defines the following
macro, which is used as an error-return value:

HUGE_VAL |
Specifies the maximum value of a double-precision floating-point
number: +infinity on machines that support IEEE-754 and DBL_MAX otherwise. |

If you define the __MATH__ preprocessor variable
before including the math.h file, the math.h file defines macros that make the names of certain math subroutines
appear to the compiler as __xxxx. The following names are redefined to have the __ (double underscore)
prefix:

exp |
sin |

asin |
log |

cos |
acos |

log10 |
tan |

atan |
sqrt |

fabs |
atan2 |

These special names instruct the C compiler to generate code that avoids
the overhead of the Math library subroutines and issues compatible-mode floating-point
subroutines directly. The __MATH__ variable is defined
by default.

If _XOPEN_SOURCE variable is defined, the following
mathematical constants are defined for your convenience. The values are of
type double and are accurate to the precision of this type. That is, the
machine value is the mathematical value rounded to double precision.

M_E |
Base of natural logarithms (e) |

M_LOG2E |
Base-2 logarithm of e |

M_LOG10E |
Base-10 logarithm of e |

M_LN2 |
Natural logarithm of 2 |

M_LN10 |
Natural logarithm of 10 |

M_PI |
Pi, the ratio of the circumference of a circle to its diameter |

M_PI_2 |
Value of pi divided by 2 |

M_PI_4 |
Value of pi divided by 4 |

M_1_PI |
Value of 1 divided by pi |

M_2_PI |
Value of 2 divided by pi |

M_2_SQRTPI |
Value of 2 divided by the positive square root of pi |

M_SQRT2 |
Positive square root of 2 |

M_SQRT1_2 |
Positive square root of 1/2 |

### Related Information

The values.h file.

Header Files Overview defines
header files, describes how they are used, and lists several of the header
files for which information is provided in this documentation.

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