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20.8.1 Absolute Value

These functions are provided for obtaining the absolute value (or magnitude) of a number. The absolute value of a real number x is x if x is positive, −x if x is negative. For a complex number z, whose real part is x and whose imaginary part is y, the absolute value is sqrt (x*x + y*y).

Prototypes for abs, labs and llabs are in stdlib.h; imaxabs is declared in inttypes.h; fabs, fabsf and fabsl are declared in math.h. cabs, cabsf and cabsl are declared in complex.h.

— Function: int abs (int number)
— Function: long int labs (long int number)
— Function: long long int llabs (long long int number)
— Function: intmax_t imaxabs (intmax_t number)

These functions return the absolute value of number.

Most computers use a two's complement integer representation, in which the absolute value of INT_MIN (the smallest possible int) cannot be represented; thus, abs (INT_MIN) is not defined.

llabs and imaxdiv are new to ISO C99.

See Integers for a description of the intmax_t type.

— Function: double fabs (double number)
— Function: float fabsf (float number)
— Function: long double fabsl (long double number)

This function returns the absolute value of the floating-point number number.

— Function: double cabs (complex double z)
— Function: float cabsf (complex float z)
— Function: long double cabsl (complex long double z)

These functions return the absolute value of the complex number z (see Complex Numbers). The absolute value of a complex number is: 

          sqrt (creal (z) * creal (z) + cimag (z) * cimag (z))

This function should always be used instead of the direct formula because it takes special care to avoid losing precision. It may also take advantage of hardware support for this operation. See hypot in Exponents and Logarithms.