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fabs - Man Page

absolute value function


This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.


#include <math.h>

double fabs(double x);
float fabsf(float x);
long double fabsl(long double x);


The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1-2017 defers to the ISO C standard.

These functions shall compute the absolute value of their argument x,|x|.

Return Value

Upon successful completion, these functions shall return the absolute value of x.

If x is NaN, a NaN shall be returned.

If x is ±0, +0 shall be returned.

If x is ±Inf, +Inf shall be returned.


No errors are defined.

The following sections are informative.


Computing the 1-Norm of a Floating-Point Vector

This example shows the use of fabs() to compute the 1-norm of a vector defined as follows:

norm1(v) = |v[0]| + |v[1]| + ... + |v[n-1]|

where |x| denotes the absolute value of x, n denotes the vector's dimension v[i] denotes the i-th component of v (0≤i<n).

#include <math.h>

norm1(const double v[], const int n)
    int     i;
    double  n1_v;  /* 1-norm of v */

    n1_v = 0;
    for (i=0; i<n; i++) {
        n1_v += fabs (v[i]);

    return n1_v;

Application Usage




Future Directions


See Also


The Base Definitions volume of POSIX.1-2017, <math.h>

Referenced By

abs(3p), math.h(0p), tgmath.h(0p).

2017 IEEE/The Open Group POSIX Programmer's Manual