FABS(3) Linux Programmer's Manual FABS(3)

fabs, fabsf, fabsl - absolute value of floating-point number

#include <math.h>
double fabs(double x);
float fabsf(float x);
long double fabsl(long double x);
Link with -lm.

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
fabsf(), fabsl():
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

These functions return the absolute value of the floating-point number x.

These functions return the absolute value of x.
If x is a NaN, a NaN is returned.
If x is -0, +0 is returned.
If x is negative infinity or positive infinity, positive infinity is returned.

No errors occur.

For an explanation of the terms used in this section, see attributes(7).
Interface Attribute Value
fabs (), fabsf (), fabsl () Thread safety MT-Safe

C99, POSIX.1-2001, POSIX.1-2008.
The variant returning double also conforms to SVr4, 4.3BSD, C89.

abs(3), cabs(3), ceil(3), floor(3), labs(3), rint(3)