llabs (3)
Leading comments
Copyright 1993 David Metcalfe (david@prism.demon.co.uk) %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Sin...
NAME
abs, labs, llabs, imaxabs  compute the absolute value of an integerSYNOPSIS
#include <stdlib.h> int abs(int j);
long int labs(long int j);
long long int llabs(long long int j); #include <inttypes.h> intmax_t imaxabs(intmax_t j);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
llabs():

_XOPEN_SOURCE >= 600  _ISOC99_SOURCE 
_POSIX_C_SOURCE >= 200112L;
or cc std=c99
DESCRIPTION
The abs() function computes the absolute value of the integer argument j. The labs(), llabs() and imaxabs() functions compute the absolute value of the argument j of the appropriate integer type for the function.RETURN VALUE
Returns the absolute value of the integer argument, of the appropriate integer type for the function.ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).Interface  Attribute  Value 
abs(), labs(), llabs(), imaxabs()  Thread safety  MTSafe 
CONFORMING TO
POSIX.12001, POSIX.12008, C99, SVr4, 4.3BSD. C89 only includes the abs() and labs() functions; the functions llabs() and imaxabs() were added in C99.NOTES
Trying to take the absolute value of the most negative integer is not defined.The llabs() function is included in glibc since version 2.0. The imaxabs() function is included in glibc since version 2.1.1.
For llabs() to be declared, it may be necessary to define _ISOC99_SOURCE or _ISOC9X_SOURCE (depending on the version of glibc) before including any standard headers.
By default, GCC handles abs(), labs(), and (since GCC 3.0) llabs() and imaxabs() as builtin functions.