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.
SVID 3, POSIX.1, BSD 4.3, ISO/IEC 9899 (C99). POSIX.1 (1996 edition) only
requires the abs() function. ISO/IEC 9899:1990 (C89) only includes the
abs() and labs() functions; the functions llabs() and
imaxabs() were added in C99.
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, but is not
in libc5 or libc4. The imaxabs() function is included in glibc since
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.
GCC handles abs() and labs() as builtin functions. GCC 3.0 also
handles llabs() and imaxabs() as builtins.