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.
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 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.
GCC handles abs() and labs() as builtin functions. GCC
3.0 also handles llabs() and imaxabs() as builtins.