ABS(3) Library Functions Manual ABS(3)
abs - integer absolute value
abs() returns the absolute value of its integer operand.
ieee_functions(3M) for fabs()
Applying the abs() function to the most negative integer generates a
result which is the most negative integer. That is, abs(0x80000000)
returns 0x80000000 as a result.
6 October 1987 ABS(3)