The qsort() function shall sort an array of nel objects, the
initial element of which is pointed to by base. The size of each
object, in bytes, is specified by the width argument. If the nel
argument has the value zero, the comparison function pointed to by
compar shall not be called and no rearrangement shall take place.
The application shall ensure that the comparison function pointed to by
compar does not alter the contents of the array. The implementation may
reorder elements of the array between calls to the comparison function, but
shall not alter the contents of any individual element.
When the same objects (consisting of width bytes, irrespective of their current
positions in the array) are passed more than once to the comparison function,
the results shall be consistent with one another. That is, they shall define a
total ordering on the array.
The contents of the array shall be sorted in ascending order according to a
comparison function. The compar argument is a pointer to the comparison
function, which is called with two arguments that point to the elements being
compared. The application shall ensure that the function returns an integer
less than, equal to, or greater than 0, if the first argument is considered
respectively less than, equal to, or greater than the second. If two members
compare as equal, their order in the sorted array is unspecified.
The requirement that each argument (hereafter referred to as p) to the
comparison function is a pointer to elements of the array implies that for
every call, for each argument separately, all of the following expressions are
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.opengroup.org/unix/online.html