This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
The functionality described on this reference page is aligned with
the ISO C standard. Any conflict between the requirements described
here and the ISO C standard is unintentional. This volume of
POSIX.1‐2008 defers to the ISO C standard.
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
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 non-zero:
((char *)p − (char *)base) % width == 0(char *)p >= (char *)base(char *)p < (char *)base + nel * width
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the
source files to man page format. To report such errors, see
IEEE/The Open Group 2013 QSORT(3P)