PROLOG

       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.

NAME

       qsort — sort a table of data

SYNOPSIS

       #include <stdlib.h>

       void qsort(void *base, size_t nel, size_t width,
           int (*compar)(const void *, const void *));

DESCRIPTION

       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 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.

RETURN VALUE

       The qsort() function shall not return a value.

ERRORS

       No errors are defined.

       The following sections are informative.

EXAMPLES

       None.

APPLICATION USAGE

       The comparison function need not compare every byte, so arbitrary data may be contained in  the  elements
       in addition to the values being compared.

RATIONALE

       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

FUTURE DIRECTIONS

       None.

SEE ALSO

       alphasort()

       The Base Definitions volume of POSIX.1‐2008, <stdlib.h>

COPYRIGHT

       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
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .