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 .