This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface

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

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
       for Information  Technology  --  Portable  Operating  System  Interface  (POSIX),  The  Open  Group  Base
       Specifications  Issue  7, 2018 Edition, Copyright (C) 2018 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 .

       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 .

IEEE/The Open Group                                   2017                                         QSORT(3POSIX)

       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.1‐2017 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.

       No errors are defined.

       Thefollowingsectionsareinformative.

       None.

       None.

       qsort — sort a table of data

       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 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

       The qsort() function shall not return a value.

alphasort()

       The Base Definitions volume of POSIX.1‐2017, <stdlib.h>

       #include <stdlib.h>

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