QSORT(3P)                                           POSIX Programmer's Manual                                          QSORT(3P)



PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux implementation of this interface may differ (con-
       sult 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  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  func-
       tion 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 argu-
       ment 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 nonzero:


              ((char *)p - (char *)base) % width == 0
              (char *)p >= (char *)base
              (char *)p < (char *)base + nel * width

FUTURE DIRECTIONS
       None.

SEE ALSO
       The Base Definitions volume of IEEE Std 1003.1-2001, <stdlib.h>

COPYRIGHT
       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,  Copy-
       right (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.open-
       group.org/unix/online.html .



IEEE/The Open Group                                           2003                                                     QSORT(3P)