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



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

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.1-2017
       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.

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.1-2017, <stdlib.h>

COPYRIGHT
       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(3)
