.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
.\"
.\" %%%LICENSE_START(VERBATIM)
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.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Permission is granted to copy and distribute modified versions of this
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one.
.\"
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date.  The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein.  The author(s) may not
.\" have taken the same level of care in the production of this manual,
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.\" %%%LICENSE_END
.\"
.\" References consulted:
.\"     Linux libc source code
.\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
.\"     386BSD man pages
.\"
.\" Modified 1993-03-29, David Metcalfe
.\" Modified 1993-07-24, Rik Faith (faith@cs.unc.edu)
.\" 2006-01-15, mtk, Added example program.
.\" Modified 2012-03-08, Mark R. Bannister <cambridge@users.sourceforge.net>
.\"                  and Ben Bacarisse <software@bsb.me.uk>
.\"     Document qsort_r()
.\"
.TH QSORT 3 2015-08-08 "" "Linux Programmer's Manual"
.SH NAME
qsort, qsort_r \- sort an array
.SH SYNOPSIS
.nf
.B #include <stdlib.h>
.sp
.BI "void qsort(void *" base ", size_t " nmemb ", size_t " size ,
.BI "           int (*" compar ")(const void *, const void *));"
.sp
.BI "void qsort_r(void *" base ", size_t " nmemb ", size_t " size ,
.BI "           int (*" compar ")(const void *, const void *, void *),"
.BI "           void *" arg ");"
.fi
.sp
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.sp
.ad l
.BR qsort_r ():
_GNU_SOURCE
.ad b
.SH DESCRIPTION
The
.BR qsort ()
function sorts an array with \fInmemb\fP elements of
size \fIsize\fP.
The \fIbase\fP argument points to the start of the
array.
.PP
The contents of the array are sorted in ascending order according to a
comparison function pointed to by \fIcompar\fP, which is called with two
arguments that point to the objects being compared.
.PP
The comparison function must return an integer less than, equal to, or
greater than zero if the first argument is considered to be respectively
less than, equal to, or greater than the second.
If two members compare as equal,
their order in the sorted array is undefined.
.PP
The
.BR qsort_r ()
function is identical to
.BR qsort ()
except that the comparison function
.I compar
takes a third argument.
A pointer is passed to the comparison function via
.IR arg .
In this way, the comparison function does not need to use global variables to
pass through arbitrary arguments, and is therefore reentrant and safe to
use in threads.
.SH RETURN VALUE
The
.BR qsort ()
and
.BR qsort_r ()
functions return no value.
.SH VERSIONS
.BR qsort_r ()
was added to glibc in version 2.8.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbw18 lb lb
l l l.
Interface	Attribute	Value
T{
.BR qsort (),
.BR qsort_r ()
T}	Thread safety	MT-Safe
.TE

.SH CONFORMING TO
.BR qsort ():
POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.
.SH NOTES
To compare C strings, the comparison function can call
.BR strcmp (3),
as shown in the example below.
.SH EXAMPLE
For one example of use, see the example under
.BR bsearch (3).

Another example is the following program,
which sorts the strings given in its command-line arguments:
.sp
.nf
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

static int
cmpstringp(const void *p1, const void *p2)
{
    /* The actual arguments to this function are "pointers to
       pointers to char", but strcmp(3) arguments are "pointers
       to char", hence the following cast plus dereference */

    return strcmp(* (char * const *) p1, * (char * const *) p2);
}

int
main(int argc, char *argv[])
{
    int j;

    if (argc < 2) {
        fprintf(stderr, "Usage: %s <string>...\\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    qsort(&argv[1], argc \- 1, sizeof(char *), cmpstringp);

    for (j = 1; j < argc; j++)
        puts(argv[j]);
    exit(EXIT_SUCCESS);
}
.fi
.SH SEE ALSO
.BR sort (1),
.BR alphasort (3),
.BR strcmp (3),
.BR versionsort (3)
.SH COLOPHON
This page is part of release 4.10 of the Linux
.I man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
\%https://www.kernel.org/doc/man\-pages/.