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.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" 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 2021-03-22 "Linux man-pages (unreleased)" "Linux Programmer's Manual"
.SH NAME
qsort, qsort_r \- sort an array
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <stdlib.h>
.PP
.BI "void qsort(void *" base ", size_t " nmemb ", size_t " size ,
.BI "           int (*" compar ")(const void *, const void *));"
.BI "void qsort_r(void *" base ", size_t " nmemb ", size_t " size ,
.BI "           int (*" compar ")(const void *, const void *, void *),"
.BI "           void *" arg ");"
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR qsort_r ():
.nf
    _GNU_SOURCE
.fi
.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).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR qsort (),
.BR qsort_r ()
T}	Thread safety	MT-Safe
.TE
.hy
.ad
.sp 1
.SH STANDARDS
.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 EXAMPLES
For one example of use, see the example under
.BR bsearch (3).
.PP
Another example is the following program,
which sorts the strings given in its command-line arguments:
.PP
.EX
#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(*(const char **) p1, *(const char **) p2);
}

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

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

    for (int j = 1; j < argc; j++)
        puts(argv[j]);
    exit(EXIT_SUCCESS);
}
.EE
.SH SEE ALSO
.BR sort (1),
.BR alphasort (3),
.BR strcmp (3),
.BR versionsort (3)