Name

qsort, qsort_r — sort an array

Synopsis

        #include <stdlib.h>
void qsort( void *base,
  size_t nmemb,
  size_t size,
  int (*compar)(const void *, const void *));
 
void qsort_r( void *base,
  size_t nmemb,
  size_t size,
  int (*compar)(const void *, const void *, void *),
  void *arg);
 
[Note] Note
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
qsort_r():
_GNU_SOURCE

DESCRIPTION

The qsort() function sorts an array with nmemb elements of size size. The base argument points to the start of the array.

The contents of the array are sorted in ascending order according to a comparison function pointed to by compar, which is called with two arguments that point to the objects being compared.

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.

The qsort_r() function is identical to qsort() except that the comparison function compar takes a third argument. A pointer is passed to the comparison function via 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.

RETURN VALUE

The qsort() and qsort_r() functions return no value.

VERSIONS

qsort_r() was added to glibc in version 2.8.

ATTRIBUTES

For an explanation of the terms used in this section, see attributes(7).

Interface Attribute Value
qsort(), qsort_r() Thread safety MT-Safe

CONFORMING TO

qsort(): POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.

NOTES

To compare C strings, the comparison function can call strcmp(3), as shown in the example below.

EXAMPLE

For one example of use, see the example under bsearch(3).

Another example is the following program, which sorts the strings given in its command-line arguments:

#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);
}

SEE ALSO

sort(1), alphasort(3), strcmp(3), versionsort(3)

COLOPHON

This page is part of release 4.16 of the Linux 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/.


  Copyright 1993 David Metcalfe (davidprism.demon.co.uk)

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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 (faithcs.unc.edu)
2006-01-15, mtk, Added example program.
Modified 2012-03-08, Mark R. Bannister <cambridgeusers.sourceforge.net>
                 and Ben Bacarisse <softwarebsb.me.uk>
    Document qsort_r()