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]](../stylesheet/note.png) |
Note | ||
|---|---|---|---|
|
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.
NOTES
Library routines suitable for use as the compar argument to qsort() include alphasort(3) and versionsort(3). 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);
}
COLOPHON
This page is part of release 3.52 of the Linux man-pages project. A
description of the project, and information about reporting
bugs, can be found at
http://www.kernel.org/doc/man−pages/.
|
Copyright 1993 David Metcalfe (davidprism.demon.co.uk) %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this 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, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. %%%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 (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() |