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QSORT(3) Linux Programmer's Manual QSORT(3)
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);
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 respec-
tively 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 compari-
son 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.
CONFORMING TO
The qsort() function conforms to SVr4, 4.3BSD, C89, C99.
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);
}
SEE ALSO
sort(1), alphasort(3), strcmp(3), versionsort(3)
COLOPHON
This page is part of release 3.69 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
http://www.kernel.org/doc/man-pages/.
2012-03-08 QSORT(3)
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