qsort, qsort_r - sort an array
#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
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.
The
qsort() and
qsort_r() functions return no value.
qsort_r() was added to glibc in version 2.8.
For an explanation of the terms used in this section, see
attributes(7).
Interface |
Attribute |
Value |
qsort (), qsort_r () |
Thread safety |
MT-Safe |
qsort(): POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.
To compare C strings, the comparison function can call
strcmp(3), as
shown in the example below.
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);
}
sort(1),
alphasort(3),
strcmp(3),
versionsort(3)