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 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>...
", 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 4.15 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/.

                                                                    2017-09-15                                                            QSORT(3)