qsort

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
       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 5.05 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/.



                                  2019-03-06                          QSORT(3)