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 4.09 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/.

                              2015-08-08                          QSORT(3)

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