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 arbi‐
       trary 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 3.74 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)