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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 compari-
       son 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 4.15 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)