NAME

       qsort, qsort_r - sort an array

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <stdlib.h>

       void qsort(void base[.size * .nmemb], size_t nmemb, size_t size,
                  int (*compar)(const void [.size], const void [.size]));
       void qsort_r(void base[.size * .nmemb], size_t nmemb, size_t size,
                  int (*compar)(const void [.size], const void [.size], 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.

ATTRIBUTES

       For an explanation of the terms used in this section, see attributes(7).

       ┌──────────────────────────────────────────────────────────────────────────────┬───────────────┬─────────┐
       │Interface                                                                     │ Attribute     │ Value   │
       ├──────────────────────────────────────────────────────────────────────────────┼───────────────┼─────────┤
       │qsort(), qsort_r()                                                            │ Thread safety │ MT-Safe │
       └──────────────────────────────────────────────────────────────────────────────┴───────────────┴─────────┘

STANDARDS

       qsort()
              C11, POSIX.1-2008.

HISTORY

       qsort()
              POSIX.1-2001, C89, SVr4, 4.3BSD.

       qsort_r()
              glibc 2.8.

NOTES

       To compare C strings, the comparison function can call strcmp(3), as shown in the example below.

EXAMPLES

       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(*(const char **) p1, *(const char **) p2);
       }

       int
       main(int argc, char *argv[])
       {
           if (argc < 2) {
               fprintf(stderr, "Usage: %s <string>...\n", argv[0]);
               exit(EXIT_FAILURE);
           }

           qsort(&argv[1], argc - 1, sizeof(char *), cmpstringp);

           for (size_t j = 1; j < argc; j++)
               puts(argv[j]);
           exit(EXIT_SUCCESS);
       }

SEE ALSO

       sort(1), alphasort(3), strcmp(3), versionsort(3)