man3/qsort.3

   1 .\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
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  23 .\" References consulted:
  24 .\"     Linux libc source code
  25 .\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
  26 .\"     386BSD man pages
  27 .\"
  28 .\" Modified 1993-03-29, David Metcalfe
  29 .\" Modified 1993-07-24, Rik Faith (faith@cs.unc.edu)
  30 .\" 2006-01-15, mtk, Added example program.
  31 .\"
  32 .TH QSORT 3 2003-11-15 "" "Linux Programmer's Manual"
  33 .SH NAME
  34 qsort \- sorts an array
  35 .SH SYNOPSIS
  36 .nf
  37 .B #include <stdlib.h>
  38 .sp
  39 .BI "void qsort(void *" base ", size_t " nmemb ", size_t " size ,
  40 .in +\w'void qsort('u
  41 .BI "int(*" compar ")(const void *, const void *));"
  42 .in
  43 .fi
  44 .SH DESCRIPTION
  45 The \fBqsort\fP() function sorts an array with \fInmemb\fP elements of
  46 size \fIsize\fP.
  47 The \fIbase\fP argument points to the start of the
  48 array.
  49 .PP
  50 The contents of the array are sorted in ascending order according to a
  51 comparison function pointed to by \fIcompar\fP, which is called with two
  52 arguments that point to the objects being compared.
  53 .PP
  54 The comparison function must return an integer less than, equal to, or
  55 greater than zero if the first argument is considered to be respectively
  56 less than, equal to, or greater than the second.
  57 If two members compare
  58 as equal, their order in the sorted array is undefined.
  59 .SH "RETURN VALUE"
  60 The \fBqsort\fP() function returns no value.
  61 .SH "CONFORMING TO"
  62 SVr4, 4.3BSD, C89, C99.
  63 .SH NOTE
  64 Library routines suitable for use as the
  65 .I compar
  66 argument include
  67 .BR alphasort ()
  68 and
  69 .BR versionsort ().
  70 To compare C strings, the comparison function can call
  71 .BR strcmp (),
  72 as shown in the example below.
  73 .SH EXAMPLE
  74 For one example of use, see the example under
  75 .BR bsearch (3).
  76
  77 Another example is the following example program,
  78 which sorts the strings given in its command-line arguments:
  79 .sp
  80 .nf
  81 #include <stdio.h>
  82 #include <stdlib.h>
  83 #include <unistd.h>
  84 #include <string.h>
  85 #include <assert.h>
  86
  87 static int
  88 cmpstringp(const void *p1, const void *p2)
  89 {
  90     /* The actual arguments to this function are "pointers to
  91        pointers to char", but strcmp() arguments are "pointers
  92        to char", hence the following cast plus dereference */
  93
  94     return strcmp(* (char * const *) p1, * (char * const *) p2);
  95 }
  96
  97 int
  98 main(int argc, char *argv[])
  99 {
 100     int j;
 101
 102     assert(argc > 1);
 103
 104     qsort(&argv[1], argc - 1, sizeof(char *), cmpstringp);
 105
 106     for (j = 1; j < argc; j++)
 107         puts(argv[j]);
 108     exit(EXIT_SUCCESS);
 109 }
 110 .fi
 111 .SH "SEE ALSO"
 112 .BR sort (1),
 113 .BR alphasort (3),
 114 .BR strcmp (3),
 115 .BR versionsort (3)