man3/qsort.3

   1 .\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
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  25 .\" References consulted:
  26 .\"     Linux libc source code
  27 .\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
  28 .\"     386BSD man pages
  29 .\"
  30 .\" Modified 1993-03-29, David Metcalfe
  31 .\" Modified 1993-07-24, Rik Faith (faith@cs.unc.edu)
  32 .\" 2006-01-15, mtk, Added example program.
  33 .\" Modified 2012-03-08, Mark R. Bannister <cambridge@users.sourceforge.net>
  34 .\"                  and Ben Bacarisse <software@bsb.me.uk>
  35 .\"     Document qsort_r()
  36 .\"
  37 .TH QSORT 3 2019-03-06 "" "Linux Programmer's Manual"
  38 .SH NAME
  39 qsort, qsort_r \- sort an array
  40 .SH SYNOPSIS
  41 .nf
  42 .B #include <stdlib.h>
  43 .PP
  44 .BI "void qsort(void *" base ", size_t " nmemb ", size_t " size ,
  45 .BI "           int (*" compar ")(const void *, const void *));"
  46 .PP
  47 .BI "void qsort_r(void *" base ", size_t " nmemb ", size_t " size ,
  48 .BI "           int (*" compar ")(const void *, const void *, void *),"
  49 .BI "           void *" arg ");"
  50 .fi
  51 .PP
  52 .in -4n
  53 Feature Test Macro Requirements for glibc (see
  54 .BR feature_test_macros (7)):
  55 .in
  56 .PP
  57 .ad l
  58 .BR qsort_r ():
  59 _GNU_SOURCE
  60 .ad b
  61 .SH DESCRIPTION
  62 The
  63 .BR qsort ()
  64 function sorts an array with \fInmemb\fP elements of
  65 size \fIsize\fP.
  66 The \fIbase\fP argument points to the start of the
  67 array.
  68 .PP
  69 The contents of the array are sorted in ascending order according to a
  70 comparison function pointed to by \fIcompar\fP, which is called with two
  71 arguments that point to the objects being compared.
  72 .PP
  73 The comparison function must return an integer less than, equal to, or
  74 greater than zero if the first argument is considered to be respectively
  75 less than, equal to, or greater than the second.
  76 If two members compare as equal,
  77 their order in the sorted array is undefined.
  78 .PP
  79 The
  80 .BR qsort_r ()
  81 function is identical to
  82 .BR qsort ()
  83 except that the comparison function
  84 .I compar
  85 takes a third argument.
  86 A pointer is passed to the comparison function via
  87 .IR arg .
  88 In this way, the comparison function does not need to use global variables to
  89 pass through arbitrary arguments, and is therefore reentrant and safe to
  90 use in threads.
  91 .SH RETURN VALUE
  92 The
  93 .BR qsort ()
  94 and
  95 .BR qsort_r ()
  96 functions return no value.
  97 .SH VERSIONS
  98 .BR qsort_r ()
  99 was added to glibc in version 2.8.
 100 .SH ATTRIBUTES
 101 For an explanation of the terms used in this section, see
 102 .BR attributes (7).
 103 .TS
 104 allbox;
 105 lbw18 lb lb
 106 l l l.
 107 Interface       Attribute       Value
 108 T{
 109 .BR qsort (),
 110 .BR qsort_r ()
 111 T}      Thread safety   MT-Safe
 112 .TE
 113 .sp 1
 114 .SH CONFORMING TO
 115 .BR qsort ():
 116 POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.
 117 .SH NOTES
 118 To compare C strings, the comparison function can call
 119 .BR strcmp (3),
 120 as shown in the example below.
 121 .SH EXAMPLE
 122 For one example of use, see the example under
 123 .BR bsearch (3).
 124 .PP
 125 Another example is the following program,
 126 which sorts the strings given in its command-line arguments:
 127 .PP
 128 .EX
 129 #include <stdio.h>
 130 #include <stdlib.h>
 131 #include <string.h>
 132
 133 static int
 134 cmpstringp(const void *p1, const void *p2)
 135 {
 136     /* The actual arguments to this function are "pointers to
 137        pointers to char", but strcmp(3) arguments are "pointers
 138        to char", hence the following cast plus dereference */
 139
 140     return strcmp(* (char * const *) p1, * (char * const *) p2);
 141 }
 142
 143 int
 144 main(int argc, char *argv[])
 145 {
 146     int j;
 147
 148     if (argc < 2) {
 149         fprintf(stderr, "Usage: %s <string>...\en", argv[0]);
 150         exit(EXIT_FAILURE);
 151     }
 152
 153     qsort(&argv[1], argc \- 1, sizeof(char *), cmpstringp);
 154
 155     for (j = 1; j < argc; j++)
 156         puts(argv[j]);
 157     exit(EXIT_SUCCESS);
 158 }
 159 .EE
 160 .SH SEE ALSO
 161 .BR sort (1),
 162 .BR alphasort (3),
 163 .BR strcmp (3),
 164 .BR versionsort (3)