man3/rand.3

   1 .\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
   2 .\"
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   7 .\" Permission is granted to copy and distribute modified versions of this
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  12 .\" Since the Linux kernel and libraries are constantly changing, this
  13 .\" manual page may be incorrect or out-of-date.  The author(s) assume no
  14 .\" responsibility for errors or omissions, or for damages resulting from
  15 .\" the use of the information contained herein.  The author(s) may not
  16 .\" have taken the same level of care in the production of this manual,
  17 .\" which is licensed free of charge, as they might when working
  18 .\" professionally.
  19 .\"
  20 .\" Formatted or processed versions of this manual, if unaccompanied by
  21 .\" the source, must acknowledge the copyright and authors of this work.
  22 .\"
  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 1993-04-28, Lars Wirzenius
  30 .\" Modified 1993-07-24, Rik Faith (faith@cs.unc.edu)
  31 .\" Modified 1995-05-18, Rik Faith (faith@cs.unc.edu) to add
  32 .\"          better discussion of problems with rand on other systems.
  33 .\"          (Thanks to Esa Hyyti{ (ehyytia@snakemail.hut.fi).)
  34 .\" Modified 1998-04-10, Nicolás Lichtmaier <nick@debian.org>
  35 .\"          with contribution from Francesco Potorti <F.Potorti@cnuce.cnr.it>
  36 .\" Modified 2003-11-15, aeb, added rand_r
  37 .\" 2010-09-13, mtk, added example program
  38 .\"
  39 .TH RAND 3 2010-10-01 "" "Linux Programmer's Manual"
  40 .SH NAME
  41 rand, rand_r, srand \- pseudo-random number generator
  42 .SH SYNOPSIS
  43 .nf
  44 .B #include <stdlib.h>
  45 .sp
  46 .B int rand(void);
  47 .sp
  48 .BI "int rand_r(unsigned int *" seedp );
  49 .sp
  50 .BI "void srand(unsigned int " seed );
  51 .fi
  52 .sp
  53 .in -4n
  54 Feature Test Macro Requirements for glibc (see
  55 .BR feature_test_macros (7)):
  56 .in
  57 .sp
  58 .BR rand_r ():
  59 _POSIX_C_SOURCE\ >=\ 1 || _XOPEN_SOURCE || _POSIX_SOURCE
  60 .SH DESCRIPTION
  61 The
  62 .BR rand ()
  63 function returns a pseudo-random integer in the range 0 to
  64 .BR RAND_MAX
  65 inclusive (i.e., the mathematical range [0,\ \fBRAND_MAX\fR]).
  66 .PP
  67 The
  68 .BR srand ()
  69 function sets its argument as the seed for a new
  70 sequence of pseudo-random integers to be returned by
  71 .BR rand ().
  72 These sequences are repeatable by calling
  73 .BR srand ()
  74 with the same seed value.
  75 .PP
  76 If no seed value is provided, the
  77 .BR rand ()
  78 function is automatically seeded with a value of 1.
  79 .PP
  80 The function
  81 .BR rand ()
  82 is not reentrant or thread-safe, since it
  83 uses hidden state that is modified on each call.
  84 This might just be the seed value to be used by the next call,
  85 or it might be something more elaborate.
  86 In order to get reproducible behavior in a threaded
  87 application, this state must be made explicit;
  88 this can be done using the reentrant function
  89 .BR rand_r ().
  90
  91 Like
  92 .BR rand (),
  93 .BR rand_r ()
  94 returns a pseudo-random integer in the range [0,\ \fBRAND_MAX\fR].
  95 The
  96 .I seedp
  97 argument is a pointer to an
  98 .IR "unsigned int"
  99 that is used to store state between calls.
 100 If
 101 .BR rand_r ()
 102 is called with the same initial value for the integer pointed to by
 103 .IR seedp ,
 104 and that value is not modified between calls,
 105 then the same pseudo-random sequence will result.
 106
 107 The value pointed to by the
 108 .I seedp
 109 argument of
 110 .BR rand_r ()
 111 provides only a very small amount of state,
 112 so this function will be a weak pseudo-random generator.
 113 Try
 114 .BR drand48_r (3)
 115 instead.
 116 .SH RETURN VALUE
 117 The
 118 .BR rand ()
 119 and
 120 .BR rand_r ()
 121 functions return a value between 0 and
 122 .BR RAND_MAX
 123 (inclusive).
 124 The
 125 .BR srand ()
 126 function returns no value.
 127 .SH CONFORMING TO
 128 The functions
 129 .BR rand ()
 130 and
 131 .BR srand ()
 132 conform to SVr4, 4.3BSD, C89, C99, POSIX.1-2001.
 133 The function
 134 .BR rand_r ()
 135 is from POSIX.1-2001.
 136 POSIX.1-2008 marks
 137 .BR rand_r ()
 138 as obsolete.
 139 .SH NOTES
 140 The versions of
 141 .BR rand ()
 142 and
 143 .BR srand ()
 144 in the Linux C Library use the same random number generator as
 145 .BR random (3)
 146 and
 147 .BR srandom (3),
 148 so the lower-order bits should be as random as the higher-order bits.
 149 However, on older
 150 .BR rand ()
 151 implementations, and on current implementations on different systems,
 152 the lower-order bits are much less random than the higher-order bits.
 153 Do not use this function in applications intended to be portable
 154 when good randomness is needed.
 155 (Use
 156 .BR random (3)
 157 instead.)
 158 .SH EXAMPLE
 159 POSIX.1-2001 gives the following example of an implementation of
 160 .BR rand ()
 161 and
 162 .BR srand (),
 163 possibly useful when one needs the same sequence on two different machines.
 164 .sp
 165 .in +4n
 166 .nf
 167 static unsigned long next = 1;
 168
 169 /* RAND_MAX assumed to be 32767 */
 170 int myrand(void) {
 171     next = next * 1103515245 + 12345;
 172     return((unsigned)(next/65536) % 32768);
 173 }
 174
 175 void mysrand(unsigned seed) {
 176     next = seed;
 177 }
 178 .fi
 179 .in
 180 .PP
 181 The following program can be used to display the
 182 pseudo-random sequence produced by
 183 .BR rand ()
 184 when given a particular seed.
 185 .in +4n
 186 .nf
 187
 188 #include <stdlib.h>
 189 #include <stdio.h>
 190
 191 int
 192 main(int argc, char *argv[])
 193 {
 194     int j, r, nloops;
 195     unsigned int seed;
 196
 197     if (argc != 3) {
 198         fprintf(stderr, "Usage: %s <seed> <nloops>\\n", argv[0]);
 199         exit(EXIT_FAILURE);
 200     }
 201
 202     seed = atoi(argv[1]);
 203     nloops = atoi(argv[2]);
 204
 205     srand(seed);
 206     for (j = 0; j < nloops; j++) {
 207         r =  rand();
 208         printf("%d\\n", r);
 209     }
 210
 211     exit(EXIT_SUCCESS);
 212 }
 213 .fi
 214 .in
 215 .SH SEE ALSO
 216 .BR drand48 (3),
 217 .BR random (3)