man3/rand.3

   1 '\" t
   2 .\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
   3 .\"
   4 .\" SPDX-License-Identifier: Linux-man-pages-copyleft
   5 .\"
   6 .\" References consulted:
   7 .\"     Linux libc source code
   8 .\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
   9 .\"     386BSD man pages
  10 .\"
  11 .\" Modified 1993-03-29, David Metcalfe
  12 .\" Modified 1993-04-28, Lars Wirzenius
  13 .\" Modified 1993-07-24, Rik Faith (faith@cs.unc.edu)
  14 .\" Modified 1995-05-18, Rik Faith (faith@cs.unc.edu) to add
  15 .\"          better discussion of problems with rand on other systems.
  16 .\"          (Thanks to Esa Hyyti{ (ehyytia@snakemail.hut.fi).)
  17 .\" Modified 1998-04-10, Nicolás Lichtmaier <nick@debian.org>
  18 .\"          with contribution from Francesco Potorti <F.Potorti@cnuce.cnr.it>
  19 .\" Modified 2003-11-15, aeb, added rand_r
  20 .\" 2010-09-13, mtk, added example program
  21 .\"
  22 .TH rand 3 (date) "Linux man-pages (unreleased)"
  23 .SH NAME
  24 rand, rand_r, srand \- pseudo-random number generator
  25 .SH LIBRARY
  26 Standard C library
  27 .RI ( libc ", " \-lc )
  28 .SH SYNOPSIS
  29 .nf
  30 .B #include <stdlib.h>
  31 .PP
  32 .B int rand(void);
  33 .BI "void srand(unsigned int " seed );
  34 .PP
  35 .BI "[[deprecated]] int rand_r(unsigned int *" seedp );
  36 .fi
  37 .PP
  38 .RS -4
  39 Feature Test Macro Requirements for glibc (see
  40 .BR feature_test_macros (7)):
  41 .RE
  42 .PP
  43 .BR rand_r ():
  44 .nf
  45     Since glibc 2.24:
  46         _POSIX_C_SOURCE >= 199506L
  47     glibc 2.23 and earlier
  48         _POSIX_C_SOURCE
  49 .fi
  50 .SH DESCRIPTION
  51 The
  52 .BR rand ()
  53 function returns a pseudo-random integer in the range 0 to
  54 .B RAND_MAX
  55 inclusive (i.e., the mathematical range [0,\ \fBRAND_MAX\fR]).
  56 .PP
  57 The
  58 .BR srand ()
  59 function sets its argument as the seed for a new
  60 sequence of pseudo-random integers to be returned by
  61 .BR rand ().
  62 These sequences are repeatable by calling
  63 .BR srand ()
  64 with the same seed value.
  65 .PP
  66 If no seed value is provided, the
  67 .BR rand ()
  68 function is automatically seeded with a value of 1.
  69 .PP
  70 The function
  71 .BR rand ()
  72 is not reentrant, since it
  73 uses hidden state that is modified on each call.
  74 This might just be the seed value to be used by the next call,
  75 or it might be something more elaborate.
  76 In order to get reproducible behavior in a threaded
  77 application, this state must be made explicit;
  78 this can be done using the reentrant function
  79 .BR rand_r ().
  80 .PP
  81 Like
  82 .BR rand (),
  83 .BR rand_r ()
  84 returns a pseudo-random integer in the range [0,\ \fBRAND_MAX\fR].
  85 The
  86 .I seedp
  87 argument is a pointer to an
  88 .I unsigned int
  89 that is used to store state between calls.
  90 If
  91 .BR rand_r ()
  92 is called with the same initial value for the integer pointed to by
  93 .IR seedp ,
  94 and that value is not modified between calls,
  95 then the same pseudo-random sequence will result.
  96 .PP
  97 The value pointed to by the
  98 .I seedp
  99 argument of
 100 .BR rand_r ()
 101 provides only a very small amount of state,
 102 so this function will be a weak pseudo-random generator.
 103 Try
 104 .BR drand48_r (3)
 105 instead.
 106 .SH RETURN VALUE
 107 The
 108 .BR rand ()
 109 and
 110 .BR rand_r ()
 111 functions return a value between 0 and
 112 .B RAND_MAX
 113 (inclusive).
 114 The
 115 .BR srand ()
 116 function returns no value.
 117 .SH ATTRIBUTES
 118 For an explanation of the terms used in this section, see
 119 .BR attributes (7).
 120 .TS
 121 allbox;
 122 lbx lb lb
 123 l l l.
 124 Interface       Attribute       Value
 125 T{
 126 .na
 127 .nh
 128 .BR rand (),
 129 .BR rand_r (),
 130 .BR srand ()
 131 T}      Thread safety   MT-Safe
 132 .TE
 133 .sp 1
 134 .SH VERSIONS
 135 The versions of
 136 .BR rand ()
 137 and
 138 .BR srand ()
 139 in the Linux C Library use the same random number generator as
 140 .BR random (3)
 141 and
 142 .BR srandom (3),
 143 so the lower-order bits should be as random as the higher-order bits.
 144 However, on older
 145 .BR rand ()
 146 implementations, and on current implementations on different systems,
 147 the lower-order bits are much less random than the higher-order bits.
 148 Do not use this function in applications intended to be portable
 149 when good randomness is needed.
 150 (Use
 151 .BR random (3)
 152 instead.)
 153 .SH STANDARDS
 154 .TP
 155 .BR rand ()
 156 .TQ
 157 .BR srand ()
 158 C11, POSIX.1-2008.
 159 .TP
 160 .BR rand_r ()
 161 POSIX.1-2008.
 162 .SH HISTORY
 163 .TP
 164 .BR rand ()
 165 .TQ
 166 .BR srand ()
 167 SVr4, 4.3BSD, C89, POSIX.1-2001.
 168 .TP
 169 .BR rand_r ()
 170 POSIX.1-2001.
 171 Obsolete in POSIX.1-2008.
 172 .SH EXAMPLES
 173 POSIX.1-2001 gives the following example of an implementation of
 174 .BR rand ()
 175 and
 176 .BR srand (),
 177 possibly useful when one needs the same sequence on two different machines.
 178 .PP
 179 .in +4n
 180 .EX
 181 static unsigned long next = 1;
 182 \&
 183 /* RAND_MAX assumed to be 32767 */
 184 int myrand(void) {
 185     next = next * 1103515245 + 12345;
 186     return((unsigned)(next/65536) % 32768);
 187 }
 188 \&
 189 void mysrand(unsigned int seed) {
 190     next = seed;
 191 }
 192 .EE
 193 .in
 194 .PP
 195 The following program can be used to display the
 196 pseudo-random sequence produced by
 197 .BR rand ()
 198 when given a particular seed.
 199 When the seed is
 200 .IR \-1 ,
 201 the program uses a random seed.
 202 .PP
 203 .in +4n
 204 .\" SRC BEGIN (rand.c)
 205 .EX
 206 #include <stdio.h>
 207 #include <stdlib.h>
 208 \&
 209 int
 210 main(int argc, char *argv[])
 211 {
 212     int           r;
 213     unsigned int  seed, nloops;
 214 \&
 215     if (argc != 3) {
 216         fprintf(stderr, "Usage: %s <seed> <nloops>\en", argv[0]);
 217         exit(EXIT_FAILURE);
 218     }
 219 \&
 220     seed = atoi(argv[1]);
 221     nloops = atoi(argv[2]);
 222 \&
 223     if (seed == \-1) {
 224         seed = arc4random();
 225         printf("seed: %u\en", seed);
 226     }
 227 \&
 228     srand(seed);
 229     for (unsigned int j = 0; j < nloops; j++) {
 230         r =  rand();
 231         printf("%d\en", r);
 232     }
 233 \&
 234     exit(EXIT_SUCCESS);
 235 }
 236 .EE
 237 .\" SRC END
 238 .in
 239 .SH SEE ALSO
 240 .BR drand48 (3),
 241 .BR random (3)