man3/rand.3

   1 .\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
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  24 .\"
  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 1993-04-28, Lars Wirzenius
  32 .\" Modified 1993-07-24, Rik Faith (faith@cs.unc.edu)
  33 .\" Modified 1995-05-18, Rik Faith (faith@cs.unc.edu) to add
  34 .\"          better discussion of problems with rand on other systems.
  35 .\"          (Thanks to Esa Hyyti{ (ehyytia@snakemail.hut.fi).)
  36 .\" Modified 1998-04-10, Nicolás Lichtmaier <nick@debian.org>
  37 .\"          with contribution from Francesco Potorti <F.Potorti@cnuce.cnr.it>
  38 .\" Modified 2003-11-15, aeb, added rand_r
  39 .\" 2010-09-13, mtk, added example program
  40 .\"
  41 .TH RAND 3 2017-07-13 "" "Linux Programmer's Manual"
  42 .SH NAME
  43 rand, rand_r, srand \- pseudo-random number generator
  44 .SH SYNOPSIS
  45 .nf
  46 .B #include <stdlib.h>
  47 .PP
  48 .B int rand(void);
  49 .PP
  50 .BI "int rand_r(unsigned int *" seedp );
  51 .PP
  52 .BI "void srand(unsigned int " seed );
  53 .fi
  54 .PP
  55 .in -4n
  56 Feature Test Macro Requirements for glibc (see
  57 .BR feature_test_macros (7)):
  58 .in
  59 .PP
  60 .BR rand_r ():
  61 .RS 4
  62 Since glibc 2.24:
  63     _POSIX_C_SOURCE >= 199506L
  64 .br
  65 Glibc 2.23 and earlier
  66     _POSIX_C_SOURCE
  67 .RE
  68 .SH DESCRIPTION
  69 The
  70 .BR rand ()
  71 function returns a pseudo-random integer in the range 0 to
  72 .BR RAND_MAX
  73 inclusive (i.e., the mathematical range [0,\ \fBRAND_MAX\fR]).
  74 .PP
  75 The
  76 .BR srand ()
  77 function sets its argument as the seed for a new
  78 sequence of pseudo-random integers to be returned by
  79 .BR rand ().
  80 These sequences are repeatable by calling
  81 .BR srand ()
  82 with the same seed value.
  83 .PP
  84 If no seed value is provided, the
  85 .BR rand ()
  86 function is automatically seeded with a value of 1.
  87 .PP
  88 The function
  89 .BR rand ()
  90 is not reentrant, since it
  91 uses hidden state that is modified on each call.
  92 This might just be the seed value to be used by the next call,
  93 or it might be something more elaborate.
  94 In order to get reproducible behavior in a threaded
  95 application, this state must be made explicit;
  96 this can be done using the reentrant function
  97 .BR rand_r ().
  98 .PP
  99 Like
 100 .BR rand (),
 101 .BR rand_r ()
 102 returns a pseudo-random integer in the range [0,\ \fBRAND_MAX\fR].
 103 The
 104 .I seedp
 105 argument is a pointer to an
 106 .IR "unsigned int"
 107 that is used to store state between calls.
 108 If
 109 .BR rand_r ()
 110 is called with the same initial value for the integer pointed to by
 111 .IR seedp ,
 112 and that value is not modified between calls,
 113 then the same pseudo-random sequence will result.
 114 .PP
 115 The value pointed to by the
 116 .I seedp
 117 argument of
 118 .BR rand_r ()
 119 provides only a very small amount of state,
 120 so this function will be a weak pseudo-random generator.
 121 Try
 122 .BR drand48_r (3)
 123 instead.
 124 .SH RETURN VALUE
 125 The
 126 .BR rand ()
 127 and
 128 .BR rand_r ()
 129 functions return a value between 0 and
 130 .BR RAND_MAX
 131 (inclusive).
 132 The
 133 .BR srand ()
 134 function returns no value.
 135 .SH ATTRIBUTES
 136 For an explanation of the terms used in this section, see
 137 .BR attributes (7).
 138 .TS
 139 allbox;
 140 lbw25 lb lb
 141 l l l.
 142 Interface       Attribute       Value
 143 T{
 144 .BR rand (),
 145 .BR rand_r (),
 146 .BR srand ()
 147 T}      Thread safety   MT-Safe
 148 .TE
 149 .SH CONFORMING TO
 150 The functions
 151 .BR rand ()
 152 and
 153 .BR srand ()
 154 conform to SVr4, 4.3BSD, C89, C99, POSIX.1-2001.
 155 The function
 156 .BR rand_r ()
 157 is from POSIX.1-2001.
 158 POSIX.1-2008 marks
 159 .BR rand_r ()
 160 as obsolete.
 161 .SH NOTES
 162 The versions of
 163 .BR rand ()
 164 and
 165 .BR srand ()
 166 in the Linux C Library use the same random number generator as
 167 .BR random (3)
 168 and
 169 .BR srandom (3),
 170 so the lower-order bits should be as random as the higher-order bits.
 171 However, on older
 172 .BR rand ()
 173 implementations, and on current implementations on different systems,
 174 the lower-order bits are much less random than the higher-order bits.
 175 Do not use this function in applications intended to be portable
 176 when good randomness is needed.
 177 (Use
 178 .BR random (3)
 179 instead.)
 180 .SH EXAMPLE
 181 POSIX.1-2001 gives the following example of an implementation of
 182 .BR rand ()
 183 and
 184 .BR srand (),
 185 possibly useful when one needs the same sequence on two different machines.
 186 .PP
 187 .in +4n
 188 .EX
 189 static unsigned long next = 1;
 190
 191 /* RAND_MAX assumed to be 32767 */
 192 int myrand(void) {
 193     next = next * 1103515245 + 12345;
 194     return((unsigned)(next/65536) % 32768);
 195 }
 196
 197 void mysrand(unsigned int seed) {
 198     next = seed;
 199 }
 200 .EE
 201 .in
 202 .PP
 203 The following program can be used to display the
 204 pseudo-random sequence produced by
 205 .BR rand ()
 206 when given a particular seed.
 207 .PP
 208 .in +4n
 209 .EX
 210 #include <stdlib.h>
 211 #include <stdio.h>
 212
 213 int
 214 main(int argc, char *argv[])
 215 {
 216     int j, r, nloops;
 217     unsigned int seed;
 218
 219     if (argc != 3) {
 220         fprintf(stderr, "Usage: %s <seed> <nloops>\\n", argv[0]);
 221         exit(EXIT_FAILURE);
 222     }
 223
 224     seed = atoi(argv[1]);
 225     nloops = atoi(argv[2]);
 226
 227     srand(seed);
 228     for (j = 0; j < nloops; j++) {
 229         r =  rand();
 230         printf("%d\\n", r);
 231     }
 232
 233     exit(EXIT_SUCCESS);
 234 }
 235 .EE
 236 .in
 237 .SH SEE ALSO
 238 .BR drand48 (3),
 239 .BR random (3)