1 .\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
3 .\" %%%LICENSE_START(VERBATIM)
4 .\" Permission is granted to make and distribute verbatim copies of this
5 .\" manual provided the copyright notice and this permission notice are
6 .\" preserved on all copies.
8 .\" Permission is granted to copy and distribute modified versions of this
9 .\" manual under the conditions for verbatim copying, provided that the
10 .\" entire resulting derived work is distributed under the terms of a
11 .\" permission notice identical to this one.
13 .\" Since the Linux kernel and libraries are constantly changing, this
14 .\" manual page may be incorrect or out-of-date. The author(s) assume no
15 .\" responsibility for errors or omissions, or for damages resulting from
16 .\" the use of the information contained herein. The author(s) may not
17 .\" have taken the same level of care in the production of this manual,
18 .\" which is licensed free of charge, as they might when working
21 .\" Formatted or processed versions of this manual, if unaccompanied by
22 .\" the source, must acknowledge the copyright and authors of this work.
25 .\" References consulted:
26 .\" Linux libc source code
27 .\" Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
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
41 .TH RAND 3 2017-07-13 "" "Linux Programmer's Manual"
43 rand, rand_r, srand \- pseudo-random number generator
46 .B #include <stdlib.h>
50 .BI "int rand_r(unsigned int *" seedp );
52 .BI "void srand(unsigned int " seed );
56 Feature Test Macro Requirements for glibc (see
57 .BR feature_test_macros (7)):
63 _POSIX_C_SOURCE >= 199506L
65 Glibc 2.23 and earlier
71 function returns a pseudo-random integer in the range 0 to
73 inclusive (i.e., the mathematical range [0,\ \fBRAND_MAX\fR]).
77 function sets its argument as the seed for a new
78 sequence of pseudo-random integers to be returned by
80 These sequences are repeatable by calling
82 with the same seed value.
84 If no seed value is provided, the
86 function is automatically seeded with a value of 1.
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
102 returns a pseudo-random integer in the range [0,\ \fBRAND_MAX\fR].
105 argument is a pointer to an
107 that is used to store state between calls.
110 is called with the same initial value for the integer pointed to by
112 and that value is not modified between calls,
113 then the same pseudo-random sequence will result.
115 The value pointed to by the
119 provides only a very small amount of state,
120 so this function will be a weak pseudo-random generator.
129 functions return a value between 0 and
134 function returns no value.
136 For an explanation of the terms used in this section, see
142 Interface Attribute Value
147 T} Thread safety MT-Safe
154 conform to SVr4, 4.3BSD, C89, C99, POSIX.1-2001.
157 is from POSIX.1-2001.
166 in the Linux C Library use the same random number generator as
170 so the lower-order bits should be as random as the higher-order bits.
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.
181 POSIX.1-2001 gives the following example of an implementation of
185 possibly useful when one needs the same sequence on two different machines.
189 static unsigned long next = 1;
191 /* RAND_MAX assumed to be 32767 */
193 next = next * 1103515245 + 12345;
194 return((unsigned)(next/65536) % 32768);
197 void mysrand(unsigned int seed) {
203 The following program can be used to display the
204 pseudo-random sequence produced by
206 when given a particular seed.
214 main(int argc, char *argv[])
220 fprintf(stderr, "Usage: %s <seed> <nloops>\\n", argv[0]);
224 seed = atoi(argv[1]);
225 nloops = atoi(argv[2]);
228 for (j = 0; j < nloops; j++) {