1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
|
'\" t
.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" References consulted:
.\" Linux libc source code
.\" Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
.\" 386BSD man pages
.\"
.\" Modified 1993-03-29, David Metcalfe
.\" Modified 1993-04-28, Lars Wirzenius
.\" Modified 1993-07-24, Rik Faith (faith@cs.unc.edu)
.\" Modified 1995-05-18, Rik Faith (faith@cs.unc.edu) to add
.\" better discussion of problems with rand on other systems.
.\" (Thanks to Esa Hyyti{ (ehyytia@snakemail.hut.fi).)
.\" Modified 1998-04-10, Nicolás Lichtmaier <nick@debian.org>
.\" with contribution from Francesco Potorti <F.Potorti@cnuce.cnr.it>
.\" Modified 2003-11-15, aeb, added rand_r
.\" 2010-09-13, mtk, added example program
.\"
.TH rand 3 2024-05-02 "Linux man-pages (unreleased)"
.SH NAME
rand, rand_r, srand \- pseudo-random number generator
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <stdlib.h>
.P
.B int rand(void);
.BI "void srand(unsigned int " seed );
.P
.BI "[[deprecated]] int rand_r(unsigned int *" seedp );
.fi
.P
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.P
.BR rand_r ():
.nf
Since glibc 2.24:
_POSIX_C_SOURCE >= 199506L
glibc 2.23 and earlier
_POSIX_C_SOURCE
.fi
.SH DESCRIPTION
The
.BR rand ()
function returns a pseudo-random integer in the range 0 to
.B RAND_MAX
inclusive (i.e., the mathematical range [0,\ \fBRAND_MAX\fR]).
.P
The
.BR srand ()
function sets its argument as the seed for a new
sequence of pseudo-random integers to be returned by
.BR rand ().
These sequences are repeatable by calling
.BR srand ()
with the same seed value.
.P
If no seed value is provided, the
.BR rand ()
function is automatically seeded with a value of 1.
.P
The function
.BR rand ()
is not reentrant, since it
uses hidden state that is modified on each call.
This might just be the seed value to be used by the next call,
or it might be something more elaborate.
In order to get reproducible behavior in a threaded
application, this state must be made explicit;
this can be done using the reentrant function
.BR rand_r ().
.P
Like
.BR rand (),
.BR rand_r ()
returns a pseudo-random integer in the range [0,\ \fBRAND_MAX\fR].
The
.I seedp
argument is a pointer to an
.I unsigned int
that is used to store state between calls.
If
.BR rand_r ()
is called with the same initial value for the integer pointed to by
.IR seedp ,
and that value is not modified between calls,
then the same pseudo-random sequence will result.
.P
The value pointed to by the
.I seedp
argument of
.BR rand_r ()
provides only a very small amount of state,
so this function will be a weak pseudo-random generator.
Try
.BR drand48_r (3)
instead.
.SH RETURN VALUE
The
.BR rand ()
and
.BR rand_r ()
functions return a value between 0 and
.B RAND_MAX
(inclusive).
The
.BR srand ()
function returns no value.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbx lb lb
l l l.
Interface Attribute Value
T{
.na
.nh
.BR rand (),
.BR rand_r (),
.BR srand ()
T} Thread safety MT-Safe
.TE
.SH VERSIONS
The versions of
.BR rand ()
and
.BR srand ()
in the Linux C Library use the same random number generator as
.BR random (3)
and
.BR srandom (3),
so the lower-order bits should be as random as the higher-order bits.
However, on older
.BR rand ()
implementations, and on current implementations on different systems,
the lower-order bits are much less random than the higher-order bits.
Do not use this function in applications intended to be portable
when good randomness is needed.
(Use
.BR random (3)
instead.)
.SH STANDARDS
.TP
.BR rand ()
.TQ
.BR srand ()
C11, POSIX.1-2008.
.TP
.BR rand_r ()
POSIX.1-2008.
.SH HISTORY
.TP
.BR rand ()
.TQ
.BR srand ()
SVr4, 4.3BSD, C89, POSIX.1-2001.
.TP
.BR rand_r ()
POSIX.1-2001.
Obsolete in POSIX.1-2008.
.SH EXAMPLES
POSIX.1-2001 gives the following example of an implementation of
.BR rand ()
and
.BR srand (),
possibly useful when one needs the same sequence on two different machines.
.P
.in +4n
.EX
static unsigned long next = 1;
\&
/* RAND_MAX assumed to be 32767 */
int myrand(void) {
next = next * 1103515245 + 12345;
return((unsigned)(next/65536) % 32768);
}
\&
void mysrand(unsigned int seed) {
next = seed;
}
.EE
.in
.P
The following program can be used to display the
pseudo-random sequence produced by
.BR rand ()
when given a particular seed.
When the seed is
.IR \-1 ,
the program uses a random seed.
.P
.in +4n
.\" SRC BEGIN (rand.c)
.EX
#include <stdio.h>
#include <stdlib.h>
\&
int
main(int argc, char *argv[])
{
int r;
unsigned int seed, nloops;
\&
if (argc != 3) {
fprintf(stderr, "Usage: %s <seed> <nloops>\en", argv[0]);
exit(EXIT_FAILURE);
}
\&
seed = atoi(argv[1]);
nloops = atoi(argv[2]);
\&
if (seed == \-1) {
seed = arc4random();
printf("seed: %u\en", seed);
}
\&
srand(seed);
for (unsigned int j = 0; j < nloops; j++) {
r = rand();
printf("%d\en", r);
}
\&
exit(EXIT_SUCCESS);
}
.EE
.\" SRC END
.in
.SH SEE ALSO
.BR drand48 (3),
.BR random (3)
|
