1RAND(3P) POSIX Programmer's Manual RAND(3P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
12 rand, rand_r, srand — pseudo-random number generator
13
15 #include <stdlib.h>
16 int rand(void);
18 int rand_r(unsigned *seed);
19 void srand(unsigned seed);
20
22 For rand() and srand(): The functionality described on this reference
23 page is aligned with the ISO C standard. Any conflict between the
24 requirements described here and the ISO C standard is unintentional.
25 This volume of POSIX.1‐2017 defers to the ISO C standard.
26 The rand() function shall compute a sequence of pseudo-random integers
28 in the range [0,{RAND_MAX}] with a period of at least 232.
29 The rand() function need not be thread-safe.
31 The rand_r() function shall compute a sequence of pseudo-random inte‐
33 gers in the range [0,{RAND_MAX}]. (The value of the {RAND_MAX} macro
34 shall be at least 32767.)
35 If rand_r() is called with the same initial value for the object
37 pointed to by seed and that object is not modified between successive
38 returns and calls to rand_r(), the same sequence shall be generated.
39 The srand() function uses the argument as a seed for a new sequence of
41 pseudo-random numbers to be returned by subsequent calls to rand(). If
42 srand() is then called with the same seed value, the sequence of
43 pseudo-random numbers shall be repeated. If rand() is called before any
44 calls to srand() are made, the same sequence shall be generated as when
45 srand() is first called with a seed value of 1.
46 The implementation shall behave as if no function defined in this vol‐
48 ume of POSIX.1‐2017 calls rand() or srand().
49
51 The rand() function shall return the next pseudo-random number in the
52 sequence.
53 The rand_r() function shall return a pseudo-random integer.
55 The srand() function shall not return a value.
57
59 No errors are defined.
60 The following sections are informative.
62
64 Generating a Pseudo-Random Number Sequence
65 The following example demonstrates how to generate a sequence of
66 pseudo-random numbers.
67 #include <stdio.h>
70 #include <stdlib.h>
71 ...
72 long count, i;
73 char *keystr;
74 int elementlen, len;
75 char c;
76 ...
77 /* Initial random number generator. */
78 srand(1);
79 /* Create keys using only lowercase characters */
81 len = 0;
82 for (i=0; i<count; i++) {
83 while (len < elementlen) {
84 c = (char) (rand() % 128);
85 if (islower(c))
86 keystr[len++] = c;
87 }
88 keystr[len] = '\0';
90 printf("%s Element%0*ld\n", keystr, elementlen, i);
91 len = 0;
92 }
93 Generating the Same Sequence on Different Machines
95 The following code defines a pair of functions that could be incorpo‐
96 rated into applications wishing to ensure that the same sequence of
97 numbers is generated across different machines.
98 static unsigned long next = 1;
101 int myrand(void) /* RAND_MAX assumed to be 32767. */
102 {
103 next = next * 1103515245 + 12345;
104 return((unsigned)(next/65536) % 32768);
105 }
106 void mysrand(unsigned seed)
108 {
109 next = seed;
110 }
111
113 The drand48() and random() functions provide much more elaborate
114 pseudo-random number generators.
115 The limitations on the amount of state that can be carried between one
117 function call and another mean the rand_r() function can never be
118 implemented in a way which satisfies all of the requirements on a
119 pseudo-random number generator.
120 These functions should be avoided whenever non-trivial requirements
122 (including safety) have to be fulfilled.
123
125 The ISO C standard rand() and srand() functions allow per-process
126 pseudo-random streams shared by all threads. Those two functions need
127 not change, but there has to be mutual-exclusion that prevents inter‐
128 ference between two threads concurrently accessing the random number
129 generator.
130 With regard to rand(), there are two different behaviors that may be
132 wanted in a multi-threaded program:
133 1. A single per-process sequence of pseudo-random numbers that is
135 shared by all threads that call rand()
136 2. A different sequence of pseudo-random numbers for each thread that
138 calls rand()
139 This is provided by the modified thread-safe function based on whether
141 the seed value is global to the entire process or local to each thread.
142 This does not address the known deficiencies of the rand() function
144 implementations, which have been approached by maintaining more state.
145 In effect, this specifies new thread-safe forms of a deficient func‐
146 tion.
147
149 The rand_r() function may be removed in a future version.
150
152 drand48(), initstate()
153 The Base Definitions volume of POSIX.1‐2017, <stdlib.h>
155
157 Portions of this text are reprinted and reproduced in electronic form
158 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
159 table Operating System Interface (POSIX), The Open Group Base Specifi‐
160 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
161 Electrical and Electronics Engineers, Inc and The Open Group. In the
162 event of any discrepancy between this version and the original IEEE and
163 The Open Group Standard, the original IEEE and The Open Group Standard
164 is the referee document. The original Standard can be obtained online
165 at http://www.opengroup.org/unix/online.html .
166 Any typographical or formatting errors that appear in this page are
168 most likely to have been introduced during the conversion of the source
169 files to man page format. To report such errors, see https://www.ker‐
170 nel.org/doc/man-pages/reporting_bugs.html .
171IEEE/The Open Group 2017 RAND(3P)