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
13 rand, rand_r, srand — pseudo-random number generator
14
16 #include <stdlib.h>
17 int rand(void);
19 int rand_r(unsigned *seed);
20 void srand(unsigned seed);
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23 For rand() and srand(): The functionality described on this reference
24 page is aligned with the ISO C standard. Any conflict between the
25 requirements described here and the ISO C standard is unintentional.
26 This volume of POSIX.1‐2008 defers to the ISO C standard.
27 The rand() function shall compute a sequence of pseudo-random integers
29 in the range [0,{RAND_MAX}] with a period of at least 232.
30 The rand() function need not be thread-safe.
32 The rand_r() function shall compute a sequence of pseudo-random inte‐
34 gers in the range [0,{RAND_MAX}]. (The value of the {RAND_MAX} macro
35 shall be at least 32767.)
36 If rand_r() is called with the same initial value for the object
38 pointed to by seed and that object is not modified between successive
39 returns and calls to rand_r(), the same sequence shall be generated.
40 The srand() function uses the argument as a seed for a new sequence of
42 pseudo-random numbers to be returned by subsequent calls to rand(). If
43 srand() is then called with the same seed value, the sequence of
44 pseudo-random numbers shall be repeated. If rand() is called before any
45 calls to srand() are made, the same sequence shall be generated as when
46 srand() is first called with a seed value of 1.
47 The implementation shall behave as if no function defined in this vol‐
49 ume of POSIX.1‐2008 calls rand() or srand().
50
52 The rand() function shall return the next pseudo-random number in the
53 sequence.
54 The rand_r() function shall return a pseudo-random integer.
56 The srand() function shall not return a value.
58
60 No errors are defined.
61 The following sections are informative.
63
65 Generating a Pseudo-Random Number Sequence
66 The following example demonstrates how to generate a sequence of
67 pseudo-random numbers.
68 #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;
100 int myrand(void) /* RAND_MAX assumed to be 32767. */
101 {
102 next = next * 1103515245 + 12345;
103 return((unsigned)(next/65536) % 32768);
104 }
105 void mysrand(unsigned seed)
107 {
108 next = seed;
109 }
110
112 The drand48() function provides a much more elaborate random number
113 generator.
114 The limitations on the amount of state that can be carried between one
116 function call and another mean the rand_r() function can never be
117 implemented in a way which satisfies all of the requirements on a
118 pseudo-random number generator. Therefore this function should be
119 avoided whenever non-trivial requirements (including safety) have to be
120 fulfilled.
121
123 The ISO C standard rand() and srand() functions allow per-process
124 pseudo-random streams shared by all threads. Those two functions need
125 not change, but there has to be mutual-exclusion that prevents inter‐
126 ference between two threads concurrently accessing the random number
127 generator.
128 With regard to rand(), there are two different behaviors that may be
130 wanted in a multi-threaded program:
131 1. A single per-process sequence of pseudo-random numbers that is
133 shared by all threads that call rand()
134 2. A different sequence of pseudo-random numbers for each thread that
136 calls rand()
137 This is provided by the modified thread-safe function based on whether
139 the seed value is global to the entire process or local to each thread.
140 This does not address the known deficiencies of the rand() function
142 implementations, which have been approached by maintaining more state.
143 In effect, this specifies new thread-safe forms of a deficient func‐
144 tion.
145
147 The rand_r() function may be removed in a future version.
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150 drand48()
151 The Base Definitions volume of POSIX.1‐2008, <stdlib.h>
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155 Portions of this text are reprinted and reproduced in electronic form
156 from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
157 -- Portable Operating System Interface (POSIX), The Open Group Base
158 Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
159 cal and Electronics Engineers, Inc and The Open Group. (This is
160 POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
161 event of any discrepancy between this version and the original IEEE and
162 The Open Group Standard, the original IEEE and The Open Group Standard
163 is the referee document. The original Standard can be obtained online
164 at http://www.unix.org/online.html .
165 Any typographical or formatting errors that appear in this page are
167 most likely to have been introduced during the conversion of the source
168 files to man page format. To report such errors, see https://www.ker‐
169 nel.org/doc/man-pages/reporting_bugs.html .
170IEEE/The Open Group 2013 RAND(3P)