1RAND(P) POSIX Programmer's Manual RAND(P)
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6 rand, rand_r, srand - pseudo-random number generator
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9 #include <stdlib.h>
10 int rand(void);
12 int rand_r(unsigned *seed);
15 void srand(unsigned seed);
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19 The rand() function shall compute a sequence of pseudo-random integers
20 in the range [0, {RAND_MAX}] with a period of at least 2**32.
21 The rand() function need not be reentrant. A function that is not
23 required to be reentrant is not required to be thread-safe.
24 The rand_r() function shall compute a sequence of pseudo-random inte‐
26 gers in the range [0, {RAND_MAX}]. (The value of the {RAND_MAX} macro
27 shall be at least 32767.)
28 If rand_r() is called with the same initial value for the object
30 pointed to by seed and that object is not modified between successive
31 returns and calls to rand_r(), the same sequence shall be generated.
32 The srand() function uses the argument as a seed for a new sequence of
34 pseudo-random numbers to be returned by subsequent calls to rand(). If
35 srand() is then called with the same seed value, the sequence of
36 pseudo-random numbers shall be repeated. If rand() is called before any
37 calls to srand() are made, the same sequence shall be generated as when
38 srand() is first called with a seed value of 1.
39 The implementation shall behave as if no function defined in this vol‐
41 ume of IEEE Std 1003.1-2001 calls rand() or srand().
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44 The rand() function shall return the next pseudo-random number in the
45 sequence.
46 The rand_r() function shall return a pseudo-random integer.
48 The srand() function shall not return a value.
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52 No errors are defined.
53 The following sections are informative.
55
57 Generating a Pseudo-Random Number Sequence
58 The following example demonstrates how to generate a sequence of
59 pseudo-random numbers.
60 #include <stdio.h>
63 #include <stdlib.h>
64 ...
65 long count, i;
66 char *keystr;
67 int elementlen, len;
68 char c;
69 ...
70 /* Initial random number generator. */
71 srand(1);
72 /* Create keys using only lowercase characters */
75 len = 0;
76 for (i=0; i<count; i++) {
77 while (len < elementlen) {
78 c = (char) (rand() % 128);
79 if (islower(c))
80 keystr[len++] = c;
81 }
82 keystr[len] = '\0';
85 printf("%s Element%0*ld\n", keystr, elementlen, i);
86 len = 0;
87 }
88 Generating the Same Sequence on Different Machines
90 The following code defines a pair of functions that could be incorpo‐
91 rated into applications wishing to ensure that the same sequence of
92 numbers is generated across different machines.
93 static unsigned long next = 1;
96 int myrand(void) /* RAND_MAX assumed to be 32767. */
97 {
98 next = next * 1103515245 + 12345;
99 return((unsigned)(next/65536) % 32768);
100 }
101 void mysrand(unsigned seed)
104 {
105 next = seed;
106 }
107
109 The drand48() function provides a much more elaborate random number
110 generator.
111 The limitations on the amount of state that can be carried between one
113 function call and another mean the rand_r() function can never be
114 implemented in a way which satisfies all of the requirements on a
115 pseudo-random number generator. Therefore this function should be
116 avoided whenever non-trivial requirements (including safety) have to be
117 fulfilled.
118
120 The ISO C standard rand() and srand() functions allow per-process
121 pseudo-random streams shared by all threads. Those two functions need
122 not change, but there has to be mutual-exclusion that prevents inter‐
123 ference between two threads concurrently accessing the random number
124 generator.
125 With regard to rand(), there are two different behaviors that may be
127 wanted in a multi-threaded program:
128 1. A single per-process sequence of pseudo-random numbers that is
130 shared by all threads that call rand()
131 2. A different sequence of pseudo-random numbers for each thread that
133 calls rand()
134 This is provided by the modified thread-safe function based on whether
136 the seed value is global to the entire process or local to each thread.
137 This does not address the known deficiencies of the rand() function
139 implementations, which have been approached by maintaining more state.
140 In effect, this specifies new thread-safe forms of a deficient func‐
141 tion.
142
144 None.
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147 drand48() , the Base Definitions volume of IEEE Std 1003.1-2001,
148 <stdlib.h>
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151 Portions of this text are reprinted and reproduced in electronic form
152 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
153 -- Portable Operating System Interface (POSIX), The Open Group Base
154 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
155 Electrical and Electronics Engineers, Inc and The Open Group. In the
156 event of any discrepancy between this version and the original IEEE and
157 The Open Group Standard, the original IEEE and The Open Group Standard
158 is the referee document. The original Standard can be obtained online
159 at http://www.opengroup.org/unix/online.html .
160IEEE/The Open Group 2003 RAND(P)