1RAND(3) Linux Programmer's Manual RAND(3)
2
6 rand, rand_r, srand - pseudo-random number generator
7
9 #include <stdlib.h>
10 int rand(void);
12 int rand_r(unsigned int *seedp);
13 void srand(unsigned int seed);
14 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
16 rand_r():
18 Since glibc 2.24:
19 _POSIX_C_SOURCE >= 199506L
20 Glibc 2.23 and earlier
21 _POSIX_C_SOURCE
22
24 The rand() function returns a pseudo-random integer in the range 0 to
25 RAND_MAX inclusive (i.e., the mathematical range [0, RAND_MAX]).
26 The srand() function sets its argument as the seed for a new sequence
28 of pseudo-random integers to be returned by rand(). These sequences
29 are repeatable by calling srand() with the same seed value.
30 If no seed value is provided, the rand() function is automatically
32 seeded with a value of 1.
33 The function rand() is not reentrant, since it uses hidden state that
35 is modified on each call. This might just be the seed value to be used
36 by the next call, or it might be something more elaborate. In order to
37 get reproducible behavior in a threaded application, this state must be
38 made explicit; this can be done using the reentrant function rand_r().
39 Like rand(), rand_r() returns a pseudo-random integer in the range
41 [0, RAND_MAX]. The seedp argument is a pointer to an unsigned int that
42 is used to store state between calls. If rand_r() is called with the
43 same initial value for the integer pointed to by seedp, and that value
44 is not modified between calls, then the same pseudo-random sequence
45 will result.
46 The value pointed to by the seedp argument of rand_r() provides only a
48 very small amount of state, so this function will be a weak pseudo-ran‐
49 dom generator. Try drand48_r(3) instead.
50
52 The rand() and rand_r() functions return a value between 0 and RAND_MAX
53 (inclusive). The srand() function returns no value.
54
56 For an explanation of the terms used in this section, see at‐
57 tributes(7).
58 ┌────────────────────────────────────────────┬───────────────┬─────────┐
60 │Interface │ Attribute │ Value │
61 ├────────────────────────────────────────────┼───────────────┼─────────┤
62 │rand(), rand_r(), srand() │ Thread safety │ MT-Safe │
63 └────────────────────────────────────────────┴───────────────┴─────────┘
64
66 The functions rand() and srand() conform to SVr4, 4.3BSD, C89, C99,
67 POSIX.1-2001. The function rand_r() is from POSIX.1-2001.
68 POSIX.1-2008 marks rand_r() as obsolete.
69
71 The versions of rand() and srand() in the Linux C Library use the same
72 random number generator as random(3) and srandom(3), so the lower-order
73 bits should be as random as the higher-order bits. However, on older
74 rand() implementations, and on current implementations on different
75 systems, the lower-order bits are much less random than the higher-or‐
76 der bits. Do not use this function in applications intended to be por‐
77 table when good randomness is needed. (Use random(3) instead.)
78
80 POSIX.1-2001 gives the following example of an implementation of rand()
81 and srand(), possibly useful when one needs the same sequence on two
82 different machines.
83 static unsigned long next = 1;
85 /* RAND_MAX assumed to be 32767 */
87 int myrand(void) {
88 next = next * 1103515245 + 12345;
89 return((unsigned)(next/65536) % 32768);
90 }
91 void mysrand(unsigned int seed) {
93 next = seed;
94 }
95 The following program can be used to display the pseudo-random sequence
97 produced by rand() when given a particular seed.
98 #include <stdlib.h>
100 #include <stdio.h>
101 int
103 main(int argc, char *argv[])
104 {
105 int r, nloops;
106 unsigned int seed;
107 if (argc != 3) {
109 fprintf(stderr, "Usage: %s <seed> <nloops>\n", argv[0]);
110 exit(EXIT_FAILURE);
111 }
112 seed = atoi(argv[1]);
114 nloops = atoi(argv[2]);
115 srand(seed);
117 for (int j = 0; j < nloops; j++) {
118 r = rand();
119 printf("%d\n", r);
120 }
121 exit(EXIT_SUCCESS);
123 }
124
126 drand48(3), random(3)
127
129 This page is part of release 5.13 of the Linux man-pages project. A
130 description of the project, information about reporting bugs, and the
131 latest version of this page, can be found at
132 https://www.kernel.org/doc/man-pages/.
133 2021-03-22 RAND(3)