1RAND(3) Linux Programmer's Manual RAND(3)
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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);
14 void srand(unsigned int seed);
16 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
18 rand_r(): _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE
20
22 The rand() function returns a pseudo-random integer in the range 0 to
23 RAND_MAX inclusive (i.e., the mathematical range [0, RAND_MAX]).
24 The srand() function sets its argument as the seed for a new sequence
26 of pseudo-random integers to be returned by rand(). These sequences
27 are repeatable by calling srand() with the same seed value.
28 If no seed value is provided, the rand() function is automatically
30 seeded with a value of 1.
31 The function rand() is not reentrant or thread-safe, since it uses hid‐
33 den state that is modified on each call. This might just be the seed
34 value to be used by the next call, or it might be something more elabo‐
35 rate. In order to get reproducible behavior in a threaded application,
36 this state must be made explicit; this can be done using the reentrant
37 function rand_r().
38 Like rand(), rand_r() returns a pseudo-random integer in the range
40 [0, RAND_MAX]. The seedp argument is a pointer to an unsigned int that
41 is used to store state between calls. If rand_r() is called with the
42 same initial value for the integer pointed to by seedp, and that value
43 is not modified between calls, then the same pseudo-random sequence
44 will result.
45 The value pointed to by the seedp argument of rand_r() provides only a
47 very small amount of state, so this function will be a weak pseudo-ran‐
48 dom generator. Try drand48_r(3) instead.
49
51 The rand() and rand_r() functions return a value between 0 and RAND_MAX
52 (inclusive). The srand() function returns no value.
53
55 The functions rand() and srand() conform to SVr4, 4.3BSD, C89, C99,
56 POSIX.1-2001. The function rand_r() is from POSIX.1-2001.
57 POSIX.1-2008 marks rand_r() as obsolete.
58
60 The versions of rand() and srand() in the Linux C Library use the same
61 random number generator as random(3) and srandom(3), so the lower-order
62 bits should be as random as the higher-order bits. However, on older
63 rand() implementations, and on current implementations on different
64 systems, the lower-order bits are much less random than the higher-
65 order bits. Do not use this function in applications intended to be
66 portable when good randomness is needed. (Use random(3) instead.)
67
69 POSIX.1-2001 gives the following example of an implementation of rand()
70 and srand(), possibly useful when one needs the same sequence on two
71 different machines.
72 static unsigned long next = 1;
74 /* RAND_MAX assumed to be 32767 */
76 int myrand(void) {
77 next = next * 1103515245 + 12345;
78 return((unsigned)(next/65536) % 32768);
79 }
80 void mysrand(unsigned seed) {
82 next = seed;
83 }
84 The following program can be used to display the pseudo-random sequence
86 produced by rand() when given a particular seed.
87 #include <stdlib.h>
89 #include <stdio.h>
90 int
92 main(int argc, char *argv[])
93 {
94 int j, r, nloops;
95 unsigned int seed;
96 if (argc != 3) {
98 fprintf(stderr, "Usage: %s <seed> <nloops>\n", argv[0]);
99 exit(EXIT_FAILURE);
100 }
101 seed = atoi(argv[1]);
103 nloops = atoi(argv[2]);
104 srand(seed);
106 for (j = 0; j < nloops; j++) {
107 r = rand();
108 printf("%d\n", r);
109 }
110 exit(EXIT_SUCCESS);
112 }
113
115 drand48(3), random(3)
116
118 This page is part of release 3.53 of the Linux man-pages project. A
119 description of the project, and information about reporting bugs, can
120 be found at http://www.kernel.org/doc/man-pages/.
121 2010-10-01 RAND(3)