srand(3) - f34

1RAND(3)                    Linux Programmer's Manual                   RAND(3)
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NAME

6       rand, rand_r, srand - pseudo-random number generator
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SYNOPSIS

9       #include <stdlib.h>
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11       int rand(void);
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13       int rand_r(unsigned int *seedp);
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15       void srand(unsigned int seed);
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17   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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19       rand_r():
20           Since glibc 2.24:
21               _POSIX_C_SOURCE >= 199506L
22           Glibc 2.23 and earlier
23               _POSIX_C_SOURCE
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DESCRIPTION

26       The  rand()  function returns a pseudo-random integer in the range 0 to
27       RAND_MAX inclusive (i.e., the mathematical range [0, RAND_MAX]).
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29       The srand() function sets its argument as the seed for a  new  sequence
30       of  pseudo-random  integers  to be returned by rand().  These sequences
31       are repeatable by calling srand() with the same seed value.
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33       If no seed value is provided,  the  rand()  function  is  automatically
34       seeded with a value of 1.
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36       The  function  rand() is not reentrant, since it uses hidden state that
37       is modified on each call.  This might just be the seed value to be used
38       by the next call, or it might be something more elaborate.  In order to
39       get reproducible behavior in a threaded application, this state must be
40       made explicit; this can be done using the reentrant function rand_r().
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42       Like  rand(),  rand_r()  returns  a  pseudo-random integer in the range
43       [0, RAND_MAX].  The seedp argument is a pointer to an unsigned int that
44       is  used  to store state between calls.  If rand_r() is called with the
45       same initial value for the integer pointed to by seedp, and that  value
46       is  not  modified  between  calls, then the same pseudo-random sequence
47       will result.
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49       The value pointed to by the seedp argument of rand_r() provides only  a
50       very small amount of state, so this function will be a weak pseudo-ran‐
51       dom generator.  Try drand48_r(3) instead.
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RETURN VALUE

54       The rand() and rand_r() functions return a value between 0 and RAND_MAX
55       (inclusive).  The srand() function returns no value.
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ATTRIBUTES

58       For  an  explanation  of  the  terms  used  in  this  section,  see at‐
59       tributes(7).
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61       ┌──────────────────────────┬───────────────┬─────────┐
62       │Interface                 │ Attribute     │ Value   │
63       ├──────────────────────────┼───────────────┼─────────┤
64       │rand(), rand_r(), srand() │ Thread safety │ MT-Safe │
65       └──────────────────────────┴───────────────┴─────────┘

CONFORMING TO

67       The functions rand() and srand() conform to  SVr4,  4.3BSD,  C89,  C99,
68       POSIX.1-2001.     The   function   rand_r()   is   from   POSIX.1-2001.
69       POSIX.1-2008 marks rand_r() as obsolete.
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NOTES

72       The versions of rand() and srand() in the Linux C Library use the  same
73       random number generator as random(3) and srandom(3), so the lower-order
74       bits should be as random as the higher-order bits.  However,  on  older
75       rand()  implementations,  and  on  current implementations on different
76       systems, the lower-order bits are much less random than the  higher-or‐
77       der bits.  Do not use this function in applications intended to be por‐
78       table when good randomness is needed.  (Use random(3) instead.)
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EXAMPLES

81       POSIX.1-2001 gives the following example of an implementation of rand()
82       and  srand(),  possibly  useful when one needs the same sequence on two
83       different machines.
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85           static unsigned long next = 1;
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87           /* RAND_MAX assumed to be 32767 */
88           int myrand(void) {
89               next = next * 1103515245 + 12345;
90               return((unsigned)(next/65536) % 32768);
91           }
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93           void mysrand(unsigned int seed) {
94               next = seed;
95           }
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97       The following program can be used to display the pseudo-random sequence
98       produced by rand() when given a particular seed.
99
100           #include <stdlib.h>
101           #include <stdio.h>
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103           int
104           main(int argc, char *argv[])
105           {
106               int r, nloops;
107               unsigned int seed;
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109               if (argc != 3) {
110                   fprintf(stderr, "Usage: %s <seed> <nloops>\n", argv[0]);
111                   exit(EXIT_FAILURE);
112               }
113
114               seed = atoi(argv[1]);
115               nloops = atoi(argv[2]);
116
117               srand(seed);
118               for (int j = 0; j < nloops; j++) {
119                   r =  rand();
120                   printf("%d\n", r);
121               }
122
123               exit(EXIT_SUCCESS);
124           }
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COLOPHON

130       This  page  is  part of release 5.10 of the Linux man-pages project.  A
131       description of the project, information about reporting bugs,  and  the
132       latest     version     of     this    page,    can    be    found    at
133       https://www.kernel.org/doc/man-pages/.
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137                                  2020-11-01                           RAND(3)