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rand(3) — Linux manual page

rand(3) Library Functions Manual rand(3)

NAME top

 rand, rand_r, srand - pseudo-random number generator

LIBRARY top

 Standard C library (libc, -lc)

SYNOPSIS top

 #include <stdlib.h>
 int rand(void);
 void srand(unsigned int seed);
 [[deprecated]] int rand_r(unsigned int *seedp);
 Feature Test Macro Requirements for glibc (see
 feature_test_macros(7)):
 rand_r():
 Since glibc 2.24:
 _POSIX_C_SOURCE >= 199506L
 glibc 2.23 and earlier
 _POSIX_C_SOURCE

DESCRIPTION top

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

RETURN VALUE top

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

ATTRIBUTES top

 For an explanation of the terms used in this section, see
 attributes(7).
 ┌──────────────────────────────────────┬───────────────┬─────────┐
 │ Interface │ Attribute │ Value │
 ├──────────────────────────────────────┼───────────────┼─────────┤
 │ rand(), rand_r(), srand() │ Thread safety │ MT-Safe │
 └──────────────────────────────────────┴───────────────┴─────────┘

VERSIONS top

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

STANDARDS top

 rand()
 srand()
 C11, POSIX.1-2008.
 rand_r()
 POSIX.1-2008.

HISTORY top

 rand()
 srand()
 SVr4, 4.3BSD, C89, POSIX.1-2001.
 rand_r()
 POSIX.1-2001. Obsolete in POSIX.1-2008.

EXAMPLES top

 POSIX.1-2001 gives the following example of an implementation of
 rand() and srand(), possibly useful when one needs the same
 sequence on two different machines.
 static unsigned long next = 1;
 /* RAND_MAX assumed to be 32767 */
 int myrand(void) {
 next = next * 1103515245 + 12345;
 return((unsigned) (next/65536) % 32768);
 }
 void mysrand(unsigned int seed) {
 next = seed;
 }
 The following program can be used to display the pseudo-random
 sequence produced by rand() when given a particular seed. When
 the seed is -1, the program uses a random seed.
 #include <stdio.h>
 #include <stdlib.h>
 int
 main(int argc, char *argv[])
 {
 int r;
 unsigned int seed, nloops;
 if (argc != 3) {
 fprintf(stderr, "Usage: %s <seed> <nloops>\n", argv[0]);
 exit(EXIT_FAILURE);
 }
 seed = atoi(argv[1]);
 nloops = atoi(argv[2]);
 if (seed == -1) {
 seed = arc4random();
 printf("seed: %u\n", seed);
 }
 srand(seed);
 for (unsigned int j = 0; j < nloops; j++) {
 r = rand();
 printf("%d\n", r);
 }
 exit(EXIT_SUCCESS);
 }

SEE ALSO top

 drand48(3), random(3)

COLOPHON top

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 user-space interface documentation) project. Information about
 the project can be found at 
 ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
 for this manual page, see
 ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
 This page was obtained from the tarball man-pages-6.15.tar.gz
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 ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
 2025年08月11日. If you discover any rendering problems in this HTML
 version of the page, or you believe there is a better or more up-
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 man-pages@man7.org
Linux man-pages 6.15 2025年05月17日 rand(3)

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Pages that refer to this page: mcookie(1), arc4random(3), drand48(3), drand48_r(3), random(3), random_r(3)

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