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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | VERSIONS | STANDARDS | HISTORY | EXAMPLES | SEE ALSO | COLOPHON |
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rand(3) Library Functions Manual rand(3)
rand, rand_r, srand - pseudo-random number generator
Standard C library (libc, -lc)
#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
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.
The rand() and rand_r() functions return a value between 0 and
RAND_MAX (inclusive). The srand() function returns no value.
For an explanation of the terms used in this section, see
attributes(7).
┌─────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├─────────────────────────────────────┼───────────────┼─────────┤
│ rand(), rand_r(), srand() │ Thread safety │ MT-Safe │
└─────────────────────────────────────┴───────────────┴─────────┘
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.)
rand()
srand()
C11, POSIX.1-2008.
rand_r()
POSIX.1-2008.
rand()
srand()
SVr4, 4.3BSD, C89, POSIX.1-2001.
rand_r()
POSIX.1-2001. Obsolete in POSIX.1-2008.
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);
}
drand48(3), random(3)
This page is part of the man-pages (Linux kernel and C library
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-6.9.1.tar.gz
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⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2024-06-26. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.9.1 2024-06-15 rand(3)
Pages that refer to this page: mcookie(1), arc4random(3), drand48(3), drand48_r(3), random(3), random_r(3)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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