| NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON | The Linux Programming Interface |
RAND(3) Linux Programmer's Manual RAND(3)
rand, rand_r, srand - pseudo-random number generator
#include <stdlib.h>
int rand(void);
int rand_r(unsigned int *seedp);
void srand(unsigned int seed);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
rand_r(): _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_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 or thread-safe, 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.
The functions rand() and srand() conform to SVr4, 4.3BSD, C89, C99,
POSIX.1-2001. The function rand_r() is from POSIX.1-2001. POSIX.1-2008 marks
rand_r() as obsolete.
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.)
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 seed) {
next = seed;
}
The following program can be used to display the pseudo-random sequence
produced by rand() when given a particular seed.
#include <stdlib.h>
#include <stdio.h>
int
main(int argc, char *argv[])
{
int j, r, nloops;
unsigned int seed;
if (argc != 3) {
fprintf(stderr, "Usage: %s <seed> <nloops>\n", argv[0]);
exit(EXIT_FAILURE);
}
seed = atoi(argv[1]);
nloops = atoi(argv[2]);
srand(seed);
for (j = 0; j < nloops; j++) {
r = rand();
printf("%d\n", r);
}
exit(EXIT_SUCCESS);
}
drand48(3), random(3)
This page is part of release 3.32 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
2010-10-01 RAND(3)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface