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.

RETURN VALUE

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

CONFORMING TO

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.

NOTES

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.)

EXAMPLE

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>
#include <stdlib.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);
}

This page is part of release 3.27 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/.