SYNOPSIS

DESCRIPTION

RETURN VALUE

ATTRIBUTES

CONFORMING TO

NOTES

EXAMPLE

SEE ALSO

COLOPHON

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, 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).

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)

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