NAME
rand, rand_r, srand - pseudo-random number generator
SYNOPSIS
#include <stdlib.h>
int rand(void);
I int rand_r(unsigned int * seedp );
I void srand(unsigned int seed );
DESCRIPTION
The
R rand ()
function returns a pseudo-random integer between 0
and RAND_MAX.
The
R srand ()
function sets its argument as the seed for a new
sequence of pseudo-random integers to be returned by
R rand ().
These sequences are repeatable by calling
R srand ()
with the same
seed value.
If no seed value is provided, the
R rand ()
function is automatically
seeded with a value of 1.
The function
R 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.
The function
R rand_r ()
is supplied with a pointer to an unsigned int, to be used as state.
This is a very small amount of state, so this function will be a weak
pseudo-random generator.
Try
drand48_r(3)
instead.
RETURN VALUE
The
R rand ()
and
R rand_r ()
functions return a value
between 0 and
R RAND_MAX .
The
R srand ()
function returns no value.
CONFORMING TO
The functions
R rand ()
and
R srand ()
conform to SVr4, 4.3BSD, C89, C99, POSIX.1-2001.
The function
R rand_r ()
is from POSIX.1-2001.
NOTES
The versions of
R rand ()
and
R 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
R 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.
In
Numerical Recipes in C: The Art of Scientific Computing
(William H. Press, Brian P. Flannery, Saul A. Teukolsky, William
T. Vetterling; New York: Cambridge University Press, 1992 (2nd ed.,
p. 277)), the following comments are made:
"If you want to generate a random integer between 1 and 10, you should
always do it by using high-order bits, as in
j = 1 + (int) (10.0 * (rand() / (RAND_MAX + 1.0)));
and never by anything resembling
j = 1 + (rand() % 10);
(which uses lower-order bits)."
Random-number generation is a complex topic.
The
Numerical Recipes in C
book (see reference above)
provides an excellent discussion of practical random-number generation
issues in Chapter 7 (Random Numbers).
For a more theoretical discussion which also covers many practical issues
in depth, see Chapter 3 (Random Numbers) in Donald E. Knuth's
R The Art of Computer Programming ,
volume 2 (Seminumerical Algorithms), 2nd ed.; Reading, Massachusetts:
Addison-Wesley Publishing Company, 1981.
EXAMPLE
POSIX.1-2001 gives the following example of an implementation of
R rand ()
and
R 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;
}
SEE ALSO
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