The
rand()
function returns a pseudo-random integer in
the 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.
The function
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
rand()
and
rand_r()
functions return a value
between 0 and
RAND_MAX.
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;
}
This page is part of release 3.14 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/.
