NAME
rand, rand_r, srand - pseudo-random number generator
SYNOPSIS
#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
DESCRIPTION
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;
}
SEE ALSO
drand48(3), random(3)
COLOPHON
This page is part of release 3.24 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/.
2008-08-29 RAND(3)