rand (3)
Leading comments
Copyright 1993 David Metcalfe (david@prism.demon.co.uk) %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Sin...
NAME
rand, rand_r, srand - pseudo-random number generatorSYNOPSIS
#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 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.
RETURN VALUE
The rand() and rand_r() functions return a value between 0 and RAND_MAX (inclusive). The srand() function returns no value.ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).| Interface | Attribute | Value |
| rand(), rand_r(), srand() | Thread safety | MT-Safe |
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 int 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); }