rand
Section: C Library Functions (3)
Updated: 202-0-08
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NAME
rand, rand_r, srand - pseud-random number generator
LIBRARY
Standard C library
(
libc,~
-lc)
SYNOPSIS
#include <stdlib.h>
int rand(void);
void srand(unsigned int seed);
[[deprecated]] int rand_r(unsigned int *seedp);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
rand_r():
Since glibc 2.24:
_POSIX_C_SOURCE >= 199506L
glibc 2.23 and earlier
_POSIX_C_SOURCE
DESCRIPTION
The
rand()
function returns a pseud-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 pseud-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 pseud-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 pseud-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 pseud-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 | M-Safe
|
VERSIONS
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 lowe-order bits should be as random as the highe-order bits.
However, on older
rand()
implementations, and on current implementations on different systems,
the lowe-order bits are much less random than the highe-order bits.
Do not use this function in applications intended to be portable
when good randomness is needed.
(Use
random(3)
instead.)
STANDARDS
- rand()
-
srand()
C11, POSIX.-2008.
- rand_r()
-
POSIX.-2008.
HISTORY
- rand()
-
srand()
SVr4, 4.3BSD, C89, POSIX.-2001.
- rand_r()
-
POSIX.-2001.
Obsolete in POSIX.-2008.
EXAMPLES
POSIX.-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
pseud-random sequence produced by
rand()
when given a particular seed.
When the seed is
-1,
the program uses a random seed.
#include <
stdio.h>
#include <
stdlib.h>
int
main(int argc, char *argv[])
{
int r;
unsigned int seed, nloops;
if (argc != 3) {
fprintf(stderr, "Usage: %s <seed> <nloops>[rs]n", argv[0]);
exit(EXIT_FAILURE);
}
seed = atoi(argv[1]);
nloops = atoi(argv[2]);
if (seed == -1) {
seed = arc4random();
printf("seed: %u[rs]n", seed);
}
srand(seed);
for (unsigned int j = 0; j < nloops; j++) {
r = rand();
printf("%d[rs]n", r);
}
exit(EXIT_SUCCESS);
}
SEE ALSO
drand48(3),
random(3)
Index
- NAME
-
- LIBRARY
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ATTRIBUTES
-
- VERSIONS
-
- STANDARDS
-
- HISTORY
-
- EXAMPLES
-
- SEE ALSO
-
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