rand, rand_r, srand - pseudo-random number generator
#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():
Since glibc 2.24:
_POSIX_C_SOURCE >= 199506L
Glibc 2.23 and earlier
_POSIX_C_SOURCE
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.
The
rand() and
rand_r() functions return a value between 0 and
RAND_MAX (inclusive). The
srand() function returns no value.
For an explanation of the terms used in this section, see
attributes(7).
Interface |
Attribute |
Value |
rand (), rand_r (), srand () |
Thread safety |
MT-Safe |
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.
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.)
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);
}
drand48(3),
random(3)