#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
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; 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.
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);
}