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RAND(3) Linux Programmer's Manual RAND(3)


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);



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 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.


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.





static unsigned long next = 1;
/* RAND_MAX assumed to be 32767 */
int myrand(void) {
    next = next * 1103515245 + 12345;
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>
main(int argc, char *argv[])
    int j, r, nloops;
    unsigned int seed;
    if (argc != 3) {
        fprintf(stderr, "Usage: %s <seed> <nloops>\n", argv[0]);
    seed = atoi(argv[1]);
    nloops = atoi(argv[2]);
        r =  rand();
        printf("%d\n", r);


drand48(3), random(3)

