rand() / srand()

// Returns a pseudo-random integer between 0 and RAND_MAX (inclusive).
// RAND_MAX is implementation-defined but is at least 32767.
int rand(void);

// Sets the seed for the random number generator.
// Passing the same seed always produces the same sequence of random numbers.
void srand(unsigned int seed);

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

int main(void) {
    // Seed with the current time so each run produces a different sequence.
    srand((unsigned int)time(NULL));

    // Roll a six-sided die 5 times.
    printf("Dice rolls: ");
    for (int i = 0; i < 5; i++) {
        int dice = 1 + rand() % 6;
        printf("%d ", dice);
    }
    printf("\n");

    // Generate 3 random floating-point values between 0.0 and 1.0.
    printf("Floating-point randoms: ");
    for (int i = 0; i < 3; i++) {
        double r = (double)rand() / RAND_MAX;
        printf("%.4f ", r);
    }
    printf("\n");

    // Using the same seed always produces the same sequence (reproducibility check).
    srand(42);
    printf("seed=42: ");
    for (int i = 0; i < 5; i++) {
        printf("%d ", rand() % 100);
    }
    printf("\n");

    return 0;
}

The quality of values returned by rand() — including distribution bias and cycle length — is implementation-defined. Do not use rand() for security-sensitive purposes such as password generation or cryptography. For those use cases, rely on a secure random source provided by the OS (such as /dev/urandom on Unix-like systems).

When using rand() % N, small values may appear slightly more often if RAND_MAX is not a multiple of N. If you need high-quality random numbers, consider alternatives from C11 Annex K or an external library.

To get the current time for seeding, see time().