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);

sample_rand_srand.c

#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;
}

Common Mistake: Calling srand Every Time

Calling srand() every time before rand() reseeds with the same value and produces the same number repeatedly. Call srand() only once at the beginning of the program.

rand_srand_ng.c

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

int main(void) {
    printf("NG (srand every time): ");
    for (int i = 0; i < 5; i++) {
        srand((unsigned int)time(NULL)); /* NG: same seed set every iteration */
        printf("%d ", rand() % 100);
    }
    printf("\n");
    return 0;
}

Run the following command:

gcc rand_srand_ng.c -o rand_srand_ng
./rand_srand_ng
NG (srand every time): 42 42 42 42 42

rand_srand_ok.c

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

int main(void) {
    srand((unsigned int)time(NULL)); /* OK: call only once at the start */
    printf("OK: ");
    for (int i = 0; i < 5; i++) {
        printf("%d ", rand() % 100);
    }
    printf("\n");
    return 0;
}

Run the following command:

gcc rand_srand_ok.c -o rand_srand_ok
./rand_srand_ok
OK: 73 28 91 47 62

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().