How to Generate Random Number in C

1. Use the rand and srand Functions to Generate Random Number in C
2. Use the random and srandom Functions to Generate Random Number in C
3. Use the getrandom Function to Generate Random Number in C

This article will introduce several methods of how to generate random numbers in C.

Use the rand and srand Functions to Generate Random Number in C

The rand function implements a pseudo-random number generator that can provide an integer in the range of [0, RAND_MAX], where RAND_MAX is 2³¹-1 on modern systems. Note that the generator algorithm behind the rand function is deterministic. Thus it should be seeded with random bits.

The srand function is used to seed the pseudo-random number generator, and subsequent calls to rand will produce random integer sequences. On the downside, rand implementations are not expected to produce uniformly random bits.

Thus, the rand function is not recommended to be utilized in cryptographically highly sensitive applications. The following example seeds the generator with the value of current time, which is not a good source of randomness.

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

#define MAX 100000
#define SIZE 100
#define NUMS_TO_GENERATE 10

int main() {
  srand(time(NULL));
  for (int i = 0; i < NUMS_TO_GENERATE; i++) {
    printf("%d\n", rand() % MAX);
  }

  exit(EXIT_SUCCESS);
}

Output:

85084
91989
85251
85016
43001
54883
8122
84491
6195
54793

Use the random and srandom Functions to Generate Random Number in C

Another pseudo-random pseudo-random number generator available in the C standard library is implemented under the random function. This method is the preferred method compared to the rand, but cryptographic applications should not utilize the random function in sensitive code.

random takes no arguments and returns long int type integer in the range of [0, RAND_MAX]. The function should preferably be seeded with the srandom function to generate relatively good quality random numbers.

Note that, like the previous example, we use the time function to pass the current time value as seed, which is not recommended in security-sensitive applications.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/random.h>

#define MAX 100000
#define SIZE 100
#define NUMS_TO_GENERATE 10

int main() {
  srandom(time(NULL));
  for (int i = 0; i < NUMS_TO_GENERATE; i++) {
    printf("%ld\n", random() / MAX);
  }
  printf("\n");

  exit(EXIT_SUCCESS);
}

Output:

91
2019
2410
11784
9139
5858
5293
17558
16625
3069

Use the getrandom Function to Generate Random Number in C

getrandom is a Linux specific function to obtain random bits that are of far better quality than two previous methods provided. getrandom takes three arguments - void pointer that points to the buffer where random bits should be stored, the size of the buffer in bytes, and flags for special features.

In the following example, we generate a single unsigned integer, the address of which &tmp is passed as the buffer to store random bits, and the size is calculated with the sizeof operator.

The source of randomness from where the getrandom retrieves the bits can be uninitialized in rare scenarios. The call to the getrandom function will block the program execution. Thus, the GRND_NONBLOCK macro definition is passed as the third argument for the function to return the error value -1 immediately in these cases.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/random.h>
#include <time.h>

#define MAX 100000
#define SIZE 100
#define NUMS_TO_GENERATE 10

int main() {
  unsigned int tmp;

  getrandom(&tmp, sizeof(unsigned int), GRND_NONBLOCK) == -1
      ? perror("getrandom")
      : "";
  printf("%u\n", tmp);

  exit(EXIT_SUCCESS);
}

934103271