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Generate Random Number in C

Created: January-x, 2021 | Updated: Jan-25, 2021

1. Utilize 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 Role to Generate Random Number in C

This commodity 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 part implements a pseudo-random number generator that can provide an integer in the range of [0, RAND_MAX], where RAND_MAX is ii³¹-1 on modern systems. Note that the generator algorithm behind the rand role 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 role is non recommended to be utilized in cryptographically highly sensitive applications. The post-obit example seeds the generator with the value of current time, which is not a skillful source of randomness.

              #include <stdio.h> #include <string.h> #include <stdlib.h> #include <time.h>  #define MAX 100000 #define SIZE 100 #define NUMS_TO_GENERATE ten  int main() {     srand(fourth dimension(Cypher));     for (int i = 0; i < NUMS_TO_GENERATE; i++){         printf("%d\northward", rand() % MAX);     }      leave(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 bachelor in the C standard library is implemented nether the random role. This method is the preferred method compared to the rand, just cryptographic applications should not utilize the random function in sensitive code.

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

Note that, like the previous example, we use the time function to laissez passer the electric current time value as seed, which is non recommended in security-sensitive applications.

              #include <stdio.h> #include <string.h> #include <stdlib.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\north", random() / MAX);     }     printf("\north");      exit(EXIT_SUCCESS); }                          

Output:

              91 2019 2410 11784 9139 5858 5293 17558 16625 3069                          

Use the getrandom Part to Generate Random Number in C

getrandom is a Linux specific role to obtain random bits that are of far better quality than two previous methods provided. getrandom takes 3 arguments - void arrow that points to the buffer where random bits should exist stored, the size of the buffer in bytes, and flags for special features.

In the following example, we generate a unmarried unsigned integer, the accost of which &tmp is passed every bit the buffer to store random $.25, and the size is calculated with the sizeof operator.

The source of randomness from where the getrandom retrieves the bits tin can be uninitialized in rare scenarios. The call to the getrandom function will cake the plan execution. Thus, the GRND_NONBLOCK macro definition is passed as the tertiary argument for the function to render the error value -one immediately in these cases.

              #include <stdio.h> #include <string.h> #include <stdlib.h> #include <time.h> #include <sys/random.h>  #ascertain MAX 100000 #define SIZE 100 #define NUMS_TO_GENERATE 10  int main() {     unsigned int tmp;      getrandom(&tmp, sizeof(unsigned int), GRND_NONBLOCK) == -one ?         perror("getrandom") : "";     printf("%u\n", tmp);      exit(EXIT_SUCCESS); }                          

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