rand

int rand();

Returns a pseudo-random integer value between ​0​ and RAND_MAX (0 and RAND_MAX included).

srand() seeds the pseudo-random number generator used by rand(). If rand() is used before any calls to srand(), rand() behaves as if it was seeded with srand(1). Each time rand() is seeded with srand(), it must produce the same sequence of values.

rand() is not guaranteed to be thread-safe.

Parameters

(none).

Return value

Pseudo-random integer value between ​0​ and RAND_MAX, inclusive.

Notes

There are no guarantees as to the quality of the random sequence produced. In the past, some implementations of rand() have had serious shortcomings in the randomness, distribution and period of the sequence produced (in one well-known example, the low-order bit simply alternated between 1 and 0 between calls). rand() is not recommended for serious random-number generation needs, like cryptography.

POSIX requires that the period of the pseudo-random number generator used by rand be at least 232
.

POSIX offered a thread-safe version of rand called rand_r, which is obsolete in favor of the drand48 family of functions.

Example

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
 
int main(void)
{
    srand(time(NULL)); // use current time as seed for random generator
    int random_variable = rand();
    printf("Random value on [0,%d]: %d\n", RAND_MAX, random_variable);
 
    // roll a 6-sided die 20 times
    for (int n=0; n != 20; ++n) {
        int x = 7;
        while(x > 6) 
            x = 1 + rand()/((RAND_MAX + 1u)/6); // Note: 1+rand()%6 is biased
        printf("%d ",  x); 
    }
}

Random value on [0,2147483647]: 448749574
3 1 3 1 4 2 2 1 3 6 4 4 3 1 6 2 3 2 6 1

References

• C11 standard (ISO/IEC 9899:2011):
• 7.22.2.1 The rand function (p: 346)
• C99 standard (ISO/IEC 9899:1999):
• 7.20.2.1 The rand function (p: 312)
• C89/C90 standard (ISO/IEC 9899:1990):
• 4.10.2.1 The rand function

See also