# rand

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< c‎ | numeric‎ | random

Defined in header `<stdlib.h>` |  |
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` is 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POSIX offered a thread-safe version of rand called rand_r, which is obsolete in favor of the drand48 family of functions.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

Run this code

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

    int main(void)
    {
        srand(time(0)); //use current time as seed for random generator
        int random_variable = rand();
        printf("Random value on [0,%d]: %d\n", RAND_MAX, random_variable);
    }

Possible output:

    Random value on [0 2147483647]: 1373858591

### 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

srand | seeds pseudo-random number generator (function)
RAND_MAX | maximum possible value generated by rand() (macro constant)
C++ documentation for rand

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