##### cppreference.com

##### Search

Create account

##### Namespaces

- Page
- Discussion

##### Variants

##### Views

- View
- Edit
- History

##### Actions

# srand

From cppreference.com
< c‎ | numeric‎ | random

Defined in header `<stdlib.h>` |  |
void srand( unsigned seed ); |  |

Seeds the pseudo-random number generator used by rand() with the value `seed`.

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.

`srand()` is not guaranteed to be thread-safe.

### [edit] Parameters

seed | - | the seed value

### [edit] Return value

(none)

### [edit] Notes

Generally speaking, the pseudo-random number generator should only be seeded once, before any calls to `rand()`, and the start of the program. It should not be repeatedly seeded, or reseeded every time you wish to generate a new batch of pseudo-random numbers.

Standard practice is to use the result of a call to time(0) as the seed. However, `time()` returns a time_t value, and `time_t` is not guaranteed to be an integral type. In practice, though, every major implementation defines `time_t` to be an integral type, and this is also what POSIX requires.

### [edit] 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

### [edit] See also

rand | generates a pseudo-random number (function) [edit]
RAND_MAX | maximum possible value generated by rand() (macro constant) [edit]
C++ documentation for srand

Retrieved from "http://en.cppreference.com/mwiki/index.php?title=c/numeric/random/srand&oldid=71060"