# std::srand

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< cpp‎ | numeric‎ | random

Defined in header `<cstdlib>` |  |
void srand( unsigned seed ); |  |

Seeds the pseudo-random number generator used by std::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 the same `seed`, it must produce the same sequence of values.

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

### Parameters

seed | - | the seed value

### Return value

(none)

### Notes

Generally speaking, the pseudo-random number generator should only be seeded once, before any calls to `rand()`, at 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.

### Example

Run this code

    #include <cstdlib>
    #include <iostream>
    #include <ctime>

    int main()
    {
        std::srand(std::time(0)); //use current time as seed for random generator
        int random_variable = std::rand();
        std::cout << "Random value on [0 " << RAND_MAX << "]: "
                  << random_variable << '\n';
    }

Possible output:

    Random value on [0 2147483647]: 1373858591

### See also

rand | generates a pseudo-random number (function)
RAND_MAX | maximum possible value generated by std::rand (macro constant)
reseed | reseeds the per-thread random engine (function)

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