Defined in header <cstdlib>  

void srand( unsigned seed ); 
Seeds the pseudorandom number generator used by std::rand()
with the value seed
.
If std::rand()
is used before any calls to srand()
, std::rand()
behaves as if it was seeded with srand(1)
.
Each time std::rand()
is seeded with the same seed
, it must produce the same sequence of values.
srand()
is not guaranteed to be threadsafe.
seed    the seed value 
(none).
Generally speaking, the pseudorandom 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 pseudorandom numbers.
Standard practice is to use the result of a call to std::time(0)
as the seed. However, std::time
returns a std::time_t
value, and std::time_t
is not guaranteed to be an integral type. In practice, though, every major implementation defines std::time_t
to be an integral type, and this is also what POSIX requires.
#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
generates a pseudorandom number (function) 

maximum possible value generated by std::rand (macro constant) 

reseeds the perthread random engine (function) 

C documentation for srand 
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