Defined in header <stdlib.h>  

void srand( unsigned seed ); 
Seeds the pseudorandom 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 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()
, and 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 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.
#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
generates a pseudorandom number (function) 

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

C++ documentation for srand 
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