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 the same seed, it must produce the same sequence of values.

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


seed - the seed value

Return value



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.


#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int main(void)
    srand(time(NULL)); //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


  • C17 standard (ISO/IEC 9899:2018):
    • The srand function (p: 252-253)
  • C11 standard (ISO/IEC 9899:2011):
    • The srand function (p: 346-347)
  • C99 standard (ISO/IEC 9899:1999):
    • The srand function (p: 312-313)
  • C89/C90 standard (ISO/IEC 9899:1990):
    • The srand function

See also

generates a pseudo-random number
maximum possible value generated by rand()
(macro constant)
C++ documentation for srand

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