Seeding a random number generator C++ [duplicate]

Answer 1

You should see random once at the begining of you program:

int main()
{
    // When testing you probably want your code to be deterministic
    // Thus don't see random and you will get the same set of results each time
    // This will allow you to use unit tests on code that use rand().
    #if !defined(TESTING)
    srand(time(NULL));  // Never call again
    #endif

    // Your code here.

}

Answer 2

Tl;dr but, most likely, you're doing it wrong. You're only supposed to set the seed once, whereas you might have something like:

for ( ... )
{
   srand(time(NULL));
   whatever = rand();
}

when it should be

srand(time(NULL));
for ( ... )
{
   whatever = rand();
}

Answer 3

1.Not really. You can ask user to input random seed, for example. Or use some other system parameters, but this won't make a difference.

2.To rid of this warning you have to do explicit conversion. Like:

unsigned int time_ui = unsigned int( time(NULL) );
srand( time_ui );

or

unsigned int time_ui = static_cast<unsigned int>( time(NULL) );

or

unsigned int time_ui = static_cast<unsigned int>( time(NULL)%1000 );

to check whether this is really conversion problem you can simply output your time on the screen and see yourself

std::cout << time(NULL);

Answer 4

For x86, direct call to the CPU time stamp counter rdtsc, instead of a library function TIME(NULL), could be used. Below 1) reads timestamp 2) seed RAND in assembly:

rdtsc
mov edi, eax
call    srand

For C++, the following would do the job with g++ compiler.

asm("rdtsc\n"
    "mov edi, eax\n"
    "call   srand");

NOTE: But may not be recommended if code is running in virtual machine.