Recommended way to initialize srand?


I need a 'good' way to initialize the pseudo-random number generator in C++. I've found an article that states:

In order to generate random-like numbers, srand is usually initialized to some distinctive value, like those related with the execution time. For example, the value returned by the function time (declared in header ctime) is different each second, which is distinctive enough for most randoming needs.

Unixtime isn't distinctive enough for my application. What's a better way to initialize this? Bonus points if it's portable, but the code will primarily be running on Linux hosts.

I was thinking of doing some pid/unixtime math to get an int, or possibly reading data from /dev/urandom.



Yes, I am actually starting my application multiple times a second and I've run into collisions.

1/29/2010 6:25:36 PM

Accepted Answer

The best answer is to use the Boost random number stuff. Or if you have access to C++11 use the <random> header.

But if we are talking about rand() and srand()
The best way is just to use time():

int main()


Be sure to do this at the beginning of your program, and not every time you call rand()!

Every time you start up, time() will return a unique value (unless you start the application multiple times a second). In 32 bit systems, it will only repeat every 60 years or so.

I know you don't think time is unique enough but I find that hard to believe. But I have been known to be wrong.

If you are starting a lot of copies of your application simultaneously you could use a timer with a finer resolution. But then you run the risk of a shorter time period before the value repeats.

OK, so if you really think you are starting multiple applications a second.
Then use a finer grain on the timer.

 int main()
     struct timeval time; 

     // microsecond has 1 000 000
     // Assuming you did not need quite that accuracy
     // Also do not assume the system clock has that accuracy.
     srand((time.tv_sec * 1000) + (time.tv_usec / 1000));

     // The trouble here is that the seed will repeat every
     // 24 days or so.

     // If you use 100 (rather than 1000) the seed repeats every 248 days.

     // Do not make the MISTAKE of using just the tv_usec
     // This will mean your seed repeats every second.
8/1/2016 8:44:44 AM

This is what I've used for small command line programs that can be run frequently (multiple times a second):

unsigned long seed = mix(clock(), time(NULL), getpid());

Where mix is:

unsigned long mix(unsigned long a, unsigned long b, unsigned long c)
    a=a-b;  a=a-c;  a=a^(c >> 13);
    b=b-c;  b=b-a;  b=b^(a << 8);
    c=c-a;  c=c-b;  c=c^(b >> 13);
    a=a-b;  a=a-c;  a=a^(c >> 12);
    b=b-c;  b=b-a;  b=b^(a << 16);
    c=c-a;  c=c-b;  c=c^(b >> 5);
    a=a-b;  a=a-c;  a=a^(c >> 3);
    b=b-c;  b=b-a;  b=b^(a << 10);
    c=c-a;  c=c-b;  c=c^(b >> 15);
    return c;

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