Recommended way to initialize srand?

Question

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.

Thanks!

EDIT

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

Answer 1

if you need a better random number generator, don't use the libc rand. Instead just use something like /dev/random or /dev/urandom directly (read in an int directly from it or something like that).

The only real benefit of the libc rand is that given a seed, it is predictable which helps with debugging.

Answer 2

Best way is to use another pseudorandom number generator. Mersenne twister (and Wichmann-Hill) is my recommendation.

http://en.wikipedia.org/wiki/Mersenne_twister

Answer 3

Best answer is to use the Boost randum number stuff.

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

int main()
{
    srand(time(NULL));

    ...
}

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 shorter time period before the value repeats.

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

 int main()
 {
     struct timeval;
     gettimeofday(&time,NULL);

     // 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.
 }

Answer 4

i suggest you see unix_random.c file in mozilla code. ( guess it is mozilla/security/freebl/ ...) it should be in freebl library.

there it uses system call info ( like pwd, netstat ....) to generate noise for the random number;it is written to support most of the platforms (which can gain me bonus point :D ).

Answer 5

On windows:

srand(GetTickCount());

provides a better seed than time() since its in milliseconds.

Answer 6

#include <stdio.h>
#include <sys/time.h>
main()
{
     struct timeval tv;
     gettimeofday(&tv,NULL);
     printf("%d\n",  tv.tv_usec);
     return 0;
}

tv.tv_usec is in microseconds. This should be acceptable seed.

Answer 7

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:

// http://www.concentric.net/~Ttwang/tech/inthash.htm
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;
}

Answer 8

The real question you must ask yourself is what randomness quality you need.

libc random is a LCG

The quality of randomness will be low whatever input you provide srand with.

If you simply need to make sure that different instances will have different initializations, you can mix process id (getpid), thread id and a timer. Mix the results with xor. Entropy should be sufficient for most applications.

Example :

struct timeb tp;
ftime(&tp);   
srand(static_cast<unsigned int>(getpid()) ^ 
static_cast<unsigned int>(pthread_self()) ^ 
static_cast<unsigned int >(tp.millitm));

For better random quality, use /dev/urandom. You can make the above code portable in using boost::thread and boost::date_time.