Creating random numbers in a multi-threaded program in C

Question

I am writing a program where there are worker threads that create random numbers from 0 to 3 from 0 to x-1 (variable)

what I need to know is how I can produce these random numbers in C.

I am using gcc compiler and working on Ubuntu 11.10

Answer 1

rand() and srand() cannot be used safely in this case.

Both of them are neither repeaters nor thread safe . Generally speaking, neither C standards nor C ++ standards impose any thread safety requirements in any of the standard library functions.
Some implementations may indeed provide thread safe versions, but a standard is not provided.

To use a random number generator in a multi-threaded environment, you will need an implementation that allows you to pass to state. Thus, you can save a single state value for the stream and generate high-quality random numbers without the need for synchronization.

The standard C library does not provide any options. This makes portability 100% impossible. The choice of use will depend on your environment, which you must indicate as part of your question in order to get accurate answers.

Check out the GNU Science Library , which claims to provide the MultiThreaded Random Number Generator .

Answer 2

If you just need some random numbers and don't care if the serial numbers of the random numbers are generated independently or not, you can still use rand() and srand() .

  #include <stdlib.h> #include <stdio.h> #include <pthread.h> #include <unistd.h> void* cb1(void*) { while(1) { printf("cb1, rand:%d\n",rand()); sleep(1); } } void* cb2(void*) { while(1) { printf("cb2, rand:%d\n",rand()); sleep(1); } } int main() { pthread_t th1, th2; srand(1); pthread_create(&th1, NULL, cb1, NULL); pthread_create(&th2, NULL, cb2, NULL); pthread_join(th1, NULL); pthread_join(th2, NULL); return 0; } 

Answer 3

Use rand_r() (see rand (3))

The rand_r () function is supplied with a pointer to an unsigned int, for use as a state.

It is reentrant and accepts a seed as input, so threads can manage their seeds separately.

Answer 4

A thing of truth and beauty;

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

Write your own RNG.

(Apologies ... I just looked at this wiki page. The last thing I looked a while ago was great, you could read it and implement a twister. Now this is a bunch of math level that doesn't explain anything to anyone , with the exception of a subgroup of the population that worked at the university for four years studying math. I see this happening on the wiki:

Answer 5

A simple version without error checking using C11 threads.h would look like this:

 mtx_t rand_mtx; //run only once. int rand_init(int seed) { srand(seed); mtx_init(&rand_mtx, mtx_plain); } int synced_rand() { mtx_lock(&rand_mtx); int r = rand(); mtx_unlock(&rand_mtx); return r; }