c++ - Is random_shuffle threadsafe? and using rand_r if it is not

Question

Is std::random_shuffle threadsafe? I presume not since the regular rand() is not threadsafe. If that is the case, how would I use rand_r with random_shuffle so that I can give each thread a unique seed. I've seen examples of using custom random generators with random_shuffle, but it is still unclear to me.

Thanks.

Answer 1

To use rand_r with std::random_shuffle, you'll need to write a (fairly trivial) wrapper. The random number generator you pass to random_shuffle needs to accept a parameter that specifies the range of numbers to be produced, which rand_r does not.

Your wrapper would look something like this:

class rand_x { 
    unsigned int seed;
public:
    rand_x(int init) : seed(init) {}

    int operator()(int limit) {
        int divisor = RAND_MAX/(limit+1);
        int retval;

        do { 
            retval = rand_r(&seed) / divisor;
        } while (retval > limit);

        return retval;
    }        
};

You'd use it with random_shuffle something like:

std::random_shuffle(whatever.begin(), whatever.end(), rand_x(some_seed));