Using stdlib's rand() from multiple threads

srand() seeds the random number generator. You should only have to call srand(time(NULL)) once during startup.

That said, the documentation states:

The function rand() is not reentrant or thread-safe, since it uses hidden state that is modified on each call. This might just be the seed value to be used by the next call, or it might be something more elaborate. In order to get reproducible behaviour in a threaded application, this state must be made explicit. The function rand_r() is supplied with a pointer to an unsigned int, to be used as state. This is a very small amount of state, so this function will be a weak pseudo-random generator. Try drand48_r(3) instead.

The emphasized part of the above is probably the reason why all your threads get the same number.

As you are using C++, rather than C, you may be able to avoid the threading problems often associated with srand/rand by using c++11. This depends on using a recent compiler which supports these features. You would use a separate engine and distribution on each thread.The example acts like a dice.

#include <random>#include <functional>std::uniform_int_distribution<int> dice_distribution(1, 6);std::mt19937 random_number_engine; // pseudorandom number generatorauto dice_roller = std::bind(dice_distribution, random_number_engine);int random_roll = dice_roller();  // Generate one of the integers 1,2,3,4,5,6.

I referred to Wikipedia C++11 and Boost random when answering this question.

If you are launching the threads all at the same time, the time sent to srand is probably the same for each thread. Since they all have the same seed, they all return the same sequence. Try using something else like a memory address from a local variable.