How Can I Generate Unique Random Numbers in Multiple Threads Using C's `rand()` Function?

Multithreaded Use of stdlib's rand()

In multithreaded programming scenarios, it is often necessary to generate random numbers in separate threads. However, when using the standard library's rand() function, you may encounter issues if not handled correctly.

Issue

Multiple threads executing the same function produce the same random number repeatedly. This is because each thread typically has its own copy of global data, including rand()'s internal state, which is shared by all threads.

Solution

To avoid generating the same random numbers, it is necessary to explicitly manage the internal state of rand(). The documentation recommends the following:

• Use rand_r() with an Explicit State Pointer: rand_r() provides a thread-safe version of rand() by allowing you to pass a pointer to an unsigned integer that will be used as the state.
• Use drand48_r(3) for a Weaker but Thread-Safe Option: drand48_r(3) is an alternative thread-safe pseudo-random generator that uses a larger internal state.

Example Usage

For example, to use rand_r() in a multithreaded environment, you could:

#include <stdlib.h>

void* thread_function(void* arg) {
  unsigned int state = 0;  // Thread-specific state
  while (1) {
    int random_number = rand_r(&state);
    // Do something with random_number
  }
  return NULL;
}

Important Note

It should be noted that using rand() or rand_r() without proper thread synchronization can lead to unpredictable behavior and incorrect results. Always ensure proper synchronization mechanisms are in place to handle multithreaded access to shared resources, including random number generators.

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