How Can I Generate Unique Random Numbers in Multithreaded C Applications?

Multithreading and Random Number Generation with stdlib's rand()

Multithreaded applications often require each thread to generate a unique sequence of random numbers. However, using the standard library function srand(time(0)) to seed the random number generator (RNG) may lead to non-random outcomes.

The Thread-Safety Issue

srand(time(0)) initializes the RNG's internal state, which is used to generate pseudo-random numbers. In a multithreaded environment, multiple threads may access the same RNG state simultaneously, leading to a shared seed value and identical number sequences.

Solution: Thread-Safe Seeding

To ensure thread-safe random number generation, it is recommended to use the rand_r() function, which takes an explicit state argument. By passing a unique state variable to each thread, the RNG state can be isolated, allowing each thread to generate its own random sequence.

Alternative: Thread-Safe Generator

Alternatively, consider using the drand48() function, which is specifically designed for multithreaded applications. It provides a larger and more reliable source of entropy than rand(), and it is thread-safe by default.

Example Usage:

#include <stdlib.h>

unsigned int thread_state;

void thread_function() {
  // Seed the state for this thread
  rand_r(&thread_state);

  // Generate random numbers using the seeded state
  int random_number = rand_r(&thread_state);
}

Conclusion:

When using rand() in a multithreaded environment, it is essential to seed the RNG with a thread-safe function like rand_r(). Alternatively, consider using thread-safe random number generators like drand48(). This ensures that each thread generates its own unique sequence of random numbers, avoiding the issue of identical numbers across threads.

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