In multithreading, a thread ID is a unique identifier assigned to each thread within a process. This ID is used to manage and control the thread's execution. In POSIX Threads (Pthreads), each thread has a unique identifier (ID), which can be obtained using the pthread_self() function.

pthread_t pthread_self(void)

The pthread_self() function returns the thread ID of the calling thread. This ID is of type pthread_t, which is an opaque data type.

Example:

#include <stdio.h>

#include <pthread.h>

void *thread_function(void *arg)

{

pthread_t tid = pthread_self(); // Get the thread ID

printf("Thread ID: %lu\n", (unsigned long)tid); // Print the thread ID

return (NULL);

}

int main()

{

pthread_t thread_id1, thread_id2;

pthread_t main_tid = pthread_self(); // Get the main thread ID

printf("Main Thread ID: %lu\n", (unsigned long)main_tid); // Print the main thread ID

pthread_create(&thread_id1, NULL, thread_function, NULL); // Create the first new thread

pthread_create(&thread_id2, NULL, thread_function, NULL); // Create the second new thread

pthread_join(thread_id1, NULL); // Wait for the first thread to finish

pthread_join(thread_id2, NULL); // Wait for the second thread to finish

return (0);

}

Sample Output (Actual numbers will vary.):

Main Thread ID: 135356936001344

Thread ID: 135356930328256

Thread ID: 135356919842496

NOTE: pthread_self() is mainly used when a thread needs to retrieve its own ID because it doesn't already know it. However, when a thread is created successfully using pthread_create(), the pthread_t variable passed to it already contains the new thread's ID. This can be useful in case one want to use the thread id in the creator function i.e., the point where the new thread was created. The example below shows printing the thread ID of a successfully created thread in the main function.

#include <stdio.h>

#include <pthread.h>

void *thread_function(void *arg)

{

pthread_t tid = pthread_self(); // Get thread ID within the thread itself

printf("Inside thread: My ID is %lu\n", (unsigned long)tid);

return (NULL);

}

int main()

{

pthread_t thread_id; // This will hold the thread ID after creation

// Create a new thread

if (pthread_create(&thread_id, NULL, thread_function, NULL) == 0)

{

printf("Main: Created thread with ID %lu\n", (unsigned long)thread_id);

}

else

{

printf("Failed to create thread\n");

}

pthread_join(thread_id, NULL); // Wait for the thread to finish

return 0;

}

Explanation:

In main():
pthread_create() is called with &thread_id, and if successful, thread_id now contains the ID of the newly created thread.
We immediately print this ID because it is already known at this point.
Inside thread_function():
The thread retrieves its ID using pthread_self() because it does not know thread_id directly.
It then prints its ID.

Thread IDs are essential reasons:

Thread Management: Thread IDs allow the program to manage and control individual threads. For example, the pthread_join() function uses thread IDs to wait for specific threads to complete their execution.
Debugging: Knowing the thread IDs can help in debugging multithreaded programs. By printing thread IDs, you can track which threads are executing and identify any issues related to thread synchronization or concurrency.
Synchronization: It is crucial in multithreaded programs to avoid race conditions and ensure data consistency. Thread IDs can implement synchronization mechanisms, such as mutexes or condition variables, to coordinate the execution of threads.