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ussing barriers in pthreads

pthreads allows us to create and manage threads in a program. When multiple threads are working together, it might be required that the threads wait for each other at a certain event or point in the program before proceeding ahead.

Let us say we have four threads, each of which is going to initialize a global variable. The 4 variables in turn might be used by all the four threads. Thus it would be feasible that all the threads wait for each other to finish the initialization of the variables before proceeding.

Such operations can be implemented by adding a barrier in the thread. A barrier is a point where the thread is going to wait for other threads and will proceed further only when predefined number of threads reach the same barrier in their respective programs.

To use a barrier we need to use variables of the type pthread_barrier_t. A barrier can be initialized using the function pthread_barrier_init, whose syntax is



Once a barrier in initialized, a thread can be made to wait on the barrier for other threads using the function pthread_barrier_wait whose syntax is



Where the barrier is the same variable initialized using pthread_barrier_init.

A thread will keep waiting till the "count" number of threads passed during init do not call the wait function on the same barrier.

In the example below, we create 4 threads, and each thread assigns a value to one of he global variables and then later all the threads print the value of all the four variables. Thus to be able to print the value of all the variables the threads have to wait until all threads have not finished assigning the values. Thus we can make use of barriers to make the threads wait.

The init function :



initializes the barrier "our_barrier" to wait for four threads.

Thus till all the four threads have not called the pthread_barrier_wait the other threads will continue to wait.

Once pthread_barrier_wait has been called by 4 threads, all the threads will continue their execution of program.

A barrier can be destroyed using the function pthread_barrier_destroy, whose syntax is :



But note that the barrier should be destroyed only when no thread is executing a wait on the barrier.



Save the program as pthread_barrier.c and compile it by passing the -lpthread flag.



To stress on usefulness of the barrier, we can change barrier initialization in the above code to wait for only 3 threads i.e.


And remove the pthread_barrier_wait from the the 4th thread.

After the above changes, if we compile and execute the code the output would be



The first three threads could not get the value entered by the fourth thread as they moved over the barrier as soon as three threads executed the wait function.

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