Simply described, it’s when you are doing more than one thing at the same time. POSIX provides the posix_memalign() function to allocate bytes aligned on a desired boundary. One of the threads, chosen randomly, will see the PTHREAD_BARRIER_SERIAL_THREAD return value, which nominates that thread to do any cleanup or preparation between stages. DRD and Helgrind are Valgrind tools for detecting errors in multithreaded C and C++ programs. Here’s a timeline where the debit made by one thread can be undone by that made by another. When I ran the script next time, I received 511010. An example of a program that has an accidental non-determinism. Then when you build programs, they will be modified to detect data races and print statistics to stderr. To give threads mutually exclusive access to a critical section, pthreads provides the mutually exclusive lock (mutex for short). We notified threads of a new event with pthread_cond_broadcast(&stats_cnd). To make a program with this model, it is necessary to make an actor have the value of the counter and receive message to set and retrieve the value of the counter, and have two actors who will simultaneously increase the value of the counter. The, weird signature is required for a thread function */, /* idiom to tell compiler arg is unused */, /* pick distinct 'from' and 'to' accounts */, /* go nuts sending money, try not to overdraft */, /* start the threads, using whatever parallelism the, system happens to offer. Note that pthread_create, is the *only* function that creates concurrency */, /* wait for the threads to all finish, using the, pthread_t handles pthread_create gave us */, -std=c99 -pedantic -D_POSIX_C_SOURCE=200809L -Wall -Wextra, /* ... do things in the critical section ... */, /* inefficient but effective way to protect a function */, /* we're safe in here, but it's a bottleneck */, /* add a mutex to prevent races on balance */, /* get an exclusive lock on both balances before, updating (there's a problem with this, see below) */, /* set the initial balance, but also create a, /* the original way to lock mutexes, which caused deadlock */, /* lock mutexes in earlier accounts first */, /* using pthread_mutex_trylock to dodge deadlock */, /* didn't get the second one, so unlock the first */, /* force a sleep so another thread can try --, /* increase the accounts and threads, but make sure there are, * "too many" threads so they tend to block each other */, /* keep a special mutex and condition variable, /* use this interface to modify the stats */, /* a dedicated thread to update the scoreboard UI */, /* go to sleep until stats change, and always, * check that they actually have changed */, /* overwrite current line with new score */, /* notice we still have a lock hierarchy, because, * we call stats_change() after locking all account, /* start thread to update the user on how many bankers, * are in the disburse() critical section at once */. The pattern is: Any thread calling pthread_mutex_lock on a previously locked mutex will go to sleep and not be scheduled until the mutex is unlocked (and any other threads already waiting on the mutex have gone first). In our Life example, we could have used an array of pointers to dynamically allocated rows rather than a contiguous two-dimensional array. Threading and Concurrent Programming Examples. But this needs not be the case. To increase the counter, program needs to get the current value, increase it by 1 and set the increased value. Travel reservation systems 6. The bankers don’t communicate with one another, so this is a demonstration of concurrency without synchronization. In this way, we don’t need to have blocks of code that need to synchronize. First thread decreases amount from Bob’s account. It aims for maximum parallelism searching for a preimage of an MD5 hash. You could say it provides the “illusion of parallelism.” However, true parallelism has the potential for greater processor throughput for problems that can be broken into independent subtasks.