Re: C
Posted by: James Donovan James Donovan
Date: September 12, 2012 12:50PM

Multithreaded applications often require synchronization objects. These objects are used to protect memory from being modified by multiple threads at the same time, which might make the data incorrect.

The first, and simplest, is an object called a mutex. A mutex is like a lock. A thread can lock it, and then any subsequent attempt to lock it, by the same thread or any other, will cause the attempting thread to block until the mutex is unlocked. These are very handy for keeping data structures correct from all the threads' points of view. For example, imagine a very large linked list. If one thread deletes a node at the same time that another thread is trying to walk the list, it is possible for the walking thread to fall off the list, so to speak, if the node is deleted or changed. Using a mutex to "lock" the list keeps this from happening.

Computer Scientist people will tell you that Mutex stands for Mutual Exclusion.
In Java, Mutex-like behaviour is accomplished using the synchronized keyword.

Technically speaking, only the thread that locks a mutex can unlock it, but sometimes operating systems will allow any thread to unlock it. Doing this is, of course, a Bad Idea. If you need this kind of functionality, read on about the semaphore in the next paragraph.

Similar to the mutex is the semaphore. A semaphore is like a mutex that counts instead of locks. If it reaches zero, the next attempt to access the semaphore will block until someone else increases it. This is useful for resource management when there is more than one resource, or if two separate threads are using the same resource in coordination. Common terminology for using semaphores is "uping" and "downing", where upping increases the count and downing decreases and blocks on zero.
Java provides a Class called Semaphore which does the same thing, but uses acquire() and release() methods instead of uping and downing.

With a name as cool-sounding as semaphore, even Computer Scientists couldn't think up what this is short for. (Yes, I know that a semaphore is a signal or flag ;)

Unlike mutexes, semaphores are designed to allow multiple threads to up and down them all at once. If you create a semaphore with a count of 1, it will act just like a mutex, with the ability to allow other threads to unlock it.

The third and final structure is the thread itself. More specifically, thread identifiers. These are useful for getting certain threads to wait for other threads, or for getting threads to tell other threads interesting things.

Computer Scientists like to refer to the pieces of code protected by mutexes and semaphores as Critical Sections. In general, it's a good idea to keep Critical Sections as short as possible to allow the application to be as parallel as possible. The larger the critical section, the more likely it is that multiple threads will hit it at the same time, causing stalls.

In POSIX, the types we'll be dealing with are pthread_t for thread identifiers, pthread_mutex_t for mutexes, and sem_t for semaphores. We use the word "pthread" a lot because it stands for POSIX Threads.

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