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The counting semaphore concept can be extended with the ability to claim or return more than one "unit" from the semaphore, a technique implemented in Unix. The modified V and P operations are as follows, using square brackets to indicate atomic operations , i.e., operations that appear indivisible to other processes:
The software thread running on processor #1 loops while the value of f is zero, then it prints the value of x. The software thread running on processor #2 stores the value 42 into x and then stores the value 1 into f. Pseudo-code for the two program fragments is shown below. The steps of the program correspond to individual processor instructions.
Based on the definition of barrier, we need to implement a function like thread_barrier_wait() in this program which will "monitor" the total number of thread in the program in order to life the barrier. In this program, every thread calls thread_barrier_wait() will be blocked until THREAD_BARRIERS_NUMBER threads reach the thread barrier.
This prevents runaway code. Avoid heap memory allocation. Restrict functions to a single printed page. Use a minimum of two runtime assertions per function. Restrict the scope of data to the smallest possible. Check the return value of all non-void functions, or cast to void to indicate the return value is useless. Use the preprocessor sparingly.
The original semaphore bounded buffer solution was written in ALGOL style. The buffer can store N portions or elements. The "number of queueing portions" semaphore counts the filled locations in the buffer, the "number of empty positions" semaphore counts the empty locations in the buffer and the semaphore "buffer manipulation" works as mutex for the buffer put and get operations.
Two examples that demonstrate the use of Grand Central Dispatch can be found in John Siracusa's Ars Technica Snow Leopard review. [18] Initially, a document-based application has a method called analyzeDocument which may do something like count the number of words and paragraphs in the document. Normally, this would be a quick process, and may ...
A common use might be to control access to a data structure in memory that cannot be updated atomically and is invalid (and should not be read by another thread) until the update is complete. Readers–writer locks are usually constructed on top of mutexes and condition variables , or on top of semaphores .
In the C programming language, Duff's device is a way of manually implementing loop unrolling by interleaving two syntactic constructs of C: the do-while loop and a switch statement. Its discovery is credited to Tom Duff in November 1983, when Duff was working for Lucasfilm and used it to speed up a real-time animation program.