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Semaphore Class Reference

A semaphore is generally used as a synchronization object between multiple threads or to protect a limited and finite resource such as a memory or thread pool. Semaphore counter for thread synchronization. More...

#include <thread.h>

List of all members.

Public Methods
	Semaphore (size_t resource=0)
	The initial value of the semaphore can be specified. More...
virtual	~Semaphore ()
	Destroying a semaphore also removes any system resources associated with it. More...
void	Wait (void)
	Wait is used to keep a thread held until the semaphore counter is greater than 0. More...
bool	TryWait (void)
	TryWait is a non-blocking variant of Wait. More...
void	Post (void)
	Posting to a semaphore increments its current value and releases the first thread waiting for the semaphore if it is currently at 0. More...
int	getValue (void)
	Get the current value of a semaphore. More...

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Detailed Description

A semaphore is generally used as a synchronization object between multiple threads or to protect a limited and finite resource such as a memory or thread pool. Semaphore counter for thread synchronization.

The semaphore has a counter which only permits access by one or more threads when the value of the semaphore is non-zero. Each access reduces the current value of the semaphore by 1. One or more threads can wait on a semaphore until it is no longer 0, and hence the semaphore can be used as a simple thread synchronization object to enable one thread to pause others until the thread is ready or has provided data for them. Semaphores are typically used as a counter for protecting or limiting concurrent access to a given resource, such as to permitting at most "x" number of threads to use resource "y", for example.

Author:

David Sugar <dyfet@ostel.com>

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Constructor & Destructor Documentation

Semaphore::Semaphore (  size_t   resource = 0 )

The initial value of the semaphore can be specified. An initial value is often used When used to lock a finite resource or to specify the maximum number of thread instances that can access a specified resource. Parameters: resource   specify initial resource count or 0 default.

virtual Semaphore::~Semaphore (    )  [virtual]

Destroying a semaphore also removes any system resources associated with it. If a semaphore has threads currently waiting on it, those threads will all continue when a semaphore is destroyed.

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Member Function Documentation

void Semaphore::Post (  void   )

Posting to a semaphore increments its current value and releases the first thread waiting for the semaphore if it is currently at 0. Interestingly, there is no support to increment a semaphore by any value greater than 1 to release multiple waiting threads in either pthread or the win32 API. Hence, if one wants to release a semaphore to enable multiple threads to execute, one must perform multiple post operations. See also: Wait

bool Semaphore::TryWait (  void   )

TryWait is a non-blocking variant of Wait. If the semaphore counter is greater than 0, then the thread is accepted and the semaphore counter is decreased. If the semaphore counter is 0 TryWait returns immediately with false. Returns: true if thread is accepted otherwise false See also: Wait , Post

void Semaphore::Wait (  void   )

Wait is used to keep a thread held until the semaphore counter is greater than 0. If the current thread is held, then another thread must increment the semaphore. Once the thread is accepted, the semaphore is automatically decremented, and the thread continues execution. The pthread semaphore object does not support a timed "wait", and hence to maintain consistancy, neither the posix nor win32 source trees support "timed" semaphore objects. See also: Post

int Semaphore::getValue (  void   )

Get the current value of a semaphore. Returns: current value.

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The documentation for this class was generated from the following file:

• thread.h

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