Qt Toolkit - QSemaphore Class

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# QSemaphore Class Reference

The QSemaphore class provides a robust integer semaphore. More...

`#include <qthread.h>`

Inherits Qt.

List of all member functions.

## Public Members

- QSemaphore ( int )
- virtual ~QSemaphore ()
- int available () const
- int total () const
- int operator++ ( int )
- int operator-- ( int )
- int operator+= ( int )
- int operator-= ( int )

## Detailed Description

The QSemaphore class provides a robust integer semaphore.

QSemaphore can be used to serialize thread execution, similar to a QMutex. A semaphore differs from a mutex, in that a semaphore can be accessed by more than one thread at a time.

An example would be an application that stores data in a large tree structure. The application creates 10 threads (commonly called a thread pool) to do searches on the tree. When the application searches the tree for some piece of data, it uses one thread per base node to do the searching. A semaphore could be used to make sure that 2 threads don't try to search the same branch of the tree.

A real world example of a semaphore would be dining at a restuarant. A semaphore initialized to have a maximum count equal to the number of chairs in the restuarant. As people arrive, they want a seat. As seats are filled, the semaphore is accessed, once per person. As people leave, the access is released, allowing more people to enter. If a party of 10 people want to be seated, but there are only 9 seats, those 10 people will wait, but a party of 4 people would be seated (taking the available seats to 5, making the party of 10 people wait longer).

## Member Function Documentation

### QSemaphore::QSemaphore ( int maxcount )

Creates a new semaphore. The semaphore can be concurrently accessed at most maxcount times.

### QSemaphore::~QSemaphore () `[virtual]`

Destroys the semaphore.

### int QSemaphore::available () const

This function returns the number of accesses currently available to the semaphore.

### int QSemaphore::operator++ ( int )

Postfix ++ operator.

Try to get access to the semaphore. If available() is >= total(), this call will block until it can get access.

### int QSemaphore::operator+= ( int n )

Try to get access to the semaphore. If available() is >= total(), the calling thread blocks until it can get access. The calling will only get access from the semaphore if it can get all n accesses at once.

### int QSemaphore::operator-- ( int )

Postfix -- operator.

Release access of the semaphore. This wakes all threads waiting for access to the semaphore.

### int QSemaphore::operator-= ( int n )

Release n accesses to the semaphore.

### int QSemaphore::total () const

This function returns the total number of accesses to the semaphore.

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