wxSemaphore Class Reference

#include <wx/thread.h>

Detailed Description

wxSemaphore is a counter limiting the number of threads concurrently accessing a shared resource.

This counter is always between 0 and the maximum value specified during the semaphore creation. When the counter is strictly greater than 0, a call to wxSemaphore::Wait() returns immediately and decrements the counter. As soon as it reaches 0, any subsequent calls to wxSemaphore::Wait block and only return when the semaphore counter becomes strictly positive again as the result of calling wxSemaphore::Post which increments the counter.

In general, semaphores are useful to restrict access to a shared resource which can only be accessed by some fixed number of clients at the same time. For example, when modeling a hotel reservation system a semaphore with the counter equal to the total number of available rooms could be created. Each time a room is reserved, the semaphore should be acquired by calling wxSemaphore::Wait and each time a room is freed it should be released by calling wxSemaphore::Post.

Library:  wxBase

Category:  Threading

Public Member Functions
	wxSemaphore (int initialcount=0, int maxcount=0)
	Specifying a maxcount of 0 actually makes wxSemaphore behave as if there is no upper limit.
	~wxSemaphore ()
	Destructor is not virtual, don't use this class polymorphically.
wxSemaError	Post ()
	Increments the semaphore count and signals one of the waiting threads in an atomic way.
wxSemaError	TryWait ()
	Same as Wait(), but returns immediately.
wxSemaError	Wait ()
	Wait indefinitely until the semaphore count becomes strictly positive and then decrement it and return.
wxSemaError	WaitTimeout (unsigned long timeout_millis)
	Same as Wait(), but with a timeout limit.

Constructor & Destructor Documentation

wxSemaphore::wxSemaphore	(	int	initialcount = 0,
		int	maxcount = 0
	)

Specifying a maxcount of 0 actually makes wxSemaphore behave as if there is no upper limit.

If maxcount is 1, the semaphore behaves almost as a mutex (but unlike a mutex it can be released by a thread different from the one which acquired it).

initialcount is the initial value of the semaphore which must be between 0 and maxcount (if it is not set to 0).

wxSemaphore::~wxSemaphore

(

)

Destructor is not virtual, don't use this class polymorphically.

Member Function Documentation

wxSemaError wxSemaphore::Post

(

)

Increments the semaphore count and signals one of the waiting threads in an atomic way.

Returns wxSEMA_OVERFLOW if the count would increase the counter past the maximum.

Returns

One of:

• wxSEMA_NO_ERROR: There was no error.
• wxSEMA_INVALID : Semaphore hasn't been initialized successfully.
• wxSEMA_OVERFLOW: Post() would increase counter past the max.
• wxSEMA_MISC_ERROR: Miscellaneous error.

wxSemaError wxSemaphore::TryWait

(

)

Same as Wait(), but returns immediately.

Returns

One of:

• wxSEMA_NO_ERROR: There was no error.
• wxSEMA_INVALID: Semaphore hasn't been initialized successfully.
• wxSEMA_BUSY: Returned by TryWait() if Wait() would block, i.e. the count is zero.
• wxSEMA_MISC_ERROR: Miscellaneous error.

wxSemaError wxSemaphore::Wait

(

)

Wait indefinitely until the semaphore count becomes strictly positive and then decrement it and return.

Returns

One of:

• wxSEMA_NO_ERROR: There was no error.
• wxSEMA_INVALID: Semaphore hasn't been initialized successfully.
• wxSEMA_MISC_ERROR: Miscellaneous error.

wxSemaError wxSemaphore::WaitTimeout

(

unsigned long

timeout_millis

)

Same as Wait(), but with a timeout limit.

Returns

One of:

• wxSEMA_NO_ERROR: There was no error.
• wxSEMA_INVALID: Semaphore hasn't been initialized successfully.
• wxSEMA_TIMEOUT: Timeout occurred without receiving semaphore.
• wxSEMA_MISC_ERROR: Miscellaneous error.