Microsoft KB Archive/151893

= DOC: CCriticalSection::Lock Doesn't Do a Timed Wait =

Article ID: 151893

Article Last Modified on 11/21/2006

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APPLIES TO

 Microsoft Foundation Class Library 4.2, when used with:  Microsoft Visual C++ 4.0 Standard Edition

 Microsoft Visual C++ 4.1 Subscription 

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This article was previously published under Q151893



SUMMARY
The CCriticalSection class contains a member function, Lock, that can be used by a thread to gain ownership of a Critical Section object. There are two versions of the Lock function, one taking no arguments and the other taking a DWORD argument. The documentation for the latter version of Lock states that the DWORD argument specifies the amount of time, in milliseconds, to wait for the critical section to become available.

This information is not correct. The CCriticalSection class wraps the SDK CRITICAL_SECTION object. This object does not have the capability for timed waits. As a consequence, entering or leaving the Critical Section always results in blocked waits. Hence, when CCriticalSection::Lock is called, the argument passed is ignored and an infinite wait is executed.

The Mutex object has the capability for timed waits. The functionality of this object is wrapped by the MFC class CMutex. An MFC application should use the CMutex class when timed waits are desired.



MORE INFORMATION
CCriticalSection::Lock makes a call to ::EnterCriticalSection. This is a blocking call that does an infinite wait. For example, if another thread currently owns the Critical Section object, ::EnterCriticalSection blocks the execution of the calling thread indefinitely until the other thread releases its ownership. The classes, such as CEvent, CMutex, and CSemaphore, that wrap other synchronization objects, make a call to ::WaitForSingleObject from their Lock functions. ::WaitForSingleObject accepts an argument that identifies the time-out interval and executes a true timed wait. A Critical Section object cannot be used with ::WaitForSingleObject. Therefore, it is not possible to do a timed wait with Critical Section objects.

This documentation error was corrected in Visual C++ 4.2 and later versions.

Additional query words: kbVC400bug

Keywords: kbbug kbdocerr kbdocfix kbfix kbthread kbvc420fix KB151893

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