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Posted 20 Sep 2002

A TryEnterCriticalSection that works on Windows 9x

, 24 Sep 2002
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TryEnterCriticalSection is a useful tool to determine if some protected resource is busy, but it's only available on the NT Platform... until now.
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It wasn't too long ago that I needed to write an application where several threads were required. One of the threads in this application would be responsible for some shared resource, and the other threads would have to ask permission to gain temporary exclusive access to that shared resource. I knew that the resource did not need to be shared across multiple processes, so I wanted to avoid using expensive kernel objects. I decided to use a critical section to protect the resource where the several consuming threads would call the ::TryEnterCriticalSection() API to ask for permission and if so granted gain exclusive access to the shared resource. The only one problem with this solution was ::TryEnterCriticalSection() is only available on NT platforms, and I needed to support 9x.

The Solution

Several solutions to this problem ran through my mind. The first solution I considered was using a mutex object where the consuming threads would call the ::WaitForSingleObject() API passing 0 to the second parameter of the function (indicating that the function should try to get the mutex, but not wait for it), and checking against a return value of WAIT_OBJECT_0. The problem with this solution is it required use of the mutex kernel object, and it is a very expensive thing to switch from user to kernel mode. I decided to shelf that solution. After pondering a few other possibilities, I decided to write my own critical section class that allows for "try-enter" logic.

The CTryEnterCS class

CTryEnterCS is the fruit of that effort. It's actually quite simple to use as there are only two public methods in the class. It operates 100% in user mode, making use of the ::InterlockedExchange() and ::GetCurrentThreadId() API's. Below is a description of the public methods:

bool TryEnter()

This locks the critical section for the current thread if no other thread already owns the critical section. If the current thread already owns the critical section (reentry), the current thread is allowed to pass. Returns true if the critical section was entered; else false.
bool Leave()
This releases (leaves) ownership of the critical section if the current thread entered it (using TryEnter()). Returns true if this was successful; else false if this failed or the current thread did not own the critical section.


  • 21-Sept-2002 Initial revision.
  • 25-Sept-2002 TryEnter now reference counts multiple entry from the same thread; Leave decrements the count and when zero releases the lock.


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    About the Author

    Olan Patrick Barnes
    Web Developer
    United States United States
    Olan Patrick Barnes is a Senior Software Engineer at Open Text Corporation. He is currently working towards a Masters of Computer Science at Eastern Michigan University.

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    Comments and Discussions

    GeneralQuestionable Implementation Pin
    Toby Opferman19-May-05 16:28
    memberToby Opferman19-May-05 16:28 
    GeneralRe: Questionable Implementation Pin
    Toby Opferman19-May-05 16:38
    memberToby Opferman19-May-05 16:38 
    GeneralRe: Questionable Implementation Pin
    Anonymous30-May-05 22:41
    sussAnonymous30-May-05 22:41 
    GeneralRe: Questionable Implementation Pin
    Toby Opferman31-May-05 16:09
    memberToby Opferman31-May-05 16:09 
    GeneralRe: Questionable Implementation Pin
    Sreekanth Muralidharan31-May-05 20:58
    memberSreekanth Muralidharan31-May-05 20:58 
    GeneralRe: Questionable Implementation Pin
    Toby Opferman1-Jun-05 5:21
    memberToby Opferman1-Jun-05 5:21 
    GeneralRe: Questionable Implementation Pin
    Sreekanth Muralidharan1-Jun-05 17:47
    memberSreekanth Muralidharan1-Jun-05 17:47 
    GeneralRe: Questionable Implementation Pin
    Toby Opferman1-Jun-05 17:57
    memberToby Opferman1-Jun-05 17:57 
    GeneralRe: Questionable Implementation Pin
    Sreekanth Muralidharan1-Jun-05 19:21
    memberSreekanth Muralidharan1-Jun-05 19:21 
    QuestionWhat about EnterCriticalSection? Pin
    aufeuer22-Aug-04 18:09
    memberaufeuer22-Aug-04 18:09 
    AnswerRe: What about EnterCriticalSection? Pin
    Toby Opferman19-May-05 16:32
    memberToby Opferman19-May-05 16:32 
    GeneralMetered Sections Pin
    Neville Franks21-Sep-02 12:04
    memberNeville Franks21-Sep-02 12:04 
    GeneralRe: Metered Sections Pin
    Olan Patrick Barnes21-Sep-02 12:52
    memberOlan Patrick Barnes21-Sep-02 12:52 
    GeneralRe: Metered Sections Pin
    mystro_AKA_kokie15-Jun-03 11:21
    membermystro_AKA_kokie15-Jun-03 11:21 
    GeneralRe: Metered Sections Pin
    Neville Franks16-Jun-03 11:01
    memberNeville Franks16-Jun-03 11:01 

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