CCriticalSection Class

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Represents a "critical section" — a synchronization object that allows one thread at a time to access a resource or section of code.

Syntax

class CCriticalSection : public CSyncObject

Members

Public Constructors

Name Description
CCriticalSection::CCriticalSection Constructs a CCriticalSection object.

Public Methods

Name Description
CCriticalSection::Lock Use to gain access to the CCriticalSection object.
CCriticalSection::Unlock Releases the CCriticalSection object.

Public Operators

Name Description
CCriticalSection::operator CRITICAL_SECTION* Retrieves a pointer to the internal CRITICAL_SECTION object.

Public Data Members

Name Description
CCriticalSection::m_sect A CRITICAL_SECTION object.

Remarks

Critical sections are useful when only one thread at a time can be allowed to modify data or some other controlled resource. For example, adding nodes to a linked list is a process that should only be allowed by one thread at a time. By using a CCriticalSection object to control the linked list, only one thread at a time can gain access to the list.

Note

The functionality of the CCriticalSection class is provided by an actual Win32 CRITICAL_SECTION object.

Critical sections are used instead of mutexes (see CMutex) when speed is critical and the resource will not be used across process boundaries.

There are two methods for using a CCriticalSection object: stand-alone and embedded in a class.

  • Stand-alone method To use a stand-alone CCriticalSection object, construct the CCriticalSection object when it is needed. After a successful return from the constructor, explicitly lock the object with a call to Lock. Call Unlock when you are done accessing the critical section. This method, while clearer to someone reading your source code, is more prone to error as you must remember to lock and unlock the critical section before and after access.

    A more preferable method is to use the CSingleLock class. It also has a Lock and Unlock method, but you don't have to worry about unlocking the resource if an exception occurs.

  • Embedded method You can also share a class with multiple threads by adding a CCriticalSection-type data member to the class and locking the data member when needed.

For more information on using CCriticalSection objects, see the article Multithreading: How to Use the Synchronization Classes.

Inheritance Hierarchy

CObject

CSyncObject

CCriticalSection

Requirements

Header: afxmt.h

CCriticalSection::CCriticalSection

Constructs a CCriticalSection object.

CCriticalSection();

Remarks

To access or release a CCriticalSection object, create a CSingleLock object and call its Lock and Unlock member functions. If the CCriticalSection object is being used stand-alone, call its Unlock member function to release it.

If the constructor fails to allocate the required system memory, a memory exception (of type CMemoryException) is automatically thrown.

Example

See the example for CCriticalSection::Lock.

CCriticalSection::Lock

Call this member function to gain access to the critical section object.

BOOL Lock();  
BOOL Lock(DWORD dwTimeout);
```  
  
### Parameters  
 `dwTimeout`  
 `Lock` ignores this parameter value.  
  
### Return Value  
 Nonzero if the function was successful; otherwise 0.  
  
### Remarks  
 `Lock` is a blocking call that will not return until the critical section object is signaled (becomes available).  
  
 If timed waits are necessary, you can use a [CMutex](../Topic/CMutex%20Class.md) object instead of a `CCriticalSection` object.  
  
 If `Lock` fails to allocate the necessary system memory, a memory exception (of type [CMemoryException](../Topic/CMemoryException%20Class.md)) is automatically thrown.  
  
### Example  
 This example demonstrates the nested critical section approach by controlling access to a shared resource (the static `_strShared` object) using a shared `CCriticalSection` object. The `SomeMethod` function demonstrates updating a shared resource in a safe manner.  
  
 [!CODE [NVC_MFC_Utilities#11](../CodeSnippet/VS_Snippets_Cpp/NVC_MFC_Utilities#11)]  
  
##  <a name="ccriticalsection__m_sect"></a>  CCriticalSection::m_sect  
 Contains a critical section object that is used by all `CCriticalSection` methods.

CRITICAL_SECTION m_sect;

  
##  <a name="ccriticalsection__operator_critical_section_star"></a>  CCriticalSection::operator CRITICAL_SECTION*  
 Retrieves a **CRITICAL_SECTION** object.

operator CRITICAL_SECTION*();

  
### Remarks  
 Call this function to retrieve a pointer to the internal **CRITICAL_SECTION** object.  
  
##  <a name="ccriticalsection__unlock"></a>  CCriticalSection::Unlock  
 Releases the `CCriticalSection` object for use by another thread.

BOOL Unlock();

  
### Return Value  
 Nonzero if the `CCriticalSection` object was owned by the thread and the release was successful; otherwise 0.  
  
### Remarks  
 If the `CCriticalSection` is being used stand-alone, `Unlock` must be called immediately after completing use of the resource controlled by the critical section. If a [CSingleLock](../Topic/CSingleLock%20Class.md) object is being used, `CCriticalSection::Unlock` will be called by the lock object's `Unlock` member function.  
  
### Example  
  See the example for [CCriticalSection::Lock](#ccriticalsection__lock).  
  
## See Also  
 [CSyncObject Class](../Topic/CSyncObject%20Class.md)   
 [Hierarchy Chart](../Topic/Hierarchy%20Chart.md)   
 [CMutex Class](../Topic/CMutex%20Class.md)