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c++windowsexceptionsynchronization
Nick
Nick

Reputation: 10499

How to properly leave a Critical Section?

I have the following C++ code where I make use of the Critical Section object:

EnterCriticalSection(&cs);

// code that may throw an exception

LeaveCriticalSection(&cs);

How can I ensure that the LeaveCriticalSection function is called even if an exception is thrown?

Upvotes: 8

Views: 2934

Answers (5)

sjdowling
sjdowling

Reputation: 3022

The other answers are correct about using RAII objects but I feel it is worth pointing out an easy way to do this with Boost.ScopeExit.

#include <boost/scope_exit.hpp>
...
EnterCriticalSection(&cs);
BOOST_SCOPE_EXIT(&cs) {
        LeaveCriticalSection(&cs);
} BOOST_SCOPE_EXIT_END
// code that may throw an exception

Upvotes: 1

Dmitry Poroh
Dmitry Poroh

Reputation: 3825

I suggest you not to use WinAPI critical sections. You can get the same by using std::mutex. When you use it you also can use RAII idiom wrapper for auto unlocking mutex (std::lock_guard ).

UPDATE: one difference between critical section and mutex that you can lock critical section multiple times on one thread but this is not true for simple std::mutex. If you need recursive behaviour of locking use std::recursive_mutex std::lock_guard<std::recursive_mutex>

UPDATE 2: Detailed difference between critical sections and mutexes are described here, performance comparison is here.

Reasons: It is better to use standard-defined mechanism whenever you can. If you use platform-specific thing - wrap it around. So if you afraid for performance - create Critical section class with lock/unlock methods (to meet BasicLocakable concept requirements) and use std::lock_guard<MyCriticalSection>.

Upvotes: 3

πάντα ῥεῖ
πάντα ῥεῖ

Reputation: 1

"How can I ensure that the LeaveCriticalSection function is called even if an exception is thrown?"

You can write a small helper class like this:

 class CsLocker {
 public:
     CsLocker(CriticalSection& cs)
     : cs_(cs) {
         EnterCriticalSection(&cs_);
     }
     ~CsLocker() {
          LeaveCriticalSection(&cs);
     }
     CsLocker(const CsLocker&) = delete;
     CsLocker& operator=(const CsLocker&) = delete;
 private:
     CriticalSection& cs_;
 };

That will guarantee that the critical section is unlocked whenever (and why ever) the scope is left.

Upvotes: 4

marcinj
marcinj

Reputation: 50016

Use RAII (Resource Acquisition Is Initialization) idiom:

struct GuardCS {
    GuardCS(CRITICAL_SECTION& p_cs) : cs(p_cs){
        EnterCriticalSection(&cs);
    }
    ~GuardCS() {
        LeaveCriticalSection(&cs);
    }
private:
    // Protect against copying, remove: =delete on pre c++11 compilers
    GuardCS(GuardCS const &) = delete;
    GuardCS& operator =(GuardCS const &) = delete;
    CRITICAL_SECTION& cs;
};

If you are using MFC by any chance there are classes that abstract such stuff: is Ccriticalsection usable in production?

Upvotes: 8

user2249683
user2249683

Reputation:

Just write a guard utilizing the destructor for clean up:

struct Guard {
  CriticalSection& cs;
  Guard(CriticalSection& cs)
  : cs(cs)
  {
    EnterCriticalSection(cs);
  }
  ~Guard() {
    LeaveCriticalSection(cs);
  }
  Guard(const Guard&) = delete;  
  Guard& operator = (const Guard&) = delete;
};

Usage:

void f() {
   Guard guard(cs);
   ...
}

Upvotes: 9

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