Saturday, December 19, 2009

RAII mutex unlocking in exception stack unwinding

Q: why adopt exception handling in my c++ app instead of traditional error handling?

I don't buy the "convenience" argument. To me the real justification is stack unwinding and RAII, including nearly guaranteed [1]
* unlock -- same as in java synchronized keyword
* dtor

Without try/catch, deadlock is more likely.

[1] actually guarantee is tough and may require careful coding. See http://ose.sourceforge.net/browse.php?group=library-manual&entry=threads.htm -->

When an exception occurs, and the stack unwound, only destructors for objects created on the stack are invoked. If you had created an object using "operator new()", and only held a pointer to that heap object via a stack ref/ptr variable, the heap object will not be destroyed, resulting in a memory leak. A similar problem can arise when using threads in that an exception occurring within the bounds of a mutex lock, can result in the mutex not being unlocked when the stack is unwound and control returned to a higher point in the call chain.

I think one way to guarantee is using a scoped mutex. As a stackvar, the object has a limited LIFETIME and the var has a limited SCOPE.

In java, synchronized keyword guarantees automatic unlocking upon exception. If you use Lock objects, then you must do it yourselves. Try/finally seems to be the way. My suggestion:

* put lock() in Method 1, in a try block
* put abnormal unlock() in finally block
* put normal unlock() statements in any subsequent method on the call stack.

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