try-finally Statement (C)

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Microsoft Specific**

The try-finally statement is a Microsoft extension to the C language that enables applications to guarantee execution of cleanup code when execution of a block of code is interrupted. Cleanup consists of such tasks as deallocating memory, closing files, and releasing file handles. The try-finally statement is especially useful for routines that have several places where a check is made for an error that could cause premature return from the routine.

try-finally-statement:
__try  compound-statement

__finally  compound-statement

The compound statement after the __try clause is the guarded section. The compound statement after the __finally clause is the termination handler. The handler specifies a set of actions that execute when the guarded section is exited, whether the guarded section is exited by an exception (abnormal termination) or by standard fall through (normal termination).

Control reaches a __try statement by simple sequential execution (fall through). When control enters the __try statement, its associated handler becomes active. Execution proceeds as follows:

1. The guarded section is executed.

2. The termination handler is invoked.

3. When the termination handler completes, execution continues after the __finally statement. Regardless of how the guarded section ends (for example, via a goto statement out of the guarded body or via a return statement), the termination handler is executed before the flow of control moves out of the guarded section.

The __leave keyword is valid within a try-finally statement block. The effect of __leave is to jump to the end of the try-finally block. The termination handler is immediately executed. Although a goto statement can be used to accomplish the same result, a goto statement causes stack unwinding. The __leave statement is more efficient because it does not involve stack unwinding.

Exiting a try-finally statement using a return statement or the longjmp run-time function is considered abnormal termination. It is illegal to jump into a __try statement, but legal to jump out of one. All __finally statements that are active between the point of departure and the destination must be run. This is called a "local unwind."

The termination handler is not called if a process is killed while executing a try-finally statement.

See the example for the try-except statement to see how the try-finally statement works.

END Microsoft Specific

See Also

try-finally Statement