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try-finally Statement

Microsoft Specific


try-finally-statement :
__try compound-statement
__finally compound-statement

The try-finally statement is a Microsoft extension to the C and C++ languages that enables 32-bit target 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.

For related information and a code sample, see try-except Statement. For more information on structured exception handling in general, see Structured Exception Handling. For more information on handling exceptions in managed applications (those using Managed Extensions for C++), see Handling Exceptions Using Managed Extensions for C++.

Note   Structured exception handling works with Win32 for both C and C++ source files. However, it is not specifically designed for C++. You can ensure that your code is more portable by using C++ exception handling. Also, C++ exception handling is more flexible, in that it can handle exceptions of any type. For C++ programs, it is recommended that you use the C++ exception-handling mechanism (try, catch, and throw statements).

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, regardless of 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, its associated handler becomes active. If no exception occurs, 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 out of the guarded body or a return statement), the termination handler is executed before the flow of control moves out of the guarded section.

    A __finally statement will not be executed if an exception is caught by a handler that is higher on the stack. A __finally statement does not block searching for an appropriate exception handler.

If an exception occurs in the __try block, the compiler must find an __except block or the program will fail. If an __except block is found, any and all __finally blocks are executed and then the __except block is executed.

For example, suppose a series of function calls links function A to function D, as shown in the following figure. Each function has one termination handler. If an exception is raised in function D and handled in A, the termination handlers are called in this order as the system unwinds the stack: D, C, B.

Order of Termination-Handler Execution

The __leave keyword

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.

Abnormal Termination

Exiting a try-finally statement using 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 (normal termination of the __try block) and the destination (the __except block that handles the exception) must be run. This is called a local unwind.

If a try block is prematurely terminated for any reason, including a jump out of the block, the system executes the associated finally block as a part of the process of unwinding the stack. In such cases, the AbnormalTermination function returns TRUE if called from within the finally block; otherwise, it returns FALSE.

The termination handler is not called if a process is killed in the middle of executing a try-finally statement.

END Microsoft Specific

See Also

Writing a Termination Handler | Structured Exception Handling | C++ Keywords