try-finally Statement


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Microsoft Specific**

The following syntax describes the try-finally statement:

__try {  
   // guarded code  
__finally {  
   // termination code  

__try compound-statement

__finally compound-statement

The try-finally statement is a Microsoft extension to the C and C++ languages that enables 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, see Exception Handling under /clr.

System_CAPS_ICON_note.jpg 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 the flow of control reaches the end of the try block, execution proceeds as follows:

  1. The termination handler is invoked.

  2. 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 does not block searching for an appropriate exception handler.

If an exception occurs in the __try block, the operating system must find a handler for the exception or the program will fail. If a handler is found, any and all __finally blocks are executed and execution resumes in the handler.

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
Order of Termination-Handler Execution

System_CAPS_ICON_note.jpg Note

The behavior of try-finally is different from some other languages that support the use of finally, such as C#. A single __try may have either, but not both, of __finally and __except. If both are to be used together, an outer try-except statement must enclose the inner try-finally statement. The rules specifying when each block executes are also different.

The __leave keyword is valid only within the guarded section of a try-finally statement, and its effect is to jump to the end of the guarded section. Execution continues at the first statement in the termination handler.

A goto statement can also jump out of the guarded section, but it degrades performance because it invokes stack unwinding. The __leave statement is more efficient because it does not cause stack unwinding.

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

Writing a Termination Handler
Structured Exception Handling (C/C++)
Termination-Handler Syntax