try-except statement (C)

Microsoft-specific

The try-except statement is a Microsoft extension to the C language that enables applications to gain control of a program when events that normally terminate execution occur. Such events are called exceptions, and the mechanism that deals with exceptions is called structured exception handling.

Exceptions may be either hardware- or software-based. Even when applications can't completely recover from hardware or software exceptions, structured exception handling makes it possible to log and display error information. It's useful to trap the internal state of the application to help diagnose the problem. In particular, it's helpful for intermittent problems that aren't easy to reproduce.

Syntax

try-except-statement:
 __try compound-statement __except ( expression ) compound-statement

The compound statement after the __try clause is the guarded section. The compound statement after the __except clause is the exception handler. The handler specifies a set of actions to take if an exception is raised during execution of the guarded section. Execution proceeds as follows:

1. The guarded section is executed.

2. If no exception occurs during execution of the guarded section, execution continues at the statement after the __except clause.

3. If an exception occurs during execution of the guarded section, or in any routine the guarded section calls, the __except expression gets evaluated. The value returned determines how the exception is handled. There are three possible values:

   • EXCEPTION_CONTINUE_SEARCH: The exception isn't recognized. Continue to search up the stack for a handler, first for containing try-except statements, then for handlers with the next highest precedence.

   • EXCEPTION_CONTINUE_EXECUTION: The exception is recognized but dismissed. Continue execution at the point where the exception occurred.

   • EXCEPTION_EXECUTE_HANDLER The exception is recognized. Transfer control to the exception handler by executing the __except compound statement, then continue execution at the point the exception occurred.

Because the __except expression is evaluated as a C expression, it's limited to either a single value, the conditional-expression operator, or the comma operator. If more extensive processing is required, the expression can call a routine that returns one of the three values listed above.

Each routine in an application can have its own exception handler. The __except expression executes in the scope of the __try body. It has access to any local variables declared there.

The __leave keyword is valid within a try-except statement block. The effect of __leave is to jump to the end of the try-except block. Execution resumes after the end of the exception handler. 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 doesn't involve stack unwinding.

Exiting a try-except statement using the longjmp run-time function is considered abnormal termination. It isn't legal to jump into a __try statement, but it's legal to jump out of one. The exception handler isn't called if a process is killed in the middle of executing a try-except statement.

Example

Here's an example of an exception handler and a termination handler. For more information about termination handlers, see try-finally statement (C).

.
.
.
puts("hello");
__try {
   puts("in try");
   __try {
      puts("in try");
      RAISE_AN_EXCEPTION();
   } __finally {
      puts("in finally");
   }
} __except( puts("in filter"), EXCEPTION_EXECUTE_HANDLER ) {
   puts("in except");
}
puts("world");

Here's the output from the example, with commentary added on the right:

hello
in try              /* fall into try                        */
in try              /* fall into nested try                 */
in filter           /* execute filter; returns 1 so accept  */
in finally          /* unwind nested finally                */
in except           /* transfer control to selected handler */
world               /* flow out of handler                  */

END Microsoft-specific

See also

try-except statement (C++)