Search This Blog

Saturday, July 08, 2006

Windows Structured Exception Handling

This post is a bit of a backgrounder for another post I've already written most of (but haven't posted yet). That post deals almost entirely with Windows Structured Exception Handling (SEH). As at least half of the people I know who read my blog are from inferior operating systems like Unix (half kidding) I thought I should start out by describing SEH.

C++ provides fairly robust error handling mechanisms. For example, you could do the following:
int *pInt = new int[some_number];
catch (bad_alloc)
cout << "Out of memory" << endl;

This would allow you to catch the out of memory exception and handle the condition. Of course, in C++ you could just as easily do:
int *pInt = (int *)malloc(some_number * sizeof(int));
if (pInt == NULL) printf("Out of memory\r\n");
In this case, you have options - you can either use the old C functions that indicate errors with special return values, or use the new C++ exception handling. However, there exists another class of exceptions - hardware exceptions. These include things like divide by zero, access violations, attempts to execute invalid instructions, etc. But neither C nor C++ provide facilities for handling these. POSIX provides a primitive method for handling these exceptions - when such an exception occurs, the OS transfers the thread in which the exception occurred to an exception handler function. This handler function can then display an error message and terminate the thread/program or attempt to recover using the tricky setjump/longjump.

Windows provides a dramatically superior method of handling hardware exceptions, and that is SEH. SEH is very similar to the C++ exception model: when a faulting instruction is executed, an exception is thrown, and the program searches for a suitable exception handler. If such a handler can't be found in the function that threw the exception, the calling function is checked; if not the calling function, the caller's calling function; and so on. Eventually either a suitable exception handler is found or the default handler is used; in either case, the stack is "unwound", destructing classes for each function, and executing cleanup code in each function in which a handler wasn't found, and the exception handler is ultimately executed, with the machine in a state as if the function containing the handler had jumped directly into the handler.
__try {
int *pInt = NULL;
*pInt = 0; // Boom
printf("Access violation");

The syntax of SEH exceptions (shown above) is almost identical to that of C++ exceptions, and, as a matter of fact, Windows C++ compilers use Windows SEH to implement C++ exceptions. One of the definitive guides on how SEH works is by (*shock and awe*) Matt Pietrek; but I'll give a brief summary of the process.

Each thread maintains a stack of exception handler records called EXCEPTION_REGISTRATIONs. This is a singly linked list of EXCEPTION_REGISTRATION structures constructed on the stack, where the head pointer is contained in an important thread-local storage structure called the Thread Environment Block (TEB). One of these exception handler records is created by the __try statement, and destroyed after the __except block. Each of the links contains a pointer to an _except_handler function - the exception handler for that block.

When an exception occurs, it is caught by the Windows kernel exception trap. Windows checks for certain types of exceptions, such as accessing a page of memory that is allocated, but currently paged to disk, and may handle the exception right then (in that example, Windows would respond by reading the page off disk, then retrying the instruction that caused the exception).

If it's not one of the special case exceptions, Windows transitions to user mode, and starts walking the list of exception handlers for the thread, calling each one with detailed information about the exception, looking for one that will handle the particular exception. The return value of the _except_handler tells Windows what to do next. ExceptionContinueSearch tells Windows that the exception handler does not want to handle the exception, and that Window should keep looking for a handler. ExceptionContinueExecution tells Windows to resume execution of the thread; this may or may not be at the same place that caused the exception, as the exception handler can alter the registers and memory of the thread.

A third option is that the exception handler calls the Windows RtlUnwind function. This function unwinds the stack to get to the desired stack frame - the stack frame of the function with the __except block that is going to get executed. Once this process is completed, the exception handler can JMP to the code in the desired __except block. Unwinding works by walking the exception handler stack from the top, calling each handler with the STATUS_UNWIND "exception code". This instructs the exception handler to destruct any stack variables that require destruction, or cleanup code that should be executed (think how C++ exceptions can be rethrown after doing some cleanup work). This allows Windows to ensure that the stack is cleaned up, without ever knowing anything about what's on the stack or how to clean it up. This also implies that "exception handler" functions aren't just to catch exceptions - there must also be such a function for every function that has destructible variables and in which an exception can be thrown (there is a VC++ project setting which tells the compiler to assume every function can throw SEH exceptions, or to assume only C++ exceptions can be thrown by some functions).

If no exception handler in the program handles the exception, the outermost exception handler will. This handler is located in the thread/process start stub in Kernel32.dll, where Windows wraps the call to your thread/process start function in a __try block. The __except block simply calls UnhandledExceptionFilter, which, by default, displays the "this program has performed an illegal operation" dialog you've no doubt seen before, and terminates the program.

Lastly, SEH exceptions are not strictly compatible with C++ exceptions. You cannot catch SEH exceptions with a C++ catch block (and in fact you can use SEH exception handling in a vanilla C app), although you can catch C++ exceptions with an __except block using certain compiler settings. I don't know about other compilers, but VC++ allows you to set a function the compiler's generated exception handlers will call to convert SEH exceptions to true C++ exceptions, however, which may be caught with C++ catch blocks. I haven't actually used this before, but that sounds pretty sweet.

No comments: