121 lines
4.0 KiB
C
121 lines
4.0 KiB
C
/* exceptions.h
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Copyright 1996, 1997, 1998 Cygnus Solutions.
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This file is part of Cygwin.
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This software is a copyrighted work licensed under the terms of the
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Cygwin license. Please consult the file "CYGWIN_LICENSE" for
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details. */
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#ifndef _EXCEPTIONS_H
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#define _EXCEPTIONS_H
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#ifdef __cplusplus
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extern "C" {
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#endif /* __cplusplus */
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/* Documentation on the innards of exception handling (i.e. from the
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perspective of a compiler implementor) apparently doesn't exist. Sigh.
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However, the following came from Onno Hovers <onno@stack.urc.tue.nl>
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The first pointer to the chain of handlers is in the thread environment block
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at FS:[0]. This chain has the following format:
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typedef struct __EXCEPTION_FRAME
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{
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struct __EXCEPTION_FRAME *Prev; /-* pointer to the previous frame *-/
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PEXCEPTION_HANDLER Handler; /-* handler function *-/
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}
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You register an exception handler in your compiler with this simple ASM
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sequence:
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PUSH _MyExceptionHandler
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PUSH FS:[0]
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MOV FS:[0],ESP
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An exception frame MUST be on the stack! The frame may have more fields and
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both Visual C++ and Borland C++ use more fields for themselves.
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When an exception occurs the system calls all handlers starting with the
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handler at FS:0, and then the previous etc. until one handler returns
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ExceptionContinueExecution, which is 0. If a handler does not want to handle
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the exception it should just return ExceptionContinueSearch, which is 1.
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The handler has the following parameters:
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ehandler (
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PEXCEPTION_RECORD erecord,
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PEXCEPTION_FRAME myframe,
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PCONTEXT context, /-* context before and after *-/
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PVOID dispatch ) /-* something *-/
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When a handler wants to handle the exception, it has some alternatives:
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-one is to do do something about the exception condition, like emulating
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an invalid instruction, mapping memory where there was a page fault, etc.
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If the handler wants to have the context of the thread that causes the
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exception changed, it should make that change in the context passed to the
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handler.
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-the second alternative is to call all exception handlers again, indicating
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that you want them to clean up. This way all the __finally blocks get
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executed. After doing that you change the context passed to the handler so
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the code starts executing in the except block. For this purpose you could
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call RtlUnwind. This (undocumented) function calls all exception handlers
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up to but not including the exception frame passed to it. If NULL is passed
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as exception frame RtlUnwind calls all exception handlers and then exits the
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process. The parameters to RtlUnwind are:
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RtlUnwind (
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PEXCEPTION_FRAME endframe,
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PVOID unusedEip,
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PEXCEPTION_RECORD erecord,
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DWORD returnEax)
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You should set unusedEip to the address where RtlUnwind should return like
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this:
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PUSH 0
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PUSH OFFSET ReturnUnwind
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PUSH 0
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PUSH 0
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CALL RtlUnwind
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ReturnUnwind:
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.....
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If no EXCEPTION_RECORD is passed, RtlUnwind makes a default exception
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record. In any case, the ExceptionFlags part of this record has the
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EH_UNWINDING (=2), flag set. (and EH_EXIT_UNWIND (=4), when NULL is passed as the end
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frame.).
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The handler for a exception as well as a for unwinds may be executed in the
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thread causing the exception, but may also be executed in another (special
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exception) thread. So it is not wise to make any assumptions about that!
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As an alternative you may consider the SetUnhandledExceptionFilter API
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to install your own exception filter. This one is documented.
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*/
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/* The January 1994 MSJ has an article entitled "Clearer, More Comprehensive
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Error Processing with Win32 Structured Exception Handling". It goes into
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a teensy bit of detail of the innards of exception handling (i.e. what we
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have to do). */
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typedef int (exception_handler)
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(EXCEPTION_RECORD *, void *, CONTEXT *, void *);
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typedef struct _exception_list
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{
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struct _exception_list *prev;
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exception_handler *handler;
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/* We're apparently free to add more stuff here.
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At present we don't need any. */
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} exception_list;
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void init_exceptions (exception_list *);
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#ifdef __cplusplus
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};
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#endif /* __cplusplus */
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#endif /* _EXCEPTIONS_H */
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