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The Microsoft Windows family of operating systems employ some specific
exception handling In computing and computer programming, exception handling is the process of responding to the occurrence of ''exceptions'' – anomalous or exceptional conditions requiring special processing – during the execution of a program. In general, a ...
mechanisms.


Structured Exception Handling

Microsoft Structured Exception Handling is the native exception handling mechanism for Windows and a forerunner technology to Vectored Exception Handling (VEH). It features the finally mechanism not present in standard C++ exceptions (but present in most imperative languages introduced later). SEH is set up and handled separately for each thread of execution.


Usage

Microsoft supports SEH as a programming technique at the compiler level only. MS Visual C++ compiler features three non-standard keywords: __try, __except and __finally — for this purpose. Other exception handling aspects are backed by a number of
Win32 API The Windows API, informally WinAPI, is Microsoft's core set of application programming interfaces (APIs) available in the Microsoft Windows operating systems. The name Windows API collectively refers to several different platform implementations th ...
functions, for example, RaiseException to raise SEH exceptions manually.


Implementation


IA-32

Each thread of execution in Windows IA-32 edition or the WoW64 emulation layer for the x86-64 version has a link to an undocumented
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at the start of its Thread Information Block. The __try statement essentially calls a compiler-defined EH_prolog function. That function allocates an on the stack pointing to the __except_handler3 function in msvcrt.dll, then adds the record to the list's head. At the end of the __try
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a compiler-defined EH_epilog function is called that does the reverse operation. Either of these compiler-defined routines can be inline. All the programmer-defined __except and __finally blocks are called from within __except_handler3. If the programmer-defined blocks are present, the created by EH_prolog is extended with a few additional fields used by __except_handler3. In the case of an exception in user mode code, the operating system parses the thread's list and calls each exception handler in sequence until a handler signals it has handled the exception (by
return value In computer programming, a return statement causes execution to leave the current subroutine and resume at the point in the code immediately after the instruction which called the subroutine, known as its return address. The return address is s ...
) or the list is exhausted. The last one in the list is always the kernel32!UnhandledExceptionFilter which displays the
General protection fault A general protection fault (GPF) in the x86 instruction set architectures (ISAs) is a fault (a type of interrupt) initiated by ISA-defined protection mechanisms in response to an access violation caused by some running code, either in the kernel ...
error message. Then the list is traversed once more giving handlers a chance to clean up any resources used. Finally, the execution returns to kernel mode where the process is either resumed or terminated. The patent on this mode of SEH, US5628016, expired in 2014.


x86-64

SEH on 64-bit Windows does not involve a runtime exception handler list; instead, it uses a stack unwinding table (UNWIND_INFO) interpreted by the system when an exception occurs. This means that the compiler does not have to generate extra code to manually perform stack unwinding and to call exception handlers appropriately. It merely has to emit information in the form of unwinding tables about the stack frame layout and specified exception handlers.


Support

GCC 4.8+ from
Mingw-w64 Mingw-w64 is a free and open source software development environment to create (cross-compile) Microsoft Windows PE applications. It was forked in 2005–2010 from MinGW (''Minimalist GNU for Windows''). Mingw-w64 includes a port of the GN ...
supports using 64-bit SEH for C++ exceptions.
LLVM LLVM is a set of compiler and toolchain technologies that can be used to develop a front end for any programming language and a back end for any instruction set architecture. LLVM is designed around a language-independent intermediate represe ...
clang supports __try on both x86 and x64.


Vectored Exception Handling

Vectored Exception Handling was introduced in
Windows XP Windows XP is a major release of Microsoft's Windows NT operating system. It was released to manufacturing on August 24, 2001, and later to retail on October 25, 2001. It is a direct upgrade to its predecessors, Windows 2000 for high-end and ...
. Vectored Exception Handling is made available to Windows programmers using languages such as
C++ C, or c, is the third letter in the Latin alphabet, used in the modern English alphabet, the alphabets of other western European languages and others worldwide. Its name in English is ''cee'' (pronounced ), plural ''cees''. History "C" ...
and Visual Basic. VEH does not replace Structured Exception Handling (SEH); rather, VEH and SEH coexist, with VEH handlers having priority over SEH handlers. Compared with SEH, VEH works more like kernel-delivered
Unix signals Signals are standardized messages sent to a running program to trigger specific behavior, such as quitting or error handling. They are a limited form of inter-process communication (IPC), typically used in Unix, Unix-like, and other POSIX-comp ...
.


Notes


References


External links

* * Note that the examples given there do not work as-is on modern Windows systems (post XP SP2) due to the changes Microsoft made to address the security issues present in the early SEH design. The examples still work on later versions of Windows if compiled with /link /safeseh:no. * * * Covers the obscure details needed to get low-level SEH (and particularly SafeSEH) code to work on more modern Windows. * * * * {{cite web, url=https://www.optiv.com/blog/old-meets-new-microsoft-windows-safeseh-incompatibility/, title=Old Meets New: Microsoft Windows SafeSEH Incompatibility, author=Joshua J. Drake, date=10 Jan 2012 An article explaining why Windows 7 SP1 ignores SafeSEH for some older binaries, while Windows XP SP3 honors it. Control flow Microsoft application programming interfaces