The

Microsoft Windows Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for serv ...

family of operating systems employ some specific exception handling 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 In computer science, a thread of execution is the smallest sequence of programmed instructions that can be managed independently by a scheduler, which is typically a part of the operating system. The implementation of threads and processes dif ...

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 functions, for example, RaiseException to raise SEH exceptions manually.

Implementation

IA-32

Each

in Windows IA-32 edition or the

WoW64 In computing on Microsoft platforms, WoW64 (Windows 32-bit on Windows 64-bit) is a subsystem of the Windows operating system capable of running 32-bit applications on 64-bit Windows. It is included in all 64-bit versions of Windows—including ...

emulation layer for the x86-64 version has a link to an undocumented list at the start of its

Thread Information Block In computing, the Win32 Thread Information Block (TIB) is a data structure in Win32 on x86 that stores information about the currently running thread. It is also known as the Thread Environment Block (TEB) for Win32. It descended from, and is bac ...

. 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 block 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 sa ...

) or the list is exhausted. The last one in the list is always the kernel32!UnhandledExceptionFilter which displays the General protection fault 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 GNU Co ...

supports using 64-bit SEH for C++ exceptions. LLVM clang supports __try on both x86 and x64.

Vectored Exception Handling

Vectored Exception Handling was introduced in Windows XP. Vectored Exception Handling is made available to Windows programmers using languages such as 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-compli ...

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