Microsoft Structured Exception Handling is the nativeexception handling mechanism for Windows and a forerunner technology to Vectored Exception Handling. It features the finally mechanism not present in standard C++ exceptions. 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 functions, for example, RaiseException to raise SEH exceptions manually.
Each thread of execution in Windows IA-32 edition or the WoW64emulation layer for the x86-64 version has a link to an undocumented _EXCEPTION_REGISTRATION_RECORD list at the start of its Thread Information Block. The __try statement essentially calls a compiler-defined EH_prolog function. That function allocates an _EXCEPTION_REGISTRATION_RECORD 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 __tryblock 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 _EXCEPTION_REGISTRATION_RECORD 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 _EXCEPTION_REGISTRATION_RECORD list and calls each exception handler in sequence until a handler signals it has handled the exception or the list is exhausted. The last one in the list is always the kernel32!UnhandledExceptionFilter which displays the General protection faulterror 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 is simpler, as it only involves using a table instead of having the compiler generate code.
Support
GCC 4.8+ from Mingw-w64 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, rather VEH and SEH coexist, with VEH handlers having priority over SEH handlers. Compared with SEH, VEH works more like kernel-delivered Unix signals.
