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
list
A ''list'' is any set of items in a row. List or lists may also refer to:
People
* List (surname)
Organizations
* List College, an undergraduate division of the Jewish Theological Seminary of America
* SC Germania List, German rugby union ...
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
block
Block or blocked may refer to:
Arts, entertainment and media Broadcasting
* Block programming, the result of a programming strategy in broadcasting
* W242BX, a radio station licensed to Greenville, South Carolina, United States known as ''96.3 ...
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