The C++ language provides language support for stack unwinding
with try and catch blocks and
the throw keyword.
These are very powerful constructs, and require some thought when applied to the standard library in order to yield components that work efficiently while cleaning up resources when unexpectedly killed via exceptional circumstances.
Two general topics of discussion follow: exception neutrality and exception safety.
What is exception-safe code?
Will define this as reasonable and well-defined behavior by classes and functions from the standard library when used by user-defined classes and functions that are themselves exception safe.
Please note that using exceptions in combination with templates imposes an additional requirement for exception safety. Instantiating types are required to have destructors that do no throw.
Using the layered approach from Abrahams, can classify library components as providing set levels of safety. These will be called exception guarantees, and can be divided into three categories.
One. Don't throw.
As specified in 23.2.1 general container requirements. Applicable to container and string classes.
Member
functions erase, pop_back, pop_front, swap, clear. And iterator
copy constructor and assignment operator.
Two. Don't leak resources when exceptions are thrown. This is also referred to as the “basic” exception safety guarantee.
This applicable throughout the standard library.
Three. Commit-or-rollback semantics. This is referred to as “strong” exception safety guarantee.
As specified in 23.2.1 general container requirements. Applicable to container and string classes.
Member functions insert of a single
element, push_back, push_front,
and rehash.
Simply put, once thrown an exception object should continue in
flight unless handled explicitly. In practice, this means
propagating exceptions should not be swallowed in
gratuitous catch(...) blocks. Instead,
matching try and catch
blocks should have specific catch handlers and allow un-handed
exception objects to propagate. If a
terminating catch(...) blocks exist then it
should end with a throw to re-throw the current
exception.
Why do this?
By allowing exception objects to propagate, a more flexible approach to error handling is made possible (although not required.) Instead of dealing with an error immediately, one can allow the exception to propagate up until sufficient context is available and the choice of exiting or retrying can be made in an informed manner.
Unfortunately, this tends to be more of a guideline than a strict rule as applied to the standard library. As such, the following is a list of known problem areas where exceptions are not propagated.
Input/Output
The destructor ios_base::Init::~Init()
swallows all exceptions from flush called on
all open streams at termination.
All formatted input in basic_istream or
formatted output in basic_ostream can be
configured to swallow exceptions
when exceptions is set to
ignore ios_base::badbit.
Functions that have been registered
with ios_base::register_callback swallow all
exceptions when called as part of a callback event.
When closing the underlying
file, basic_filebuf::close will swallow
(non-cancellation) exceptions thrown and return NULL.
Thread
The constructors of thread that take a
callable function argument swallow all exceptions resulting from
executing the function argument.
C++ is a language that strives to be as efficient as is possible
in delivering features. As such, considerable care is used by both
language implementer and designers to make sure unused features
not impose hidden or unexpected costs. The GNU system tries to be
as flexible and as configurable as possible. So, it should come as
no surprise that GNU C++ provides an optional language extension,
spelled -fno-exceptions, as a way to excise the
implicitly generated magic necessary to
support try and catch blocks
and thrown objects. (Language support
for -fno-exceptions is documented in the GNU
GCC manual.)
Before detailing the library support
for -fno-exceptions, first a passing note on
the things lost when this flag is used: it will break exceptions
trying to pass through code compiled
with -fno-exceptions whether or not that code
has any try or catch
constructs. If you might have some code that throws, you shouldn't
use -fno-exceptions. If you have some code that
uses try or catch, you
shouldn't use -fno-exceptions.
And what it to be gained, tinkering in the back alleys with a
language like this? Exception handling overhead can be measured
in the size of the executable binary, and varies with the
capabilities of the underlying operating system and specific
configuration of the C++ compiler. On recent hardware with GNU
system software of the same age, the combined code and data size
overhead for enabling exception handling is around 7%. Of course,
if code size is of singular concern than using the appropriate
optimizer setting with exception handling enabled
(ie, -Os -fexceptions) may save up to twice
that, and preserve error checking.
So. Hell bent, we race down the slippery track, knowing the brakes
are a little soft and that the right front wheel has a tendency to
wobble at speed. Go on: detail the standard library support
for -fno-exceptions.
In sum, valid C++ code with exception handling is transformed into
a dialect without exception handling. In detailed steps: all use
of the C++
keywords try, catch,
and throw in the standard library have been
permanently replaced with the pre-processor controlled equivalents
spelled __try, __catch,
and __throw_exception_again. They are defined
as follows.
#ifdef __EXCEPTIONS # define __try try # define __catch(X) catch(X) # define __throw_exception_again throw #else # define __try if (true) # define __catch(X) if (false) # define __throw_exception_again #endif
In addition, for every object derived from
class exception, there exists a corresponding
function with C language linkage. An example:
#ifdef __EXCEPTIONS
void __throw_bad_exception(void)
{ throw bad_exception(); }
#else
void __throw_bad_exception(void)
{ abort(); }
#endif
The last language feature needing to be transformed
by -fno-exceptions is treatment of exception
specifications on member functions. Fortunately, the compiler deals
with this by ignoring exception specifications and so no alternate
source markup is needed.
By using this combination of language re-specification by the
compiler, and the pre-processor tricks and the functional
indirection layer for thrown exception objects by the library,
libstdc++ files can be compiled
with -fno-exceptions.
User code that uses C++ keywords
like throw, try,
and catch will produce errors even if the user
code has included libstdc++ headers and is using constructs
like basic_iostream. Even though the standard
library has been transformed, user code may need modification. User
code that attempts or expects to do error checking on standard
library components compiled with exception handling disabled should
be evaluated and potentially made conditional.
Some issues remain with this approach (see bugzilla entry
25191). Code paths are not equivalent, in
particular catch blocks are not evaluated. Also
problematic are throw expressions expecting a
user-defined throw handler. Known problem areas in the standard
library include using an instance
of basic_istream
with exceptions set to specific
ios_base::iostate conditions, or
cascading catch blocks that dispatch error
handling or recovery efforts based on the type of exception object
thrown.
Oh, and by the way: none of this hackery is at all special. (Although perhaps well-deserving of a raised eyebrow.) Support continues to evolve and may change in the future. Similar and even additional techniques are used in other C++ libraries and compilers.
C++ hackers with a bent for language and control-flow purity have
been successfully consoled by grizzled C veterans lamenting the
substitution of the C language keyword
const with the uglified
doppelganger __const.
C language code that is expecting to interoperate with C++ should be
compiled with -fexceptions. This will make
debugging a C language function called as part of C++-induced stack
unwinding possible.
In particular, unwinding into a frame with no exception handling
data will cause a runtime abort. If the unwinder runs out of unwind
info before it finds a handler, std::terminate()
is called.
Please note that most development environments should take care of
getting these details right. For GNU systems, all appropriate parts
of the GNU C library are already compiled
with -fexceptions.
GNU systems re-use some of the exception handling mechanisms to
track control flow for POSIX thread cancellation.
Cancellation points are functions defined by POSIX as worthy of special treatment. The standard library may use some of these functions to implement parts of the ISO C++ standard or depend on them for extensions.
Of note:
nanosleep,
read, write, open, close,
and wait.
The parts of libstdc++ that use C library functions marked as
cancellation points should take pains to be exception neutral.
Failing this, catch blocks have been augmented to
show that the POSIX cancellation object is in flight.
This augmentation adds a catch block
for __cxxabiv1::__forced_unwind, which is the
object representing the POSIX cancellation object. Like so:
catch(const __cxxabiv1::__forced_unwind&)
{
this->_M_setstate(ios_base::badbit);
throw;
}
catch(...)
{ this->_M_setstate(ios_base::badbit); }