Exceptions

Exceptions
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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.

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.

Exception Neutrality

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.

Doing without

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.

Compatibility

With `C`

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.

With `POSIX` thread cancellation

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); }