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From: Tomas Puverle (Tomas.Puverle_at_[hidden])
Date: 2006-05-22 22:42:45


> I have been thinking about how to reconcile Boost.Asio error
> handling with the system error classes defined in the TR2
> filesystem proposal N1975. I have some concerns which I'll
> outline in the context of how I plan to use them.
> The current plan is as follows:
> - Synchronous functions would by default throw a system_error
> exception.
> - An overload of each synchronous function would take an
> error_code& as the final argument. On failure these functions
> set the error_code to the appropriate error. On success, the
> error_code is set to represent the no error state. These
> functions do not throw exceptions.

Here's a decription of the approach I've taken when designing a portable
system library:

There were several problems that I was trying to solve:
a) I was trying to minimize the amount of boilerplate error handling code
which gets written, or even worse, which doesn't get written, because it's so
boring to write it!

b) Unify the error reporting infrastructure under Unix and Windows based

c) Make it easy to handle different classes of errors.

An example of this would be the Unix read() call:

How many times have you seen people write

while (...)
  ssize_t res = ::read(...);

  if (!res)
    switch (errno)
      case EINTR: continue;
      case ...

So wrt errors, we would like some types of errors to (usually) automatically
restart the operation that was attempted, while other "exceptional cases"
(like EOF) are not critical probably should not cause an exception, while
others should just be handled as critical errors (and e.g. throw)

The other problem was that Windows and Unix have a different models of
handling errors. On Unix, errors are "syscall sensitive", i.e. there is a
limited number of error codes and they sometimes mean slightly different
things depending on which system call was invoked. On the other hand, on
Windows the error space is "flat" and any system call can essentially return
any of the (10000 or so) error values.

My design was as follows:

1) I also use an extra parameter, SysResult &, but also provided a second
parameter called ErrorHandler. This was a stateless class which would define
the behaviour in case of an error. In our programs, I've observed that in
most places return codes would get handled in a very similar fashion, so we
required only a few different variations of ErrorHandlers.

2) Upon a invocation, the function call would get executed and do its thing.
If there was an error, the error handler is to be assigned several values:
  a) The "library" error code. This was a high level translation of the
common error codes into a system independent enumeration (which, btw, was
using a "smart enum" class, which would have an associated error string with
each value)
  b) The system error code. This was the error code of the last function that
caused the problem (e.g. errno)
  c) A string that would describe the error in more detail. IMHO it is very
important for a system to provide accurate and leggible errors which make it
easy to diagnose the source of the problem even if it propagated deep from
within some library (instead of just returning "EACESS". What am I supposed
to do if the system just logs "EACESS"?) This would be a concatenation of
the "Library" error code string, the system error code string (e.g. as
returned by strerror() or FormatMessage()) and any other contextual
information (e.g. The name of the function called, what was the function
trying to do and a suggestion what could be the cause of the problem) This is
because for example when mapping a file, one has to go through several system
calls to return a memory mapped file object, and one would like to know
exactly in what part of that function the call failed.

  d) The error handler would then get invoked with the SysResult and it could
decide based either on the "System independent" or "system specific" error
code, what it should do. The error handler would return one
of "CONTINUE", "RETRY", "FAIL" enumerations. The first and second are, I
think, self-explanatory. The third would cause the surrounding code to trow
an exception, similar to system_error. My default error handlers would do
something sensible like retry on EINTR and and throw on other error
conditions. I also had a non-throwing version.

Assuming there was no exception, it was easy to check the result of an
operation, just like you describe.

This system, while far from perfect, seems to work quite well. It is not
without fault though - here are some problems (some of which, it seems, are
shared by your proposal above):

 i) Assuming that your "high-level" system operation is implemented in terms
of several system calls, what is a good way handling system-specific error
codes? My error handlers had the possibility of handling not only the "high-
level" library errors but also use the low-level system errnos. However, as
you can see, this breaks encapsulation by making assumptions about how the
function is implemented in terms of the underlying syscalls.

 ii) The same issue I described above applies equaly well to other functions
implemented in terms of several "high-level" (library) system calls. To be
able to use the SysResult effectively, you would need to know which function
generated it, again breaking encapsulation.

What do you think?


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