Re: [Boost-users] [iostreams] Error-handling with filters

19 Jan 2006

      Paul Giaccone wrote:
...
How can you deal with a corrupt file when using filters?
I have a chain of filters that includes a bzip2 and multichar filter 
have written myself.  If the read() function fails to find what it 
expects in the file, it returns 0 (meaning "no characters read").  
Otherwise it returns the number of characters read (which should be 
equal to the number of characters requested in the third function 
parameter) or -1 if the end of the file is reached.
This works fine if the file is as is required, but if it is corrupted in 
any way, the program either hangs or crashes at
if (s->strm != strm) return BZ_PARAM_ERROR;
in bzlib.c, with s as a pointer that cannot be read (set to 0xcdcdcdcd).
I'm working in Microsoft Visual C++ and gcc 3.2.3.
How should this be handled?  I haven't seen anything about it in the 
documentation.  I imagine I should throw an exception instead of 
returning 0 (perhaps an iostreams exception?).  Given that the crash is 
the bzip2 (bzip2 being the first operation done when deserialising the 
file), where would I put the associated catch block?
OK, I throw an exception if my filter finds an error, but the error is 
clearly being caused at the bzip stage.  How can I trap that?

Here's a summary of what I am doing:

Filter chain:

    try
    {
        boost::iostreams::filtering_istream in;
        in.push(boost::iostreams::bzip2_decompressor());
        in.push(InputEncryptionFilter());
        in.push(boost::iostreams::bzip2_decompressor());
        in.push(boost::iostreams::file_source(filename, BOOST_IOS::in | 
BOOST_IOS::binary));
        assert(in.is_complete());
        boost::archive::xml_iarchive input_archive(in);
        my_data >> BOOST_SERIALIZATION_NVP(my_object);
        return;
    }
    catch (MyException e)
    {
        std::cerr << "Error reading archive file " << filename << ": " 
<< e.what() << std::endl;
    }
    catch (boost::archive::archive_exception e)
    {
        std::cerr << "Error decoding archive file " << filename << ": " 
<< e.what() << std::endl;
    }
    catch (std::ios_base::failure e)
    {
        std::cerr << "Error reading from file " << filename << ": " << 
e.what() << std::endl;
    }

I am deliberatively providing the above with a corrupted file that bzip2 
will not be able to understand.  None of the "catch"es above are executed.

So how can I catch a problem at the bzip2 stage?

Paul Giaccone