[Boost-users] [iostreams] filtering streams in a chain are continually writing the same bytes

21 Feb 2017

      Perhaps someone can clarify that my use of multichar_output_filter is valid.

When I debug this, even though my write method is returning 4 bytes for
every write, the caller of my output filter keeps providing me the same
four bytes after the first sequence of 4 bytes.

This is boot version 1.62. Here is compiling code that illustrates:

class fixed_size_output_filter : public
boost::iostreams::multichar_output_filter
{
public:

fixed_size_output_filter(std::streamsize size = 4) : mSize(size) {}

template<typename Sink>
std::streamsize write(Sink &sink, const char *s, std::streamsize n)
{
std::streamsize bytesAttempted = std::min(mSize, n);
std::streamsize bytesActual = boost::iostreams::write(sink, s,
bytesAttempted);
return bytesActual;
}

private:

std::streamsize mSize;

};

BOOST_AUTO_TEST_CASE(FixedSizeOutputFilter)
{
#define BYTES 500

char sink_buffer[BYTES];
array_sink array_sink(sink_buffer, sizeof(sink_buffer));

std::streampos os_pos;
{
filtering_ostream os;
os.push(fixed_size_output_filter());
os.push(array_sink);

char buffer[BYTES];
for(char i = 0; i < sizeof(buffer); i++)
buffer[i] = i;

os.write(buffer, sizeof(buffer));
}

for(char i = 0; i < sizeof(sink_buffer); i++)
BOOST_CHECK(i == sink_buffer[i]);
}

The actual bytes that end up in the sink_buffer are as follows:

0, 1, 2, 3, 4, 5, 6, 7, 4, 5, 6, 7, 4, 5, 6, 7, 4, 5, 6, 7,
....uninitialized bytes follow through the rest of the buffer.

It would appear that the indirect_streambuf in use by the filter chain is
incorrectly flushing the bytes.

Is my understanding of the library flawed? Does this appear to be a bug?
Any help is greatly appreciated.

-- 
Andrew Ames
aames@tanzle.com