PRB: CArchive::Read Might Hang When Using CSocketFile

• The Microsoft Foundation Classes (MFC), included with:
  • Microsoft Visual C++, 32-bit Editions, versions 2.1, 2.2, 4.0, 4.1, 4.2, 5.0, 6.0

SYMPTOMS

When reading data that uses CArchive::Read and the file is a CSocketFile, certain buffer sizes and data sizes are more likely to cause a hang in CSocket::Receive at the end of transmission when the transmission isn't closed.

CAUSE

CArchive sometimes tries to maintain data in its buffer even if all of the data requested has been read. This causes an extra CSocketFile::Read call when no data is available on the socket file. The Read call hangs until more data appears or until the socket connection is closed.

RESOLUTION

The underlying socket of the socket file should be closed at the end of transmitting the data.

STATUS

This behavior is by design.

MORE INFORMATION

CArchive provides a buffered mechanism to read data from a file, which can be based on CSocketFile as well as CFile. The algorithm for CArchive::Read is implemented so that an extra call to a CFile derived class (for example, CSocketFile) Read function is made even if all of the needed data is present. For disk-based files, this presents no problem because EOF is reached and the call to Read returns immediately. For CsocketFile-based files, either the socket must be closed or more data needs to be sent for the Read call to return.

The hang is more likely to occur if the transmitted data size is a multiple of the buffer size (4k) because the last byte needed is more likely to be the last byte in the current buffer. This results in an empty buffer and causes another Read operation even though there is no more data to be read.

Normally, if the required data to be read is less than the buffer size, the Read operation returns with less than the requested size (the buffer size), however in this case, it does not cause CArchive to replenish its buffer.

The proper way to avoid this situation is to close the socket connection. (c) Microsoft Corporation 1998, All Rights Reserved. Contributions by Adam Kim, Microsoft Corporation.

Additional query words: Write hangs CAsyncSocket

Keywords : kbMFC kbVC kbVC210 kbVC220 kbVC400 kbVC410 kbVC420 kbVC500 kbVC600 kbWinsock
Version : WINNT:2.1,2.2,4.0,4.1,4.2,5.0,6.0
Platform : winnt
Issue type : kbprb