Read from socket: Is it guaranteed to at least get x bytes?

Question

I have a rare bug that seems to occur reading a socket.

It seems, that during reading of data sometimes I get only 1-3 bytes of a data package that is bigger than th

Answer 1

The simple answer to your question, "Read from socket: Is it guaranteed to at least get x bytes?", is no. Look at the doc strings for these socket methods:

>>> import socket
>>> s = socket.socket()
>>> print s.recv.__doc__
recv(buffersize[, flags]) -> data

Receive up to buffersize bytes from the socket.  For the optional flags
argument, see the Unix manual.  When no data is available, block until
at least one byte is available or until the remote end is closed.  When
the remote end is closed and all data is read, return the empty string.
>>> 
>>> print s.settimeout.__doc__
settimeout(timeout)

Set a timeout on socket operations.  'timeout' can be a float,
giving in seconds, or None.  Setting a timeout of None disables
the timeout feature and is equivalent to setblocking(1).
Setting a timeout of zero is the same as setblocking(0).
>>> 
>>> print s.setblocking.__doc__
setblocking(flag)

Set the socket to blocking (flag is true) or non-blocking (false).
setblocking(True) is equivalent to settimeout(None);
setblocking(False) is equivalent to settimeout(0.0).

From this it is clear that recv() is not required to return as many bytes as you asked for. Also, because you are calling settimeout(10.0), it is possible that some, but not all, data is received near the expiration time for the recv(). In that case recv() will return what it has read - which will be less than you asked for (but consistenty < 4 bytes does seem unlikely).

You mention datagram in your question which implies that you are using (connectionless) UDP sockets (not TCP). The distinction is described here. The posted code does not show socket creation so we can only guess here, however, this detail can be important. It may help if you could post a more complete sample of your code.

If the problem is reproducible you could disable the timeout (which incidentally you do not seem to be handling) and see if that fixes the problem.