Why do I get a string from a socket until the \ n newline escape sequence is executed in python?

Question

It is supposed to get 2 strings of a string from a tcp socket, but in fact I only get a string to a new line.

socket = socket(AF_INET,SOCK_STREAM) socket.connect((ip,port)) data = socket.recv(1024) print "%s" % data 

if the call to the receive function now gets the second part of the line after the interrupt line. This is not what I really want. It should return the entire string from the first call. When I use nc to connect to the server, I get the line usually at the beginning of the connection. Why is this happening?

Answer 1

Since TCP is a streaming protocol (as specified in SOCK_STREAM ), there are no fixed boundaries or message packets, and you can never be sure that you get everything with just one recv call. You just need to call recv in the loop, add to the buffer, and exit the loop if you think you have enough. Most text protocols use either a new line to indicate that the reading is done, and to do something with the received data. Other protocols use different characters or byte sequences.

In your case, if there is no special character saying that the end of the current data has been reached, you have two solutions:

• Use timeout: if new data has not been received for some time, print it.

• Non-blocking sockets: just read through the loop by adding data to the internal buffer. When the recv call throws an error with errno equal to errno.EWOULDBLOCK , then there is no longer any need to read and print the received data.

Alternative 2, along with the select package, is probably the best way to go.

Edit

Here is a simple example of what I mean. This probably won't work, and you need some kind of customization, but hopefully this is enough for you.

 # Need to import: package socket, package select, package errno # Create socket and connect to server # Make the socket non-blocking (see http://docs.python.org/library/socket.html#socket.socket.setblocking) socket.setblocking(0) run_main_loop = True while run_main_loop: # Wait for events... read_ready, _, _ = select.select([socket], None, None) if socket in read_ready: # The socket have data ready to be received buffer = '' continue_recv = True while continue_recv: try: # Try to receive som data buffer += socket.recv(1024) except socket.error, e: if e.errno != errno.EWOULDBLOCK: # Error! Print it and tell main loop to stop print 'Error: %r' % e run_main_loop = False # If e.errno is errno.EWOULDBLOCK, then no more data continue_recv = False # We now have all data we can in "buffer" print '%r' % buffer socket.close()