CODEWITHSUNDEEP
pythonsocketsselect
luc
luc

Reputation: 43096

How to empty a socket in python?

I need to empty the data on a socket (making sure that there is nothing to receive). Unfortunately, there is no function for this in the python socket module.

I've implemented something this way:

def empty_socket(sock):
    """remove the data present on the socket"""
    input = [sock]
    while 1:
        inputready, o, e = select.select(input,[],[], 0.0)
        if len(inputready)==0: break
        for s in inputready: s.recv(1)

What do you think? Is there a better way to do that?

Update: I don't want to change the socket timeout. What's why i prefer a select to a read.

Update: The original question was using the 'flush' term. It seems that 'empty' is a better term.

Update - 2010-02-27 : I've noticed a bug when the pair has closed. The inputready is always filled with the sockets. I fixed that by adding a maximum number of loops. Is there a better fix?

Upvotes: 17

Views: 63034

Answers (5)

Vinay Sajip
Vinay Sajip

Reputation: 99385

Using select.select is good practice, as indicated in the Socket Programming HOWTO. You'll need to set the socket as non-blocking, using sock.setblocking(0).

Just a comment about nomenclature: flush is normally associated with output operations.

Upvotes: 5

Aamir Shaikh
Aamir Shaikh

Reputation: 403

Cant we just keep on reading until buffer is empty?

def clear_buffer(sock):
    try:
        while sock.recv(1024): pass
    except:
        pass

Upvotes: 0

darda
darda

Reputation: 4165

For UDP packets, I did the following:

  1. After creating the socket, setting options, and binding, I use socket.settimeout(). Note the documentation for setblocking() gives some information which that of settimeout() doesn't - if you want your socket operations to block, you should just use settimeout() to set the timeout. setblocking() just puts an infinite timeout on it. (I had a bug calling settimeout() followed by setblocking(1).)

  2. My "buffer emptying" function is then just this ("Listener" is my socket):

    def FlushListen(self): while 1: try: PacketBytes = self.__Listener.recv(1024) except: break;

    With a timeout of 1 second, this will read all the UDP packets and then return 1 second after there's no data.

In my case I'm using it to just talk between two programs on the same PC so I could easily lower my timeout but speed is not an issue so I'm fine with this.

According to some of the links others posted, this should work with data streams as well.

Upvotes: 1

Thomas
Thomas

Reputation: 4255

If by "flush" you mean throw away any pending incoming data then you can either use select() like you do, or set the socket to nonblocking and read in a loop until you're out of data.

Also note that (from the Linux manpage):

Under Linux, select() may report a socket file descriptor as "ready for reading", while nevertheless a subsequent read blocks. This could for example happen when data has arrived but upon examination has wrong checksum and is discarded. There may be other circumstances in which a file descriptor is spuriously reported as ready. Thus it may be safer to use O_NONBLOCK on sockets that should not block.

Spurious readiness notification for Select System call

And as has been noted by others, "flush" usually refers to output.

Upvotes: 9

Steef
Steef

Reputation: 34665

Not sure if this will work, but you could attach a file object to the socket's file descriptor and call the flush() method on that file object:

import os

file_obj = os.fdopen(your_socket.fileno())
file_obj.flush()

This won't work in Windows because the descriptor returned by fileno() can't be passed to os.fdopen() in Windows

Upvotes: -1

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