Sockets – Non Blocking recv() in C Sockets

cnonblockingrecvsendsockets

I am using an infinite loop in sockets in which if it receives some data it should receive it or if it wants to send data it sends. Something like given below. I am using select. I have only one socket sd.

fd_set readsd;
int maxsd = readsd +1;
// all those actions of setting maxsd to the maximum fd +1 and FDSETing the FDs.
while(1)
{
     FD_ZERO(&read_sd);
     FD_SET(sd, &read_sd);
     if(FD_ISSET(sd, &readsd))
     {
       //recv call
     }
     else
     {
       //send call
     }
}

As far as I know, select selects one of the socket descriptors on which data arrives first. But here I have only one socket, and I want to recv if there is some data or I want to send otherwise.

In that case, is the code given above fine? Or there is some other option for me which I don't know about?

Best Answer

In that case, is the code given above fine ?

I don't see any call to select. Also, if “maxsd” is designed to be the first argument of select, its value is wrong : it must be the bigest file descriptor +1. Anyway, you could simply call recv with the flag MSG_DONTWAIT, in which case it will return an error if there is no data to read.

Related Solutions

Python – A non-blocking read on a subprocess.PIPE in Python

fcntl, select, asyncproc won't help in this case.

A reliable way to read a stream without blocking regardless of operating system is to use Queue.get_nowait():

import sys
from subprocess import PIPE, Popen
from threading  import Thread

try:
    from queue import Queue, Empty
except ImportError:
    from Queue import Queue, Empty  # python 2.x

ON_POSIX = 'posix' in sys.builtin_module_names

def enqueue_output(out, queue):
    for line in iter(out.readline, b''):
        queue.put(line)
    out.close()

p = Popen(['myprogram.exe'], stdout=PIPE, bufsize=1, close_fds=ON_POSIX)
q = Queue()
t = Thread(target=enqueue_output, args=(p.stdout, q))
t.daemon = True # thread dies with the program
t.start()

# ... do other things here

# read line without blocking
try:  line = q.get_nowait() # or q.get(timeout=.1)
except Empty:
    print('no output yet')
else: # got line
    # ... do something with line

Linux TCP/IP Non-blocking send for socket stream..what happens to the TCP recv buffer

Yes. It is quite possible (and in fact likely) that when you get EWOULDBLOCK, some data you have already sent has not yet been read by the recieving application. This buffered data will be available to the next read on the socket.

This means that if your reciever then sends a "Ooops, don't send any more" message back to the sender, the sender can't act on that message and "un-send" the data. Once it's been passed to write()/send(), it's on its way and can't be recalled.

Your receiver will have to handle this eventuality by reading out the data it's no longer interested in and discarding it, which will mean you'll need some kind of transaction delimiters in your data stream.

Best Answer

Related Solutions

Python – A non-blocking read on a subprocess.PIPE in Python

Linux TCP/IP Non-blocking send for socket stream..what happens to the TCP recv buffer

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