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Unix Domain Sockets

From the programmer’s perspective there are two essential differences between using a Unix domain socket and an TCP/IP socket. First, the address of the socket is a path on the filesystem, rather than a tuple containing servername and port. Second, the node created in the filesystem to represent the socket persists after the socket is closed, and needs to be removed each time the server starts up. The echo server example from earlier can be updated to use UDS by making a few changes in the setup section.

import socket
import sys
import os

server_address = './uds_socket'

# Make sure the socket does not already exist
try:
    os.unlink(server_address)
except OSError:
    if os.path.exists(server_address):
        raise

The socket needs to be created with address family AF_UNIX.

# Create a UDS socket
sock = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)

Binding the socket and managing the incomming connections works the same as with TCP/IP sockets.

# Bind the socket to the port
print >>sys.stderr, 'starting up on %s' % server_address
sock.bind(server_address)

# Listen for incoming connections
sock.listen(1)

while True:
    # Wait for a connection
    print >>sys.stderr, 'waiting for a connection'
    connection, client_address = sock.accept()
    try:
        print >>sys.stderr, 'connection from', client_address

        # Receive the data in small chunks and retransmit it
        while True:
            data = connection.recv(16)
            print >>sys.stderr, 'received "%s"' % data
            if data:
                print >>sys.stderr, 'sending data back to the client'
                connection.sendall(data)
            else:
                print >>sys.stderr, 'no more data from', client_address
                break
            
    finally:
        # Clean up the connection
        connection.close()

The client setup also needs to be modified to work with UDS. It should assume the filesystem node for the socket exists, since the server creates it by binding to the address.

import socket
import sys

# Create a UDS socket
sock = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)

# Connect the socket to the port where the server is listening
server_address = './uds_socket'
print >>sys.stderr, 'connecting to %s' % server_address
try:
    sock.connect(server_address)
except socket.error, msg:
    print >>sys.stderr, msg
    sys.exit(1)

Sending and receiving data works the same way in the UDS client as the TCP/IP client from before.

try:
    
    # Send data
    message = 'This is the message.  It will be repeated.'
    print >>sys.stderr, 'sending "%s"' % message
    sock.sendall(message)

    amount_received = 0
    amount_expected = len(message)
    
    while amount_received < amount_expected:
        data = sock.recv(16)
        amount_received += len(data)
        print >>sys.stderr, 'received "%s"' % data

finally:
    print >>sys.stderr, 'closing socket'
    sock.close()

The program output is mostly the same, with appropriate updates for the address information. From the server:

$ python ./socket_echo_server_uds.py

starting up on ./uds_socket
waiting for a connection
connection from
received "This is the mess"
sending data back to the client
received "age.  It will be"
sending data back to the client
received " repeated."
sending data back to the client
received ""
no more data from
waiting for a connection

and from the client:

$ python socket_echo_client_uds.py

connecting to ./uds_socket
sending "This is the message.  It will be repeated."
received "This is the mess"
received "age.  It will be"
received " repeated."
closing socket

Permissions

Since the UDS socket is represented by a node on the filesystem, standard filesystem permissions can be used to control access to the server.

$ ls -l ./uds_socket
srwxr-xr-x  1 dhellmann  dhellmann  0 Sep 20 08:24 ./uds_socket

$ sudo chown root ./uds_socket

$ ls -l ./uds_socket
srwxr-xr-x  1 root  dhellmann  0 Sep 20 08:24 ./uds_socket

Running the client as a user other than root now results in an error because the process does not have permission to open the socket.

$ python socket_echo_client_uds.py

connecting to ./uds_socket
[Errno 13] Permission denied

Communication Between Parent and Child Processes

The socketpair() function is useful for setting up UDS sockets for interprocess communication under Unix. It creates a pair of connected sockets that can be used to communicate between a parent process and a child process after the child is forked.

import socket
import os

parent, child = socket.socketpair()

pid = os.fork()

if pid:
    print 'in parent, sending message'
    child.close()
    parent.sendall('ping')
    response = parent.recv(1024)
    print 'response from child:', response
    parent.close()

else:
    print 'in child, waiting for message'
    parent.close()
    message = child.recv(1024)
    print 'message from parent:', message
    child.sendall('pong')
    child.close()

By default, a UDS socket is created, but the caller can also pass address family, socket type, and even protocol options to control how the sockets are created.

$ python socket_socketpair.py

in parent, sending message
response from child: pong
in child, waiting for message
message from parent: ping

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