Recv_fds() Function Of Socket Module In Python

Overview:

  • The socket module function recv_fds() receives file descriptors sent from a Unix Domain Socket. To send file descriptors refer to send_fds().

  • The File Descriptors are reconstructed by the Unix kernel for them to work as expected at the process where they are received.

Example - Client:

# Example Python program that requests for 
# file descriptors from a server using Unix domain sockets
# and the function socket.recv_fds() 
# to read & print the contents from the files.
import socket
import os

# Create a Unix Domain Socket
client    = socket.socket(socket.AF_UNIX)

# Bind the Unix Domain socket to a path
path     = "/Users/vinodh/pprogs/recv_fds"
client.connect(path)

# Send the file names for which the descriptors
# are needed from server
client.send("./courses.txt,./campusinfo.txt,./contacts.txt".encode())

# Receive the file descriptors
buf_size = 1024
tup = socket.recv_fds(client, buf_size, 3)

# File descriptor list
fdList = tup[1]
print(tup)

# Read from each file descriptor
# and print the contents
bytesCount = 16
for fd in fdList:
    print(os.read(fd, bytesCount))

Output:

(b'Sending the files you asked for->', [4, 5, 6], 0, None)

b'Course1\nCourse2\n'

b'Campus1\nCampus2'

b'Contacts\n'

Example - Server:

# Example Python program that sends back file 
# descriptors as requested by clients
# using Unix Domain Protocol
import socket
import os

# Create a Unix Domain Socket
svr     = socket.socket(socket.AF_UNIX)

# Bind the Unix Domain Socket to a path
path     = "/Users/vinodh/pprogs/recv_fds"
os.unlink(path)
svr.bind(path)

# Move the socket to listen state
svr.listen()

# Accept connections from clients
while(True):
    print("Server listening for file (descriptor) requests:")
    clSocket, address = svr.accept()

    # Get the names of the files
    fileNameReceived     = clSocket.recv(1024).decode()
    fileNames             = fileNameReceived.split(",")
    print(fileNames)

    fdList = []
    for fileName in fileNames:
        print(fileName)
        fd = os.open(fileName, os.O_RDONLY)
        fdList.append(fd)
    
    # Send the file descriptors
    msg     = "Sending the files you asked for->" 
    msgb    = bytes(msg, 'ascii')
    socket.send_fds(clSocket, [msgb], fdList, 3, None)        
    clSocket.close() 

 

Output:

Server listening for file (descriptor) requests:

['./courses.txt', './campusinfo.txt', './contacts.txt']

./courses.txt

./campusinfo.txt

./contacts.txt

Server listening for file (descriptor) requests:


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