Sending Binary Data¶

Sockets transmit streams of bytes. Those bytes can contain text messages encoded to bytes, as in the previous examples, or they can be made up of binary data that has been packed into a buffer with struct to prepare it for transmission.

This client program encodes an integer, a string of two characters, and a floating point value into a sequence of bytes that can be passed to the socket for transmission.

import binascii
import socket
import struct
import sys

# Create a TCP/IP socket
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
server_address = ('localhost', 10000)
sock.connect(server_address)

values = (1, b'ab', 2.7)
packer = struct.Struct('I 2s f')
packed_data = packer.pack(*values)

print('values =', values)

try:
    # Send data
    print('sending {!r}'.format(binascii.hexlify(packed_data)))
    sock.sendall(packed_data)
finally:
    print('closing socket')
    sock.close()

When sending multi-byte binary data between two systems, it is important to ensure that both sides of the connection know what order the bytes are in and how to assemble them back into the correct order for the local architecture. The server program uses the same Struct specifier to unpack the bytes it receives so they are interpreted in the correct order.

import binascii
import socket
import struct
import sys

# Create a TCP/IP socket
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
server_address = ('localhost', 10000)
sock.bind(server_address)
sock.listen(1)

unpacker = struct.Struct('I 2s f')

while True:
    print('\nwaiting for a connection')
    connection, client_address = sock.accept()
    try:
        data = connection.recv(unpacker.size)
        print('received {!r}'.format(binascii.hexlify(data)))

        unpacked_data = unpacker.unpack(data)
        print('unpacked:', unpacked_data)

    finally:
        connection.close()

Running the client produces:

$ python3 source/socket/socket_binary_client.py
values = (1, b'ab', 2.7)
sending b'0100000061620000cdcc2c40'
closing socket

And the server shows the values it receives:

$ python3 socket_binary_server.py

waiting for a connection
received b'0100000061620000cdcc2c40'
unpacked: (1, b'ab', 2.700000047683716)

waiting for a connection

The floating point value loses some precision as it is packed and unpacked, but otherwise the data is transmitted as expected. One thing to keep in mind is that depending on the value of the integer, it may be more efficient to convert it to text and then transmit, instead of using struct. The integer 1 uses one byte when represented as a string, but four when packed into the structure.