Missing one-pixel row while transfering image with TCP socket

Question

I\'m facing strange error right now, I have python script, that is sending/receiving data using TCP socket, everything works fine, but when I\'m trying to download image wit

Answer 1

The problem was with sending 'complete' string to the server, because the script had not enough time to get all bytes from the image. So one way to fix this is to add sleep(0.2) to the script.

TCP is a stream protocol with no message boundaries. This means that multiple sends can be received in one recv call, or one send can be received in multiple recv calls.

The delay work-around may not work reliably. You need to delimit messages in the stream.

There are 2 common ways of delimiting messages in a stream:

1. Prefix messages with a header.
2. End messages with a suffix.

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Since you are sending binary data any suffix can naturally be present in the payload. Unless the suffix is longer than the payload, which isn't practical.

Hence, what you may like to do here is prefix a fixed-size header to your payload. In this particular case, a header with a 4-byte binary file length would suffice. E.g.:

file_len = os.stat(filename).st_size
msg_header = struct.pack('<I', file_len)
self.s.sendall(msg_header)

The receiver needs to read the header first:

msg_header = self.s.recv(4)
while len(msg_header) != 4:
    msg_header += self.s.recv(4 - len(msg_header))
file_len = struct.unpack('<I', msg_header)

And then read exactly file_len from the socket.

Knowing the size of the file being received also allows you to preallocate the buffer to avoid memory reallocations and/or preallocate the entire file to minimize disk fragmentation or avoid out of disk space error after the file transfer has started.