Os Io Sendfile

In this tutorial, you'll learn about How to Fix sendfile Errors. We cover key concepts, practical examples, and best practices.

Fix sendfile errors when data copied between kernel and user unnecessarily.

Quick Fix

Wrong

import os
# Reading file then sending to socket:
data=open('file.bin','rb').read()
sock.send(data)  # file -> userspace -> socket buffer (2 copies!)

Data travels: disk -> page cache -> userspace -> socket buffer. Two copies.

Right

import os, socket
# Use sendfile for zero-copy:
s=socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(('example.com', 80))
with open('file.bin','rb') as f:
    os.sendfile(s.fileno(), f.fileno(), 0, 65536)  # file -> socket buffer directly
# For Python 3.8+:
import asyncio
async def send_file():
    reader,writer=await asyncio.open_connection('example.com',80)
    with open('file.bin','rb') as f:
        await writer.sendfile(f.fileno(), 0, 65536)

sendfile transfers file to socket in kernel. Zero-copy. No userspace buffer involved.

Prevention

sendfile copies between file descriptors entirely in kernel. Zero-copy file transfer.

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