Python program demonstration

sigtest.py

#! /usr/bin/env python3
#
# Demonstrates how a short write can occur when writing to a pipe, even though
# Python normally restarts system calls that fail with EINTR
#
# If you are making small writes (<= 1 page size) to a pipe, the kernel will
# wait to copy any data into the pipe buffer until there is enough space. Thus,
# when the pipe is full and the alarm signal occurs, nothing from the current
# write() call have been written, the syscall will return EINTR, and python (>=
# 3.5) will retry the system call. This mean that the script never "sees" the
# short write
#
# However if the application is writing more than one page of data, the kernel
# will do a partial copy of the data into the pipe buffer. Then, when the alarm
# signal occurs the kernel will return the number of bytes written instead of
# EINTR and python is unable to restart the syscall and will return a short
# write
#
import os
import signal

# If you change this value to <= the kernel pagesize, you'll never get a short
# write. If it's larger, the program will exit with a short write
WRITE_SIZE = 4097

(rfd, wfd) = os.pipe()

wfile = os.fdopen(wfd, "wb", 0)


def handle_alarm(sig, frame):
    print("Got SIGALRM")


signal.signal(signal.SIGALRM, handle_alarm)
signal.alarm(5)

total = 0
while True:
    msg = b"0" * WRITE_SIZE
    l = wfile.write(msg)

    total += l
    if l < len(msg):
        print("Short write of %d bytes" % l)
        break
    print("Wrote %d bytes" % total)