Os Io Select Poll

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

Fix select/poll errors when select fd set size exceeded FD_SETSIZE causing buffer overflow.

Quick Fix

Wrong

import select
# select() limited to FD_SETSIZE (1024 typically)
# If fd >= 1024, select may overflow internal fd_set buffer!

File descriptor >= 1024 corrupts select's internal bitmask. Undefined behavior.

Right

import select
# Use poll() instead of select() - no FD_SETSIZE limit:
poller=select.poll()
poller.register(fd, select.POLLIN)
events=poller.poll(timeout=1000)  # milliseconds
for fd,event in events:
    if event & select.POLLIN:
        data=os.read(fd, 4096)
# Or use epoll() for large number of fds:
epoll=select.epoll()
epoll.register(fd, select.EPOLLIN)
events=epoll.poll(timeout=1)
# poll works for up to ~millions of fds. epoll scales better with thousands+.

poll() handles any number of fds. No FD_SETSIZE limit.

Prevention

select() limited to FD_SETSIZE (1024). Use poll() or epoll() for many fds.

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