python Non-block read file

Question

I want to read a file with non-block mode. So i did like below

import fcntl
import os

fd = open(\"./filename\", \"r\")
flag = fcntl.fcntl(fd.fileno(), fcntl         


        

Answer 1

O_NONBLOCK is a status flag, not a descriptor flag. Therefore use F_SETFL to set File status flags, not F_SETFD, which is for setting File descriptor flags.

Also, be sure to pass an integer file descriptor as the first argument to fcntl.fcntl, not the Python file object. Thus use

f = open("/tmp/out", "r")
fd = f.fileno()
fcntl.fcntl(fd, fcntl.F_SETFL, flag | os.O_NONBLOCK)

rather than

fd = open("/tmp/out", "r")
...
fcntl.fcntl(fd, fcntl.F_SETFD, flag | os.O_NONBLOCK)
flag = fcntl.fcntl(fd, fcntl.F_GETFD)

---

import fcntl
import os

with open("/tmp/out", "r") as f:
    fd = f.fileno()
    flag = fcntl.fcntl(fd, fcntl.F_GETFL)
    fcntl.fcntl(fd, fcntl.F_SETFL, flag | os.O_NONBLOCK)
    flag = fcntl.fcntl(fd, fcntl.F_GETFL)
    if flag & os.O_NONBLOCK:
        print "O_NONBLOCK!!"

prints

O_NONBLOCK!!