Os Syscall Fcntl

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

Fix fcntl errors when fcntl without checking return value for errors.

Quick Fix

Wrong

import fcntl, os
f=open('file.txt','r')
fcntl.fcntl(f, fcntl.F_SETFL, fcntl.O_NONBLOCK)  # may fail!

fcntl may return -1 on error (bad fd, EACCES, etc). Not checked.

Right

import fcntl, os, errno
f=open('file.txt','r')
try:
    flags=fcntl.fcntl(f, fcntl.F_GETFL)  # get current flags
    flags|=os.O_NONBLOCK
    result=fcntl.fcntl(f, fcntl.F_SETFL, flags)
    if result==-1: raise OSError('fcntl failed')
    print('Set non-blocking')
except OSError as e:
    if e.errno==errno.EACCES:
        print('Permission denied')
    elif e.errno==errno.EBADF:
        print('Bad file descriptor')
    else: raise
finally:
    f.close()

fcntl result checked. Get flags first, modify, set. Error handling with errno.

Prevention

Always check fcntl return value. Use GETFL then SETFL pattern. Handle errors with errno.

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