Using non-blocking iNotify [duplicate]

Question

My objective: To monitor a text file for modification without the monitor blocking my program, but forming part of a loop (so checking sequentially) instead.

My head says: Either find a way to run iNotify in non-blocking mode, or thread iNotfiy.

I tried the non-blocking way, and disabled O_NONBLOCK for my iNotify instance using the following command:

fcntl (fd, F_SETFL, fcntl (fd, F_GETFL) | O_NONBLOCK);

However, when I do this and I then attempt:

length = read(fd, buffer, BUF_LEN);

It keeps on telling me that for read, a resource is temporarily unavailable.

Can anyone give me some tips on how to achieve what I want to do? Does not need to be this method, but I need the functionality as I am editing a text file with a webserver and want to read modifications into my C++ program to update variables.

Thanks in advance!

Answer 1

EAGAIN (resource temporarily unavailable) is the expected error status if there is no data available on the file descriptor being read when the file descriptor is set in non-blocking mode. Since you are using a polling loop, you can just try to read again on the next iteration.

Alternatively, you can attempt to use the signal-driven I/O notification for the inotify file descriptor:

Since Linux 2.6.25, signal-driven I/O notification is available for inotify file descriptors; see the discussion of F_SETFL (for setting the O_ASYNC flag), F_SETOWN, and F_SETSIG in fcntl(2). The siginfo_t structure (described in sigaction(2)) that is passed to the signal handler has the following fields set: si_fd is set to the inotify file descriptor number; si_signo is set to the signal number; si_code is set to POLL_IN; and POLLIN is set in si_band.