C++ catch if file closed by another program while polling

Question

I have a program that is polling a file for changes. However, if I open another Linux console and delete the file that it is polling, while it is polling, the program will continue to poll forever. Is there a way to catch this?

struct pollfd pollFileDescriptor;
pollFileDescriptor.fd = GetFileDescriptor(...returns fd
pollFileDescriptor.events = POLLPRI | POLLERR | POLLNVAL | POLLHUP;

if (poll(&pollFileDescriptor, 1, -1) > 0)
{
     ...
}

I have tried all the various events, checking them against revents, as well as just putting a printf within the polling if statement just to see if it enters the code block with no luck. It seems to hang on poll

score 2 · Answer 1 · answered May 06 '15 at 00:04

2

Deleting the file removes its reference from the filesystem. But file descriptor is another reference. As long as there is any reference, the file is not actually destroyed so no change to the file data that can be reported to the polling happened. (The file can also be hardlinked, that is another reference whose deletion would not be reported to you.)

You can watch the directory for changes. Linux provides inotify and dnotify for that.

answered May 06 '15 at 00:04

StenSoft

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Thanks for the response. If you use inotify to see when the file is removed, the polling will still have the thread blocked though, right? – user2654735 May 06 '15 at 00:08
Then you need a positive value for `timeout` – Bastien Durel May 06 '15 at 15:42
@user2654735 inotify creates another FD that you can poll. Simply poll both FDs. – StenSoft May 06 '15 at 16:03

C++ catch if file closed by another program while polling

1 Answers1