CODEWITHSUNDEEP
cnetwork-programmingepoll
DiscobarMolokai
DiscobarMolokai

Reputation: 544

networking with epoll: error handling

I am working on a networking application, written in C, using the Linux epoll facility. I am indeed implementing my own event loop, and yes, I know there are libraries that do this for me. This is just an exercise, however, and I _want_ to implement the event loop myself.

I was wondering about how to tackle error handling, specifically handling errors returned by epoll system calls. Should I really check for each return value? Because, it seems to me that some of the errors returned by e. g. epoll_ctl are logic errors rather than runtime errors; I mean, the only time I saw epoll_ctl fail was when I accidentally fed it an illegal epoll file descriptor or something else that only occurs when you write incorrect code. So my question is, can epoll_ctl actually return a 'runtime error', like, say, write does when a pipe is broken? If so, is it something I should check for? I do not intend to check for errors that indicate the kernel has run out of resources or stuff like that (in that case, what could one even do?). Additional question: can fcntl produce a runtime error of some sort when it is only used to set some flags (O_NONBLOCK)?

Upvotes: 0

Views: 1956

Answers (1)

abligh
abligh

Reputation: 25129

Yes, epoll can return a runtime error.

From the manpage:

ERRORS

EBADF epfd or fd is not a valid file descriptor.

EEXIST op was EPOLL_CTL_ADD, and the supplied file descriptor fd is already registered with this epoll instance.

EINVAL epfd is not an epoll file descriptor, or fd is the same as epfd, or the requested operation op is not supported by this interface.

ENOENT op was EPOLL_CTL_MOD or EPOLL_CTL_DEL, and fd is not registered with this epoll instance.

ENOMEM There was insufficient memory to handle the requested op control operation.

ENOSPC The limit imposed by /proc/sys/fs/epoll/max_user_watches was encountered while trying to register (EPOLL_CTL_ADD) a new file descriptor on an epoll instance. See epoll(7) for further details.

EPERM The target file fd does not support epoll.

As you can see, ENOMEM and ENOSPC are runtime errors. Depending on your logic, EPERM might also be a runtime error (e.g. if you are incorporating stdin into epoll and there is a redirect).

However, good practice suggests you check anyway - far better to fail early if you have a logic error.

Upvotes: 2

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