CODEWITHSUNDEEP
clinux
RomaValcer
RomaValcer

Reputation: 2956

Correctly processing Ctrl-C when using poll()

I am making a program, that runs like a server, so it is constantly running poll. I need to process both Ctrl-C and Ctrl-D. And while Ctrl-D is pretty easy to work with when using poll (you just also poll for POLLIN on stdin), I cannot come up with a pretty solution for signals. Do I need to create a dummy file to which my signal handler will write something when it's time to exit, or would pipes fit this purpose nicely?

Upvotes: 4

Views: 2362

Answers (2)

Aconcagua
Aconcagua

Reputation: 25526

signalfd is what you are after - connect it to SIG_INT and you can poll for ctrl+c – see the example in the link provided (quite down the page – actually, they are catching ctrl+c there...).

Upvotes: 2

Basile Starynkevitch
Basile Starynkevitch

Reputation: 1

As commented by Dietrich Epp, a usual way of handling this is the "pipe to self" trick. First, at initialization time, you set up a pipe(7): you'll call pipe(2) and you keep both read and write file descriptors of that pipe in some (e.g. global) data. Your signal handler would just write(2) onto the write-end fd some bytes (perhaps a single 0 byte ...). And your event loop around poll(2) (or the older select(2), etc...) would react by read(2)-ing bytes when the read-end file descriptor has some data.

This pipe to self trick is common and portable to all POSIX systems, and recommended e.g. by Qt.

The signalfd(2) system call is Linux specific (e.g. you don't have that on MacOSX). Some old Linux kernels might not have it.

Be aware that the set of functions usable inside a signal handler is limited to async-signal-safe functions - so you are allowed to use write(2) but forbidden to use fprintf or malloc inside a signal handler. Read carefully signal(7) and signal-safety(7).

Upvotes: 5

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