Newsgroups: fa.linux.kernel
From: Linus Torvalds <torvalds@osdl.org>
Subject: Re: BUG (non-kernel), can hurt developers.
Original-Message-ID: <Pine.LNX.4.58.0311261021400.1524@home.osdl.org>
Date: Wed, 26 Nov 2003 18:31:27 GMT
Message-ID: <fa.j4rspt8.1fi8l8o@ifi.uio.no>

On Wed, 26 Nov 2003, Richard B. Johnson wrote:
>
> Note  to hackers. Even though this is a lib-c bug

It's not.

It's a bug in your program.

You can't just randomly use library functions in signal handlers. You can
only use a very specific "signal-safe" set.

POSIX lists that set in 3.3.1.3 (3f), and says

	"All POSIX.1 functions not in the preceding table and all
	 functions defined in the C standard {2} not stated to be callable
	 from a signal-catching function are considered to be /unsafe/
	 with respect to signals. .."

typos mine.

The thing is, they have internal state that makes then non-reentrant (and
note that even the re-entrant ones might not be signal-safe, since they
may have deadlock issues: being thread-safe is _not_ the same as being
signal-safe).

In other words, if you want to do complex things from signals, you should
really just set a flag (of type "sigatomic_t") and have your main loop do
them. Or you have to be very very careful and only use stuff that is
defined to be signal-safe (mainly core system calls - stuff like <stdio.h>
is right out).

		Linus