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/* POSIX compatible signal blocking.
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Copyright (C) 2008-2011 Free Software Foundation, Inc.
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Written by Eric Blake <ebb9@byu.net>, 2008.
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This program is free software: you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 3 of the License, or
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(at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program. If not, see <http://www.gnu.org/licenses/>. */
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/* This implementation of sigaction is tailored to Woe32 behavior:
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signal() has SysV semantics (ie. the handler is uninstalled before
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it is invoked). This is an inherent data race if an asynchronous
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signal is sent twice in a row before we can reinstall our handler,
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but there's nothing we can do about it. Meanwhile, sigprocmask()
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is not present, and while we can use the gnulib replacement to
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provide critical sections, it too suffers from potential data races
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in the face of an ill-timed asynchronous signal. And we compound
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the situation by reading static storage in a signal handler, which
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POSIX warns is not generically async-signal-safe. Oh well.
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- We don't implement SA_NOCLDSTOP or SA_NOCLDWAIT, because SIGCHLD
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- We don't implement SA_ONSTACK, because sigaltstack() is not present.
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- We ignore SA_RESTART, because blocking Win32 calls are not interrupted
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anyway when an asynchronous signal occurs, and the MSVCRT runtime
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never sets errno to EINTR.
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- We don't implement SA_SIGINFO because it is impossible to do so
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POSIX states that an application should not mix signal() and
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sigaction(). We support the use of signal() within the gnulib
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sigprocmask() substitute, but all other application code linked
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with this module should stick with only sigaction(). */
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/* Check some of our assumptions. */
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#if defined SIGCHLD || defined HAVE_SIGALTSTACK || defined HAVE_SIGINTERRUPT
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# error "Revisit the assumptions made in the sigaction module"
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/* Out-of-range substitutes make a good fallback for uncatchable
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/* On native Windows, as of 2008, the signal SIGABRT_COMPAT is an alias
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for the signal SIGABRT. Only one signal handler is stored for both
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SIGABRT and SIGABRT_COMPAT. SIGABRT_COMPAT is not a signal of its own. */
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#if (defined _WIN32 || defined __WIN32__) && ! defined __CYGWIN__
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# undef SIGABRT_COMPAT
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# define SIGABRT_COMPAT 6
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/* A signal handler. */
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typedef void (*handler_t) (int signal);
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/* Set of current actions. If sa_handler for an entry is NULL, then
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that signal is not currently handled by the sigaction handler. */
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static struct sigaction volatile action_array[NSIG] /* = 0 */;
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/* Signal handler that is installed for signals. */
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sigaction_handler (int sig)
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int saved_errno = errno;
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if (sig < 0 || NSIG <= sig || !action_array[sig].sa_handler)
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/* Unexpected situation; be careful to avoid recursive abort. */
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signal (SIGABRT, SIG_DFL);
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/* Reinstall the signal handler when required; otherwise update the
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bookkeeping so that the user's handler may call sigaction and get
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accurate results. We know the signal isn't currently blocked, or
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we wouldn't be in its handler, therefore we know that we are not
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interrupting a sigaction() call. There is a race where any
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asynchronous instance of the same signal occurring before we
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reinstall the handler will trigger the default handler; oh
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handler = action_array[sig].sa_handler;
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if ((action_array[sig].sa_flags & SA_RESETHAND) == 0)
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signal (sig, sigaction_handler);
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action_array[sig].sa_handler = NULL;
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/* Block appropriate signals. */
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mask = action_array[sig].sa_mask;
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if ((action_array[sig].sa_flags & SA_NODEFER) == 0)
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sigaddset (&mask, sig);
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sigprocmask (SIG_BLOCK, &mask, &oldmask);
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/* Invoke the user's handler, then restore prior mask. */
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sigprocmask (SIG_SETMASK, &oldmask, NULL);
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/* Change and/or query the action that will be taken on delivery of
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signal SIG. If not NULL, ACT describes the new behavior. If not
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NULL, OACT is set to the prior behavior. Return 0 on success, or
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set errno and return -1 on failure. */
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sigaction (int sig, const struct sigaction *restrict act,
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struct sigaction *restrict oact)
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if (sig < 0 || NSIG <= sig || sig == SIGKILL || sig == SIGSTOP
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|| (act && act->sa_handler == SIG_ERR))
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#ifdef SIGABRT_COMPAT
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if (sig == SIGABRT_COMPAT)
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/* POSIX requires sigaction() to be async-signal-safe. In other
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words, if an asynchronous signal can occur while we are anywhere
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inside this function, the user's handler could then call
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sigaction() recursively and expect consistent results. We meet
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this rule by using sigprocmask to block all signals before
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modifying any data structure that could be read from a signal
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handler; this works since we know that the gnulib sigprocmask
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replacement does not try to use sigaction() from its handler. */
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sigprocmask (SIG_BLOCK, &mask, &oldmask);
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if (action_array[sig].sa_handler)
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*oact = action_array[sig];
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/* Safe to change the handler at will here, since all
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signals are currently blocked. */
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oact->sa_handler = signal (sig, SIG_DFL);
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if (oact->sa_handler == SIG_ERR)
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signal (sig, oact->sa_handler);
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oact->sa_flags = SA_RESETHAND | SA_NODEFER;
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sigemptyset (&oact->sa_mask);
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/* Safe to install the handler before updating action_array,
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since all signals are currently blocked. */
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if (act->sa_handler == SIG_DFL || act->sa_handler == SIG_IGN)
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if (signal (sig, act->sa_handler) == SIG_ERR)
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action_array[sig].sa_handler = NULL;
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if (signal (sig, sigaction_handler) == SIG_ERR)
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action_array[sig] = *act;
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sigprocmask (SIG_SETMASK, &oldmask, NULL);
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sigprocmask (SIG_SETMASK, &oldmask, NULL);
added
removed
lib/sigaction.c
