1 /* POSIX compatible signal blocking.
2 Copyright (C) 2008 Free Software Foundation, Inc.
3 Written by Eric Blake <ebb9@byu.net>, 2008.
5 This program is free software: you can redistribute it and/or modify
6 it under the terms of the GNU General Public License as published by
7 the Free Software Foundation; either version 3 of the License, or
8 (at your option) any later version.
10 This program is distributed in the hope that it will be useful,
11 but WITHOUT ANY WARRANTY; without even the implied warranty of
12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
13 GNU General Public License for more details.
15 You should have received a copy of the GNU General Public License
16 along with this program. If not, see <http://www.gnu.org/licenses/>. */
27 /* This implementation of sigaction is tailored to Woe32 behavior:
28 signal() has SysV semantics (ie. the handler is uninstalled before
29 it is invoked). This is an inherent data race if an asynchronous
30 signal is sent twice in a row before we can reinstall our handler,
31 but there's nothing we can do about it. Meanwhile, sigprocmask()
32 is not present, and while we can use the gnulib replacement to
33 provide critical sections, it too suffers from potential data races
34 in the face of an ill-timed asynchronous signal. And we compound
35 the situation by reading static storage in a signal handler, which
36 POSIX warns is not generically async-signal-safe. Oh well.
39 - We don't implement SA_NOCLDSTOP or SA_NOCLDWAIT, because SIGCHLD
41 - We don't implement SA_ONSTACK, because sigaltstack() is not present.
42 - We ignore SA_RESTART, because blocking Win32 calls are not interrupted
43 anyway when an asynchronous signal occurs, and the MSVCRT runtime
44 never sets errno to EINTR.
45 - We don't implement SA_SIGINFO because it is impossible to do so
48 POSIX states that an application should not mix signal() and
49 sigaction(). We support the use of signal() within the gnulib
50 sigprocmask() substitute, but all other application code linked
51 with this module should stick with only sigaction(). */
53 /* Check some of our assumptions. */
54 #if defined SIGCHLD || defined HAVE_SIGALTSTACK || defined HAVE_SIGINTERRUPT
55 # error "Revisit the assumptions made in the sigaction module"
58 /* Out-of-range substitutes make a good fallback for uncatchable
67 /* A signal handler. */
68 typedef void (*handler_t) (int signal);
70 /* Set of current actions. If sa_handler for an entry is NULL, then
71 that signal is not currently handled by the sigaction handler. */
72 static struct sigaction volatile action_array[NSIG] /* = 0 */;
74 /* Signal handler that is installed for signals. */
76 sigaction_handler (int sig)
81 int saved_errno = errno;
82 if (sig < 0 || NSIG <= sig || !action_array[sig].sa_handler)
84 /* Unexpected situation; be careful to avoid recursive abort. */
86 signal (SIGABRT, SIG_DFL);
90 /* Reinstall the signal handler when required; otherwise update the
91 bookkeeping so that the user's handler may call sigaction and get
92 accurate results. We know the signal isn't currently blocked, or
93 we wouldn't be in its handler, therefore we know that we are not
94 interrupting a sigaction() call. There is a race where any
95 asynchronous instance of the same signal occurring before we
96 reinstall the handler will trigger the default handler; oh
98 handler = action_array[sig].sa_handler;
99 if ((action_array[sig].sa_flags & SA_RESETHAND) == 0)
100 signal (sig, sigaction_handler);
102 action_array[sig].sa_handler = NULL;
104 /* Block appropriate signals. */
105 mask = action_array[sig].sa_mask;
106 if ((action_array[sig].sa_flags & SA_NODEFER) == 0)
107 sigaddset (&mask, sig);
108 sigprocmask (SIG_BLOCK, &mask, &oldmask);
110 /* Invoke the user's handler, then restore prior mask. */
114 sigprocmask (SIG_SETMASK, &oldmask, NULL);
118 /* Change and/or query the action that will be taken on delivery of
119 signal SIG. If not NULL, ACT describes the new behavior. If not
120 NULL, OACT is set to the prior behavior. Return 0 on success, or
121 set errno and return -1 on failure. */
123 sigaction (int sig, const struct sigaction *restrict act,
124 struct sigaction *restrict oact)
130 if (sig < 0 || NSIG <= sig || sig == SIGKILL || sig == SIGSTOP
131 || (act && act->sa_handler == SIG_ERR))
137 /* POSIX requires sigaction() to be async-signal-safe. In other
138 words, if an asynchronous signal can occur while we are anywhere
139 inside this function, the user's handler could then call
140 sigaction() recursively and expect consistent results. We meet
141 this rule by using sigprocmask to block all signals before
142 modifying any data structure that could be read from a signal
143 handler; this works since we know that the gnulib sigprocmask
144 replacement does not try to use sigaction() from its handler. */
148 sigprocmask (SIG_BLOCK, &mask, &oldmask);
151 if (action_array[sig].sa_handler)
152 *oact = action_array[sig];
155 /* Safe to change the handler at will here, since all
156 signals are currently blocked. */
157 oact->sa_handler = signal (sig, SIG_DFL);
158 if (oact->sa_handler == SIG_ERR)
160 signal (sig, oact->sa_handler);
161 oact->sa_flags = SA_RESETHAND | SA_NODEFER;
162 sigemptyset (&oact->sa_mask);
168 /* Safe to install the handler before updating action_array,
169 since all signals are currently blocked. */
170 if (act->sa_handler == SIG_DFL || act->sa_handler == SIG_IGN)
172 if (signal (sig, act->sa_handler) == SIG_ERR)
174 action_array[sig].sa_handler = NULL;
178 if (signal (sig, sigaction_handler) == SIG_ERR)
180 action_array[sig] = *act;
183 sigprocmask (SIG_SETMASK, &oldmask, NULL);
188 sigprocmask (SIG_SETMASK, &oldmask, NULL);