1 /* POSIX compatible signal blocking.
2 Copyright (C) 2008-2017 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 <https://www.gnu.org/licenses/>. */
27 /* This implementation of sigaction is tailored to native Windows 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 native Windows API calls are
43 not interrupted anyway when an asynchronous signal occurs, and the
44 MSVCRT runtime 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 /* On native Windows, as of 2008, the signal SIGABRT_COMPAT is an alias
68 for the signal SIGABRT. Only one signal handler is stored for both
69 SIGABRT and SIGABRT_COMPAT. SIGABRT_COMPAT is not a signal of its own. */
70 #if (defined _WIN32 || defined __WIN32__) && ! defined __CYGWIN__
71 # undef SIGABRT_COMPAT
72 # define SIGABRT_COMPAT 6
75 /* A signal handler. */
76 typedef void (*handler_t
) (int signal
);
78 /* Set of current actions. If sa_handler for an entry is NULL, then
79 that signal is not currently handled by the sigaction handler. */
80 static struct sigaction
volatile action_array
[NSIG
] /* = 0 */;
82 /* Signal handler that is installed for signals. */
84 sigaction_handler (int sig
)
89 int saved_errno
= errno
;
90 if (sig
< 0 || NSIG
<= sig
|| !action_array
[sig
].sa_handler
)
92 /* Unexpected situation; be careful to avoid recursive abort. */
94 signal (SIGABRT
, SIG_DFL
);
98 /* Reinstall the signal handler when required; otherwise update the
99 bookkeeping so that the user's handler may call sigaction and get
100 accurate results. We know the signal isn't currently blocked, or
101 we wouldn't be in its handler, therefore we know that we are not
102 interrupting a sigaction() call. There is a race where any
103 asynchronous instance of the same signal occurring before we
104 reinstall the handler will trigger the default handler; oh
106 handler
= action_array
[sig
].sa_handler
;
107 if ((action_array
[sig
].sa_flags
& SA_RESETHAND
) == 0)
108 signal (sig
, sigaction_handler
);
110 action_array
[sig
].sa_handler
= NULL
;
112 /* Block appropriate signals. */
113 mask
= action_array
[sig
].sa_mask
;
114 if ((action_array
[sig
].sa_flags
& SA_NODEFER
) == 0)
115 sigaddset (&mask
, sig
);
116 sigprocmask (SIG_BLOCK
, &mask
, &oldmask
);
118 /* Invoke the user's handler, then restore prior mask. */
122 sigprocmask (SIG_SETMASK
, &oldmask
, NULL
);
126 /* Change and/or query the action that will be taken on delivery of
127 signal SIG. If not NULL, ACT describes the new behavior. If not
128 NULL, OACT is set to the prior behavior. Return 0 on success, or
129 set errno and return -1 on failure. */
131 sigaction (int sig
, const struct sigaction
*restrict act
,
132 struct sigaction
*restrict oact
)
138 if (sig
< 0 || NSIG
<= sig
|| sig
== SIGKILL
|| sig
== SIGSTOP
139 || (act
&& act
->sa_handler
== SIG_ERR
))
145 #ifdef SIGABRT_COMPAT
146 if (sig
== SIGABRT_COMPAT
)
150 /* POSIX requires sigaction() to be async-signal-safe. In other
151 words, if an asynchronous signal can occur while we are anywhere
152 inside this function, the user's handler could then call
153 sigaction() recursively and expect consistent results. We meet
154 this rule by using sigprocmask to block all signals before
155 modifying any data structure that could be read from a signal
156 handler; this works since we know that the gnulib sigprocmask
157 replacement does not try to use sigaction() from its handler. */
161 sigprocmask (SIG_BLOCK
, &mask
, &oldmask
);
164 if (action_array
[sig
].sa_handler
)
165 *oact
= action_array
[sig
];
168 /* Safe to change the handler at will here, since all
169 signals are currently blocked. */
170 oact
->sa_handler
= signal (sig
, SIG_DFL
);
171 if (oact
->sa_handler
== SIG_ERR
)
173 signal (sig
, oact
->sa_handler
);
174 oact
->sa_flags
= SA_RESETHAND
| SA_NODEFER
;
175 sigemptyset (&oact
->sa_mask
);
181 /* Safe to install the handler before updating action_array,
182 since all signals are currently blocked. */
183 if (act
->sa_handler
== SIG_DFL
|| act
->sa_handler
== SIG_IGN
)
185 if (signal (sig
, act
->sa_handler
) == SIG_ERR
)
187 action_array
[sig
].sa_handler
= NULL
;
191 if (signal (sig
, sigaction_handler
) == SIG_ERR
)
193 action_array
[sig
] = *act
;
196 sigprocmask (SIG_SETMASK
, &oldmask
, NULL
);
201 sigprocmask (SIG_SETMASK
, &oldmask
, NULL
);