1 .\" Copyright (c) 1980, 1990, 1993
2 .\" The Regents of the University of California. All rights reserved.
4 .\" Redistribution and use in source and binary forms, with or without
5 .\" modification, are permitted provided that the following conditions
7 .\" 1. Redistributions of source code must retain the above copyright
8 .\" notice, this list of conditions and the following disclaimer.
9 .\" 2. Redistributions in binary form must reproduce the above copyright
10 .\" notice, this list of conditions and the following disclaimer in the
11 .\" documentation and/or other materials provided with the distribution.
12 .\" 3. All advertising materials mentioning features or use of this software
13 .\" must display the following acknowledgement:
14 .\" This product includes software developed by the University of
15 .\" California, Berkeley and its contributors.
16 .\" 4. Neither the name of the University nor the names of its contributors
17 .\" may be used to endorse or promote products derived from this software
18 .\" without specific prior written permission.
20 .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
21 .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
22 .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
23 .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
24 .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
25 .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
26 .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
27 .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
28 .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
29 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
32 .\" From: @(#)sigaction.2 8.2 (Berkeley) 4/3/94
33 .\" $FreeBSD: src/lib/libc/sys/sigaction.2,v 1.22.2.10 2002/12/29 16:35:34 schweikh Exp $
40 .Nd software signal facilities
48 * Signal handler function if flag SA_SIGINFO is not used and for
49 * SIG_DFL and SIG_IGN.
51 void (*sa_handler)(int);
53 /* Signal handler function if flag SA_SIGINFO is used */
54 void (*sa_sigaction)(int, siginfo_t *, void *);
56 sigset_t sa_mask; /* signal mask to apply */
57 int sa_flags; /* see signal options below */
61 .Fn sigaction "int sig" "const struct sigaction *act" "struct sigaction *oact"
63 The system defines a set of signals that may be delivered to a process.
64 Signal delivery resembles the occurrence of a hardware interrupt:
65 the signal is normally blocked from further occurrence, the current process
66 context is saved, and a new one is built.
67 A process may specify a
69 to which a signal is delivered, or specify that a signal is to be
71 A process may also specify that a default action is to be taken
72 by the system when a signal occurs.
75 in which case its delivery is postponed until it is
77 The action to be taken on delivery is determined at the time
79 Normally, signal handlers execute on the current stack
81 This may be changed, on a per-handler basis,
82 so that signals are taken on a special
85 Signal routines normally execute with the signal that caused their
88 but other signals may yet occur.
91 defines the set of signals currently blocked from delivery
93 The signal mask for a process is initialized
94 from that of its parent (normally empty).
95 It may be changed with a
97 call, or when a signal is delivered to the process.
100 condition arises for a process, the signal is added to a set of
101 signals pending for the process.
102 If the signal is not currently
104 by the process then it is delivered to the process.
105 Signals may be delivered any time a process enters the operating system
106 (e.g., during a system call, page fault or trap, or clock interrupt).
107 If multiple signals are ready to be delivered at the same time,
108 any signals that could be caused by traps are delivered first.
109 Additional signals may be processed at the same time, with each
110 appearing to interrupt the handlers for the previous signals
111 before their first instructions.
112 The set of pending signals is returned by the
116 is delivered, the current state of the process is saved,
117 a new signal mask is calculated (as described below),
118 and the signal handler is invoked.
119 The call to the handler
120 is arranged so that if the signal handling routine returns
121 normally the process will resume execution in the context
122 from before the signal's delivery.
123 If the process wishes to resume in a different context, then it
124 must arrange to restore the previous context itself.
126 When a signal is delivered to a process a new signal mask is
127 installed for the duration of the process' signal handler
131 This mask is formed by taking the union of the current signal mask set,
132 the signal to be delivered, and
133 the signal mask associated with the handler to be invoked.
136 assigns an action for a signal specified by
144 or a handler routine) and mask
145 to be used when delivering the specified signal.
148 is non-zero, the previous handling information for the signal
149 is returned to the user.
151 Once a signal handler is installed, it normally remains installed
157 A signal-specific default action may be reset by
162 The defaults are process termination, possibly with core dump;
163 no action; stopping the process; or continuing the process.
164 See the signal list below for each signal's default action.
169 the default action for the signal is to discard the signal,
170 and if a signal is pending,
171 the pending signal is discarded even if the signal is masked.
176 current and pending instances
177 of the signal are ignored and discarded.
179 Options may be specified by setting
181 The meaning of the various bits is as follows:
182 .Bl -tag -offset indent -width SA_RESETHANDXX
184 If this bit is set when installing a catching function
190 signal will be generated only when a child process exits,
191 not when a child process stops.
193 If this bit is set when calling
197 signal, the system will not create zombie processes when children of
198 the calling process exit.
199 If the calling process subsequently issues a
201 (or equivalent), it blocks until all of the calling process's child
202 processes terminate, and then returns a value of -1 with
207 If this bit is set, the system will deliver the signal to the process
213 If this bit is set, further occurrences of the delivered signal are
214 not masked during the execution of the handler.
216 If this bit is set, the handler is reset back to
218 at the moment the signal is delivered.
220 If this bit is set, the handler function is assumed to be pointed to by the
222 member of struct sigaction and should match the prototype shown above or as
225 This bit should not be set when assigning
231 If a signal is caught during the system calls listed below,
232 the call may be forced to terminate
235 the call may return with a data transfer shorter than requested,
236 or the call may be restarted.
237 Restart of pending calls is requested
242 The affected system calls include
251 on a communications channel or a slow device (such as a terminal,
252 but not a regular file)
257 However, calls that have already committed are not restarted,
258 but instead return a partial success (for example, a short read count).
264 all signals, the signal mask, the signal stack,
265 and the restart/interrupt flags are inherited by the child.
268 reinstates the default
269 action for all signals which were caught and
270 resets all signals to be caught on the user stack.
271 Ignored signals remain ignored;
272 the signal mask remains the same;
273 signals that restart pending system calls continue to do so.
275 The following is a list of all signals
276 with names as in the include file
278 .Bl -column SIGVTALARMXX "create core imagexxx"
279 .It Sy "NAME Default Action Description"
280 .It Dv SIGHUP No " terminate process" " terminal line hangup"
281 .It Dv SIGINT No " terminate process" " interrupt program"
282 .It Dv SIGQUIT No " create core image" " quit program"
283 .It Dv SIGILL No " create core image" " illegal instruction"
284 .It Dv SIGTRAP No " create core image" " trace trap"
285 .It Dv SIGABRT No " create core image" Ta Xr abort 3
288 .It Dv SIGEMT No " create core image" " emulate instruction executed"
289 .It Dv SIGFPE No " create core image" " floating-point exception"
290 .It Dv SIGKILL No " terminate process" " kill program"
291 .It Dv SIGBUS No " create core image" " bus error"
292 .It Dv SIGSEGV No " create core image" " segmentation violation"
293 .It Dv SIGSYS No " create core image" " non-existent system call invoked"
294 .It Dv SIGPIPE No " terminate process" " write on a pipe with no reader"
295 .It Dv SIGALRM No " terminate process" " real-time timer expired"
296 .It Dv SIGTERM No " terminate process" " software termination signal"
297 .It Dv SIGURG No " discard signal" " urgent condition present on socket"
298 .It Dv SIGSTOP No " stop process" " stop (cannot be caught or ignored)"
299 .It Dv SIGTSTP No " stop process" " stop signal generated from keyboard"
300 .It Dv SIGCONT No " discard signal" " continue after stop"
301 .It Dv SIGCHLD No " discard signal" " child status has changed"
302 .It Dv SIGTTIN No " stop process" " background read attempted from control terminal"
303 .It Dv SIGTTOU No " stop process" " background write attempted to control terminal"
304 .It Dv SIGIO No " discard signal" Tn " I/O"
305 is possible on a descriptor (see
307 .It Dv SIGXCPU No " terminate process" " cpu time limit exceeded (see"
309 .It Dv SIGXFSZ No " terminate process" " file size limit exceeded (see"
311 .It Dv SIGVTALRM No " terminate process" " virtual time alarm (see"
313 .It Dv SIGPROF No " terminate process" " profiling timer alarm (see"
315 .It Dv SIGWINCH No " discard signal" " Window size change"
316 .It Dv SIGINFO No " discard signal" " status request from keyboard"
317 .It Dv SIGUSR1 No " terminate process" " User defined signal 1"
318 .It Dv SIGUSR2 No " terminate process" " User defined signal 2"
325 is not allowed to block
329 Any attempt to do so will be silently ignored.
331 The following functions are either reentrant or not interruptible
332 by signals and are async-signal safe.
333 Therefore applications may
334 invoke them, without restriction, from signal-catching functions:
420 .\".Fn timer_getoverrun ,
424 .\".Fn timer_gettime ,
428 .\".Fn timer_settime .
430 All functions not in the above lists are considered to be unsafe
431 with respect to signals.
432 That is to say, the behaviour of such
433 functions when called from a signal handler is undefined.
437 There are three possible prototypes the handler may match:
438 .Bl -tag -offset indent -width short
442 .It Traditional BSD style:
444 .Fn handler int "int code" "struct sigcontext *scp" ;
445 .It POSIX SA_SIGINFO:
447 .Fn handler int "siginfo_t *info" "ucontext_t *uap" ;
450 The handler function should match the
455 It then should be pointed to by the
458 .Dv struct sigaction .
459 Note that you should not assign
467 flag is not set, the handler function should match
468 either the ANSI C or traditional
470 prototype and be pointed to by
474 .Dv struct sigaction .
477 always sends the three arguments of the latter and since the ANSI C
478 prototype is a subset, both will work.
481 member declaration in
483 include files is that of ANSI C (as required by POSIX),
484 so a function pointer of a
486 function needs to be casted to
487 compile without warning.
490 style is not portable and since its capabilities are a full subset of a
492 handler, its use is deprecated.
496 argument is the signal number, one of the
510 handler contain a numeric code explaining the
511 cause of the signal, usually one of the
515 or codes specific to a signal, i.e.\& one of the
524 handler points to an instance of struct
531 handler points to an instance of ucontext_t.
534 will fail and no new signal handler will be installed if one
535 of the following occurs:
542 points to memory that is not a valid part of the process
546 is not a valid signal number.
548 An attempt is made to ignore or supply a handler for
571 function call is expected to conform to
577 flags are Berkeley extensions,
592 Those signals are available on most
599 flags are intended for backwards compatibility with other operating
607 flags are featuring options commonly found in other operating systems.