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30 .\" @(#)sigaction.2 8.2 (Berkeley) 4/3/94
31 .\"
32 .Dd June 3, 2006
33 .Dt SIGACTION 2
34 .Os
35 .Sh NAME
36 .Nm sigaction
37 .Nd software signal facilities
38 .Sh LIBRARY
39 .Lb libc
40 .Sh SYNOPSIS
41 .In signal.h
42 .Ft int
43 .Fn sigaction "int sig" "const struct sigaction * restrict act" "struct sigaction * restrict oact"
44 .Sh DESCRIPTION
45 The system defines a set of signals that may be delivered to a process.
46 Signal delivery resembles the occurrence of a hardware interrupt:
47 the signal is blocked from further occurrence, the current process
48 context is saved, and a new one is built.
49 A process may specify a
50 .Em handler
51 to which a signal is delivered, or specify that a signal is to be
52 .Em ignored .
53 A process may also specify that a default action is to be taken
54 by the system when a signal occurs.
55 A signal may also be
56 .Em blocked ,
57 in which case its delivery is postponed until it is
58 .Em unblocked .
59 The action to be taken on delivery is determined at the time of delivery.
60 Normally, signal handlers execute on the current stack of the process.
61 This may be changed, on a per-handler basis, so that signals are
62 taken on a special
63 .Em "signal stack" .
64 .Pp
65 Signal routines execute with the signal that caused their
66 invocation
67 .Em blocked ,
68 but other signals may yet occur.
69 A global
70 .Em "signal mask"
71 defines the set of signals currently blocked from delivery
72 to a process.
73 The signal mask for a process is initialized from that of its parent
74 (normally empty).
75 It may be changed with a
76 .Xr sigprocmask 2
77 call, or when a signal is delivered to the process.
78 Signal masks are represented using the
79 .Em sigset_t
80 type; the
81 .Xr sigsetops 3
82 interface is used to modify such data.
83 .Pp
84 When a signal
85 condition arises for a process, the signal is added to a set of
86 signals pending for the process.
87 If the signal is not currently
88 .Em blocked
89 by the process then it is delivered to the process.
90 Signals may be delivered any time a process enters the operating system
91 (e.g., during a system call, page fault or trap, or clock interrupt).
92 If multiple signals are ready to be delivered at the same time,
93 any signals that could be caused by traps are delivered first.
94 Additional signals may be processed at the same time, with each
95 appearing to interrupt the handlers for the previous signals
96 before their first instructions.
97 The set of pending signals is returned by the
98 .Xr sigpending 2
99 function.
100 When a caught signal
101 is delivered, the current state of the process is saved,
102 a new signal mask is calculated (as described below),
103 and the signal handler is invoked.
104 The call to the handler is arranged so that if the signal handling
105 routine returns normally the process will resume execution in the
106 context from before the signal's delivery.
107 If the process wishes to resume in a different context, then it
108 must arrange to restore the previous context itself.
109 .Pp
110 .Em "struct sigaction"
111 includes the following members:
112 .Bd -literal -offset indent
113 void (*sa_sigaction)(int sig, siginfo_t *info, void *ctx);
114 void (*sa_handler)(int sig);
115 sigset_t sa_mask;
116 int sa_flags;
117 .Ed
118 .Pp
119 When a signal is delivered to a process a new signal mask is
120 installed for the duration of the process' signal handler
121 (or until a
122 .Xr sigprocmask 2
123 call is made).
124 This mask is formed by taking the union of the current signal mask,
125 the signal to be delivered, and
126 the signal mask associated with the handler to be invoked,
127 .Em sa_mask .
128 .Pp
129 .Fn sigaction
130 assigns an action for a specific signal.
131 If
132 .Fa act
133 is non-zero, it
134 specifies an action
135 .Pf ( Dv SIG_DFL ,
136 .Dv SIG_IGN ,
137 or a handler routine) and mask
138 to be used when delivering the specified signal.
139 If
140 .Fa oact
141 is non-zero, the previous handling information for the signal
142 is returned to the user.
143 .Pp
144 Once a signal handler is installed, it remains installed
145 until another
146 .Fn sigaction
147 call is made, or an
148 .Xr execve 2
149 is performed.
150 A signal-specific default action may be reset by
151 setting
152 .Fa sa_handler
153 to
154 .Dv SIG_DFL .
155 The defaults are process termination, possibly with core dump;
156 no action; stopping the process; or continuing the process.
157 See the signal list below for each signal's default action.
158 If
159 .Fa sa_handler
160 is set to
161 .Dv SIG_DFL ,
162 the default action for the signal is to discard the signal,
163 and if a signal is pending,
164 the pending signal is discarded even if the signal is masked.
165 If
166 .Fa sa_handler
167 is set to
168 .Dv SIG_IGN ,
169 current and pending instances
170 of the signal are ignored and discarded.
171 .Pp
172 Options may be specified by setting
173 .Em sa_flags .
174 .Bl -tag -width SA_NOKERNINFO
175 .It Dv SA_NODEFER
176 If set, then the signal that caused the handler to be executed is not added
177 to the list of block signals.
178 Please note that
179 .Fa sa_mask
180 takes precedence over
181 .Dv SA_NODEFER ,
182 so that if the specified signal is blocked in
183 .Fa sa_mask ,
184 then
185 .Dv SA_NODEFER
186 will have no effect.
187 .It Dv SA_NOCLDSTOP
188 If set when installing a catching function
189 for the
190 .Dv SIGCHLD
191 signal,
192 the
193 .Dv SIGCHLD
194 signal will be generated only when a child process exits,
195 not when a child process stops.
196 .It Dv SA_NOCLDWAIT
197 If set, the system will not create a zombie when the child exits,
198 but the child process will be automatically waited for.
199 The same effect can be achieved by setting the signal handler for
200 .Dv SIGCHLD
201 to
202 .Dv SIG_IGN .
203 .It Dv SA_ONSTACK
204 If set, the system will deliver the signal to the process on a
205 .Em "signal stack" ,
206 specified with
207 .Xr sigaltstack 2 .
208 .It Dv SA_RESETHAND
209 If set, the default action will be reinstated when the signal
210 is first posted.
211 .It Dv SA_RESTART
212 Normally, if a signal is caught during the system calls listed below,
213 the call may be forced to terminate
214 with the error
215 .Er EINTR ,
216 the call may return with a data transfer shorter than requested,
217 or the call may be restarted.
218 Restarting of pending calls is requested
219 by setting the
220 .Dv SA_RESTART
221 bit in
222 .Ar sa_flags .
223 The affected system calls include
224 .Xr open 2 ,
225 .Xr read 2 ,
226 .Xr write 2 ,
227 .Xr sendto 2 ,
228 .Xr recvfrom 2 ,
229 .Xr sendmsg 2
230 and
231 .Xr recvmsg 2
232 on a communications channel or a slow device (such as a terminal,
233 but not a regular file)
234 and during a
235 .Xr wait 2
236 or
237 .Xr ioctl 2 .
238 However, calls that have already committed are not restarted,
239 but instead return a partial success (for example, a short read count).
240 .Pp
241 After a
242 .Xr fork 2
243 or
244 .Xr vfork 2
245 all signals, the signal mask, the signal stack,
246 and the restart/interrupt flags are inherited by the child.
247 .Pp
248 The
249 .Xr execve 2
250 system call reinstates the default
251 action for all signals which were caught and
252 resets all signals to be caught on the user stack.
253 Ignored signals remain ignored;
254 the signal mask remains the same;
255 signals that restart pending system calls continue to do so.
256 .Pp
257 See
258 .Xr signal 7
259 for comprehensive list of supported signals.
260 .It Dv SA_SIGINFO
261 If set, the signal handler function will receive additional information
262 about the caught signal.
263 An alternative handler that gets passed additional arguments will
264 be called which is named
265 .Fa sa_sigaction .
266 The
267 .Ar sig
268 argument of this handler contains the signal number that was caught.
269 The
270 .Ar info
271 argument contains additional signal specific information which
272 is listed in
273 .Xr siginfo 2 .
274 The
275 .Ar ctx
276 argument
277 is a pointer to the
278 .Xr ucontext 2
279 context where the signal handler will return to.
280 .It Dv SA_NOKERNINFO
281 This flag is relevant only to
282 .Dv SIGINFO ,
283 and turns off printing kernel messages on the tty.
284 It is similar to the
285 .Dv NOKERNINFO
286 flag in
287 .Xr termios 4 .
288 .El
289 .Pp
290 Only functions that are async-signal-safe can safely be used in signal
291 handlers, see
292 .Xr signal 7
293 for a complete list.
294 .Sh NOTES
295 The mask specified in
296 .Fa act
297 is not allowed to block
298 .Dv SIGKILL
299 or
300 .Dv SIGSTOP .
301 This is enforced silently by the system.
302 .Sh RETURN VALUES
303 A 0 value indicates that the call succeeded.
304 A \-1 return value indicates an error occurred and
305 .Va errno
306 is set to indicate the reason.
307 .Sh ERRORS
308 .Fn sigaction
309 will fail and no new signal handler will be installed if one
310 of the following occurs:
311 .Bl -tag -width Er
312 .It Bq Er EFAULT
313 Either
314 .Fa act
315 or
316 .Fa oact
317 points to memory that is not a valid part of the process
318 address space.
319 .It Bq Er EINVAL
320 .Fa sig
321 is not a valid signal number.
322 .It Bq Er EINVAL
323 An attempt is made to ignore or supply a handler for
324 .Dv SIGKILL
325 or
326 .Dv SIGSTOP .
327 .It Bq Er EINVAL
328 The
329 .Em sa_flags
330 word contains bits other than
331 .Dv SA_NOCLDSTOP ,
332 .Dv SA_NOCLDWAIT ,
333 .Dv SA_NODEFER ,
334 .Dv SA_ONSTACK ,
335 .Dv SA_RESETHAND ,
336 .Dv SA_RESTART ,
337 and
338 .Dv SA_SIGINFO .
339 .El
340 .Sh SEE ALSO
341 .Xr kill 1 ,
342 .Xr kill 2 ,
343 .Xr ptrace 2 ,
344 .Xr sigaltstack 2 ,
345 .Xr siginfo 2 ,
346 .Xr sigprocmask 2 ,
347 .Xr sigsuspend 2 ,
348 .Xr setjmp 3 ,
349 .Xr sigsetops 3 ,
350 .Xr tty 4 ,
351 .Xr signal 7
352 .Sh STANDARDS
353 The
354 .Fn sigaction
355 function conforms to
356 .St -p1003.1-90 .
357 The
358 .Dv SA_ONSTACK
359 and
360 .Dv SA_RESTART
361 flags are Berkeley extensions, available on most
362 .Bx Ns \-derived
363 systems.