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28 .\" From: @(#)sigaction.2 8.2 (Berkeley) 4/3/94
31 .Dd September 27, 2012
36 .Nd software signal facilities
43 void (*sa_handler)(int);
44 void (*sa_sigaction)(int, siginfo_t *, void *);
45 int sa_flags; /* see signal options below */
46 sigset_t sa_mask; /* signal mask to apply */
52 .Fa "const struct sigaction * restrict act"
53 .Fa "struct sigaction * restrict oact"
56 The system defines a set of signals that may be delivered to a process.
57 Signal delivery resembles the occurrence of a hardware interrupt:
58 the signal is normally blocked from further occurrence, the current process
59 context is saved, and a new one is built.
60 A process may specify a
62 to which a signal is delivered, or specify that a signal is to be
64 A process may also specify that a default action is to be taken
65 by the system when a signal occurs.
68 in which case its delivery is postponed until it is
70 The action to be taken on delivery is determined at the time
72 Normally, signal handlers execute on the current stack
74 This may be changed, on a per-handler basis,
75 so that signals are taken on a special
78 Signal routines normally execute with the signal that caused their
81 but other signals may yet occur.
84 defines the set of signals currently blocked from delivery
86 The signal mask for a process is initialized
87 from that of its parent (normally empty).
88 It may be changed with a
90 call, or when a signal is delivered to the process.
93 condition arises for a process, the signal is added to a set of
94 signals pending for the process.
95 If the signal is not currently
97 by the process then it is delivered to the process.
98 Signals may be delivered any time a process enters the operating system
99 (e.g., during a system call, page fault or trap, or clock interrupt).
100 If multiple signals are ready to be delivered at the same time,
101 any signals that could be caused by traps are delivered first.
102 Additional signals may be processed at the same time, with each
103 appearing to interrupt the handlers for the previous signals
104 before their first instructions.
105 The set of pending signals is returned by the
109 is delivered, the current state of the process is saved,
110 a new signal mask is calculated (as described below),
111 and the signal handler is invoked.
112 The call to the handler
113 is arranged so that if the signal handling routine returns
114 normally the process will resume execution in the context
115 from before the signal's delivery.
116 If the process wishes to resume in a different context, then it
117 must arrange to restore the previous context itself.
119 When a signal is delivered to a process a new signal mask is
120 installed for the duration of the process' signal handler
123 system call is made).
124 This mask is formed by taking the union of the current signal mask set,
125 the signal to be delivered, and
126 the signal mask associated with the handler to be invoked.
131 assigns an action for a signal specified by
139 or a handler routine) and mask
140 to be used when delivering the specified signal.
143 is non-zero, the previous handling information for the signal
144 is returned to the user.
146 The above declaration of
147 .Vt "struct sigaction"
149 It is provided only to list the accessible members.
152 for the actual definition.
153 In particular, the storage occupied by sa_handler and sa_sigaction overlaps,
154 and an application can not use both simultaneously.
156 Once a signal handler is installed, it normally remains installed
159 system call is made, or an
162 A signal-specific default action may be reset by
167 The defaults are process termination, possibly with core dump;
168 no action; stopping the process; or continuing the process.
169 See the signal list below for each signal's default action.
174 the default action for the signal is to discard the signal,
175 and if a signal is pending,
176 the pending signal is discarded even if the signal is masked.
181 current and pending instances
182 of the signal are ignored and discarded.
184 Options may be specified by setting
186 The meaning of the various bits is as follows:
187 .Bl -tag -offset indent -width SA_RESETHANDXX
189 If this bit is set when installing a catching function
195 signal will be generated only when a child process exits,
196 not when a child process stops.
198 If this bit is set when calling
202 signal, the system will not create zombie processes when children of
203 the calling process exit.
204 If the calling process subsequently issues a
206 (or equivalent), it blocks until all of the calling process's child
207 processes terminate, and then returns a value of \-1 with
211 The same effect of avoiding zombie creation can also be achieved by setting
218 If this bit is set, the system will deliver the signal to the process
224 If this bit is set, further occurrences of the delivered signal are
225 not masked during the execution of the handler.
227 If this bit is set, the handler is reset back to
229 at the moment the signal is delivered.
233 If this bit is set, the handler function is assumed to be pointed to by the
236 .Vt "struct sigaction"
237 and should match the prototype shown above or as below in
239 This bit should not be set when assigning
245 If a signal is caught during the system calls listed below,
246 the call may be forced to terminate
249 the call may return with a data transfer shorter than requested,
250 or the call may be restarted.
251 Restart of pending calls is requested
256 The affected system calls include
265 on a communications channel or a slow device (such as a terminal,
266 but not a regular file)
271 However, calls that have already committed are not restarted,
272 but instead return a partial success (for example, a short read count).
278 all signals, the signal mask, the signal stack,
279 and the restart/interrupt flags are inherited by the child.
283 system call reinstates the default
284 action for all signals which were caught and
285 resets all signals to be caught on the user stack.
286 Ignored signals remain ignored;
287 the signal mask remains the same;
288 signals that restart pending system calls continue to do so.
290 The following is a list of all signals
291 with names as in the include file
293 .Bl -column SIGVTALARMXX "create core imagexxx"
294 .It Sy "NAME Default Action Description"
295 .It Dv SIGHUP No " terminate process" " terminal line hangup"
296 .It Dv SIGINT No " terminate process" " interrupt program"
297 .It Dv SIGQUIT No " create core image" " quit program"
298 .It Dv SIGILL No " create core image" " illegal instruction"
299 .It Dv SIGTRAP No " create core image" " trace trap"
300 .It Dv SIGABRT No " create core image" Ta Xr abort 3
303 .It Dv SIGEMT No " create core image" " emulate instruction executed"
304 .It Dv SIGFPE No " create core image" " floating-point exception"
305 .It Dv SIGKILL No " terminate process" " kill program"
306 .It Dv SIGBUS No " create core image" " bus error"
307 .It Dv SIGSEGV No " create core image" " segmentation violation"
308 .It Dv SIGSYS No " create core image" " non-existent system call invoked"
309 .It Dv SIGPIPE No " terminate process" " write on a pipe with no reader"
310 .It Dv SIGALRM No " terminate process" " real-time timer expired"
311 .It Dv SIGTERM No " terminate process" " software termination signal"
312 .It Dv SIGURG No " discard signal" " urgent condition present on socket"
313 .It Dv SIGSTOP No " stop process" " stop (cannot be caught or ignored)"
314 .It Dv SIGTSTP No " stop process" " stop signal generated from keyboard"
315 .It Dv SIGCONT No " discard signal" " continue after stop"
316 .It Dv SIGCHLD No " discard signal" " child status has changed"
317 .It Dv SIGTTIN No " stop process" " background read attempted from control terminal"
318 .It Dv SIGTTOU No " stop process" " background write attempted to control terminal"
319 .It Dv SIGIO No " discard signal" Tn " I/O"
320 is possible on a descriptor (see
322 .It Dv SIGXCPU No " terminate process" " cpu time limit exceeded (see"
324 .It Dv SIGXFSZ No " terminate process" " file size limit exceeded (see"
326 .It Dv SIGVTALRM No " terminate process" " virtual time alarm (see"
328 .It Dv SIGPROF No " terminate process" " profiling timer alarm (see"
330 .It Dv SIGWINCH No " discard signal" " Window size change"
331 .It Dv SIGINFO No " discard signal" " status request from keyboard"
332 .It Dv SIGUSR1 No " terminate process" " User defined signal 1"
333 .It Dv SIGUSR2 No " terminate process" " User defined signal 2"
340 is not allowed to block
344 Any attempt to do so will be silently ignored.
346 The following functions are either reentrant or not interruptible
347 by signals and are async-signal safe.
348 Therefore applications may
349 invoke them, without restriction, from signal-catching functions:
435 .Fn timer_getoverrun ,
452 and perhaps some others.
454 Extension Interfaces:
459 All functions not in the above lists are considered to be unsafe
460 with respect to signals.
461 That is to say, the behaviour of such
462 functions when called from a signal handler is undefined.
463 In general though, signal handlers should do little more than set a
464 flag; most other actions are not safe.
466 Also, it is good practice to make a copy of the global variable
468 and restore it before returning from the signal handler.
469 This protects against the side effect of
471 being set by functions called from inside the signal handler.
475 There are three possible prototypes the handler may match:
476 .Bl -tag -offset indent -width short
480 .It Traditional BSD style:
482 .Fn handler int "int code" "struct sigcontext *scp" ;
483 .It Tn POSIX Dv SA_SIGINFO :
485 .Fn handler int "siginfo_t *info" "ucontext_t *uap" ;
488 The handler function should match the
494 It then should be pointed to by the
497 .Vt "struct sigaction" .
498 Note that you should not assign
506 flag is not set, the handler function should match
511 prototype and be pointed to by
515 .Vt "struct sigaction" .
518 always sends the three arguments of the latter and since the
520 prototype is a subset, both will work.
523 member declaration in
525 include files is that of
529 so a function pointer of a
531 function needs to be casted to
532 compile without warning.
535 style is not portable and since its capabilities
536 are a full subset of a
539 its use is deprecated.
543 argument is the signal number, one of the
558 handler contain a numeric code explaining the
559 cause of the signal, usually one of the
563 or codes specific to a signal, i.e., one of the
572 handler points to an instance of
573 .Vt "struct sigcontext" .
580 handler points to an instance of
586 will fail and no new signal handler will be installed if one
587 of the following occurs:
593 is not a valid signal number.
595 An attempt is made to ignore or supply a handler for
619 system call is expected to conform to
625 flags are Berkeley extensions,
640 Those signals are available on most
647 flags are intended for backwards compatibility with other operating
655 flags are featuring options commonly found in other operating systems.
656 The flags are approved by
658 along with the option to avoid zombie creation by ignoring