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28 .\" From: @(#)sigaction.2 8.2 (Berkeley) 4/3/94
36 .Nd software signal facilities
44 void (*__sa_handler)(int);
45 void (*__sa_sigaction)(int, struct __siginfo *, void *);
46 } __sigaction_u; /* signal handler */
47 int sa_flags; /* see signal options below */
48 sigset_t sa_mask; /* signal mask to apply */
51 #define sa_handler __sigaction_u.__sa_handler
52 #define sa_sigaction __sigaction_u.__sa_sigaction
57 .Fa "const struct sigaction * restrict act"
58 .Fa "struct sigaction * restrict oact"
61 The system defines a set of signals that may be delivered to a process.
62 Signal delivery resembles the occurrence of a hardware interrupt:
63 the signal is normally blocked from further occurrence, the current process
64 context is saved, and a new one is built.
65 A process may specify a
67 to which a signal is delivered, or specify that a signal is to be
69 A process may also specify that a default action is to be taken
70 by the system when a signal occurs.
73 in which case its delivery is postponed until it is
75 The action to be taken on delivery is determined at the time
77 Normally, signal handlers execute on the current stack
79 This may be changed, on a per-handler basis,
80 so that signals are taken on a special
83 Signal routines normally execute with the signal that caused their
86 but other signals may yet occur.
89 defines the set of signals currently blocked from delivery
91 The signal mask for a process is initialized
92 from that of its parent (normally empty).
93 It may be changed with a
95 call, or when a signal is delivered to the process.
98 condition arises for a process, the signal is added to a set of
99 signals pending for the process.
100 If the signal is not currently
102 by the process then it is delivered to the process.
103 Signals may be delivered any time a process enters the operating system
104 (e.g., during a system call, page fault or trap, or clock interrupt).
105 If multiple signals are ready to be delivered at the same time,
106 any signals that could be caused by traps are delivered first.
107 Additional signals may be processed at the same time, with each
108 appearing to interrupt the handlers for the previous signals
109 before their first instructions.
110 The set of pending signals is returned by the
114 is delivered, the current state of the process is saved,
115 a new signal mask is calculated (as described below),
116 and the signal handler is invoked.
117 The call to the handler
118 is arranged so that if the signal handling routine returns
119 normally the process will resume execution in the context
120 from before the signal's delivery.
121 If the process wishes to resume in a different context, then it
122 must arrange to restore the previous context itself.
124 When a signal is delivered to a process a new signal mask is
125 installed for the duration of the process' signal handler
128 system call is made).
129 This mask is formed by taking the union of the current signal mask set,
130 the signal to be delivered, and
131 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
154 system call is made, or an
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
206 The same effect of avoiding zombie creation can also be achieved by setting
213 If this bit is set, the system will deliver the signal to the process
219 If this bit is set, further occurrences of the delivered signal are
220 not masked during the execution of the handler.
222 If this bit is set, the handler is reset back to
224 at the moment the signal is delivered.
228 If this bit is set, the handler function is assumed to be pointed to by the
231 .Vt "struct sigaction"
232 and should match the prototype shown above or as below in
234 This bit should not be set when assigning
240 If a signal is caught during the system calls listed below,
241 the call may be forced to terminate
244 the call may return with a data transfer shorter than requested,
245 or the call may be restarted.
246 Restart of pending calls is requested
251 The affected system calls include
260 on a communications channel or a slow device (such as a terminal,
261 but not a regular file)
266 However, calls that have already committed are not restarted,
267 but instead return a partial success (for example, a short read count).
273 all signals, the signal mask, the signal stack,
274 and the restart/interrupt flags are inherited by the child.
278 system call reinstates the default
279 action for all signals which were caught and
280 resets all signals to be caught on the user stack.
281 Ignored signals remain ignored;
282 the signal mask remains the same;
283 signals that restart pending system calls continue to do so.
285 The following is a list of all signals
286 with names as in the include file
288 .Bl -column SIGVTALARMXX "create core imagexxx"
289 .It Sy "NAME Default Action Description"
290 .It Dv SIGHUP No " terminate process" " terminal line hangup"
291 .It Dv SIGINT No " terminate process" " interrupt program"
292 .It Dv SIGQUIT No " create core image" " quit program"
293 .It Dv SIGILL No " create core image" " illegal instruction"
294 .It Dv SIGTRAP No " create core image" " trace trap"
295 .It Dv SIGABRT No " create core image" Ta Xr abort 3
298 .It Dv SIGEMT No " create core image" " emulate instruction executed"
299 .It Dv SIGFPE No " create core image" " floating-point exception"
300 .It Dv SIGKILL No " terminate process" " kill program"
301 .It Dv SIGBUS No " create core image" " bus error"
302 .It Dv SIGSEGV No " create core image" " segmentation violation"
303 .It Dv SIGSYS No " create core image" " non-existent system call invoked"
304 .It Dv SIGPIPE No " terminate process" " write on a pipe with no reader"
305 .It Dv SIGALRM No " terminate process" " real-time timer expired"
306 .It Dv SIGTERM No " terminate process" " software termination signal"
307 .It Dv SIGURG No " discard signal" " urgent condition present on socket"
308 .It Dv SIGSTOP No " stop process" " stop (cannot be caught or ignored)"
309 .It Dv SIGTSTP No " stop process" " stop signal generated from keyboard"
310 .It Dv SIGCONT No " discard signal" " continue after stop"
311 .It Dv SIGCHLD No " discard signal" " child status has changed"
312 .It Dv SIGTTIN No " stop process" " background read attempted from control terminal"
313 .It Dv SIGTTOU No " stop process" " background write attempted to control terminal"
314 .It Dv SIGIO No " discard signal" Tn " I/O"
315 is possible on a descriptor (see
317 .It Dv SIGXCPU No " terminate process" " cpu time limit exceeded (see"
319 .It Dv SIGXFSZ No " terminate process" " file size limit exceeded (see"
321 .It Dv SIGVTALRM No " terminate process" " virtual time alarm (see"
323 .It Dv SIGPROF No " terminate process" " profiling timer alarm (see"
325 .It Dv SIGWINCH No " discard signal" " Window size change"
326 .It Dv SIGINFO No " discard signal" " status request from keyboard"
327 .It Dv SIGUSR1 No " terminate process" " User defined signal 1"
328 .It Dv SIGUSR2 No " terminate process" " User defined signal 2"
335 is not allowed to block
339 Any attempt to do so will be silently ignored.
341 The following functions are either reentrant or not interruptible
342 by signals and are async-signal safe.
343 Therefore applications may
344 invoke them, without restriction, from signal-catching functions:
430 .Fn timer_getoverrun ,
447 and perhaps some others.
449 Extension Interfaces:
454 All functions not in the above lists are considered to be unsafe
455 with respect to signals.
456 That is to say, the behaviour of such
457 functions when called from a signal handler is undefined.
458 In general though, signal handlers should do little more than set a
459 flag; most other actions are not safe.
461 Also, it is good practice to make a copy of the global variable
463 and restore it before returning from the signal handler.
464 This protects against the side effect of
466 being set by functions called from inside the signal handler.
470 There are three possible prototypes the handler may match:
471 .Bl -tag -offset indent -width short
475 .It Traditional BSD style:
477 .Fn handler int "int code" "struct sigcontext *scp" ;
478 .It Tn POSIX Dv SA_SIGINFO :
480 .Fn handler int "siginfo_t *info" "ucontext_t *uap" ;
483 The handler function should match the
489 It then should be pointed to by the
492 .Vt "struct sigaction" .
493 Note that you should not assign
501 flag is not set, the handler function should match
506 prototype and be pointed to by
510 .Vt "struct sigaction" .
513 always sends the three arguments of the latter and since the
515 prototype is a subset, both will work.
518 member declaration in
520 include files is that of
524 so a function pointer of a
526 function needs to be casted to
527 compile without warning.
530 style is not portable and since its capabilities
531 are a full subset of a
534 its use is deprecated.
538 argument is the signal number, one of the
553 handler contain a numeric code explaining the
554 cause of the signal, usually one of the
558 or codes specific to a signal, i.e., one of the
567 handler points to an instance of
568 .Vt "struct sigcontext" .
575 handler points to an instance of
581 will fail and no new signal handler will be installed if one
582 of the following occurs:
589 points to memory that is not a valid part of the process
595 is not a valid signal number.
597 An attempt is made to ignore or supply a handler for
625 system call is expected to conform to
631 flags are Berkeley extensions,
646 Those signals are available on most
653 flags are intended for backwards compatibility with other operating
661 flags are featuring options commonly found in other operating systems.
662 The flags are approved by
664 along with the option to avoid zombie creation by ignoring