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32 .\" From: @(#)sigaction.2 8.2 (Berkeley) 4/3/94
40 .Nd software signal facilities
48 void (*__sa_handler)(int);
49 void (*__sa_sigaction)(int, struct __siginfo *, void *);
50 } __sigaction_u; /* signal handler */
51 int sa_flags; /* see signal options below */
52 sigset_t sa_mask; /* signal mask to apply */
55 #define sa_handler __sigaction_u.__sa_handler
56 #define sa_sigaction __sigaction_u.__sa_sigaction
61 .Fa "const struct sigaction * restrict act"
62 .Fa "struct sigaction * restrict oact"
65 The system defines a set of signals that may be delivered to a process.
66 Signal delivery resembles the occurrence of a hardware interrupt:
67 the signal is normally blocked from further occurrence, the current process
68 context is saved, and a new one is built.
69 A process may specify a
71 to which a signal is delivered, or specify that a signal is to be
73 A process may also specify that a default action is to be taken
74 by the system when a signal occurs.
77 in which case its delivery is postponed until it is
79 The action to be taken on delivery is determined at the time
81 Normally, signal handlers execute on the current stack
83 This may be changed, on a per-handler basis,
84 so that signals are taken on a special
87 Signal routines normally execute with the signal that caused their
90 but other signals may yet occur.
93 defines the set of signals currently blocked from delivery
95 The signal mask for a process is initialized
96 from that of its parent (normally empty).
97 It may be changed with a
99 call, or when a signal is delivered to the process.
102 condition arises for a process, the signal is added to a set of
103 signals pending for the process.
104 If the signal is not currently
106 by the process then it is delivered to the process.
107 Signals may be delivered any time a process enters the operating system
108 (e.g., during a system call, page fault or trap, or clock interrupt).
109 If multiple signals are ready to be delivered at the same time,
110 any signals that could be caused by traps are delivered first.
111 Additional signals may be processed at the same time, with each
112 appearing to interrupt the handlers for the previous signals
113 before their first instructions.
114 The set of pending signals is returned by the
118 is delivered, the current state of the process is saved,
119 a new signal mask is calculated (as described below),
120 and the signal handler is invoked.
121 The call to the handler
122 is arranged so that if the signal handling routine returns
123 normally the process will resume execution in the context
124 from before the signal's delivery.
125 If the process wishes to resume in a different context, then it
126 must arrange to restore the previous context itself.
128 When a signal is delivered to a process a new signal mask is
129 installed for the duration of the process' signal handler
132 system call is made).
133 This mask is formed by taking the union of the current signal mask set,
134 the signal to be delivered, and
135 the signal mask associated with the handler to be invoked.
140 assigns an action for a signal specified by
148 or a handler routine) and mask
149 to be used when delivering the specified signal.
152 is non-zero, the previous handling information for the signal
153 is returned to the user.
155 Once a signal handler is installed, it normally remains installed
158 system call is made, or an
161 A signal-specific default action may be reset by
166 The defaults are process termination, possibly with core dump;
167 no action; stopping the process; or continuing the process.
168 See the signal list below for each signal's default action.
173 the default action for the signal is to discard the signal,
174 and if a signal is pending,
175 the pending signal is discarded even if the signal is masked.
180 current and pending instances
181 of the signal are ignored and discarded.
183 Options may be specified by setting
185 The meaning of the various bits is as follows:
186 .Bl -tag -offset indent -width SA_RESETHANDXX
188 If this bit is set when installing a catching function
194 signal will be generated only when a child process exits,
195 not when a child process stops.
197 If this bit is set when calling
201 signal, the system will not create zombie processes when children of
202 the calling process exit.
203 If the calling process subsequently issues a
205 (or equivalent), it blocks until all of the calling process's child
206 processes terminate, and then returns a value of \-1 with
210 The same effect of avoiding zombie creation can also be achieved by setting
217 If this bit is set, the system will deliver the signal to the process
223 If this bit is set, further occurrences of the delivered signal are
224 not masked during the execution of the handler.
226 If this bit is set, the handler is reset back to
228 at the moment the signal is delivered.
232 If this bit is set, the handler function is assumed to be pointed to by the
235 .Vt "struct sigaction"
236 and should match the prototype shown above or as below in
238 This bit should not be set when assigning
244 If a signal is caught during the system calls listed below,
245 the call may be forced to terminate
248 the call may return with a data transfer shorter than requested,
249 or the call may be restarted.
250 Restart of pending calls is requested
255 The affected system calls include
264 on a communications channel or a slow device (such as a terminal,
265 but not a regular file)
270 However, calls that have already committed are not restarted,
271 but instead return a partial success (for example, a short read count).
277 all signals, the signal mask, the signal stack,
278 and the restart/interrupt flags are inherited by the child.
282 system call reinstates the default
283 action for all signals which were caught and
284 resets all signals to be caught on the user stack.
285 Ignored signals remain ignored;
286 the signal mask remains the same;
287 signals that restart pending system calls continue to do so.
289 The following is a list of all signals
290 with names as in the include file
292 .Bl -column SIGVTALARMXX "create core imagexxx"
293 .It Sy "NAME Default Action Description"
294 .It Dv SIGHUP No " terminate process" " terminal line hangup"
295 .It Dv SIGINT No " terminate process" " interrupt program"
296 .It Dv SIGQUIT No " create core image" " quit program"
297 .It Dv SIGILL No " create core image" " illegal instruction"
298 .It Dv SIGTRAP No " create core image" " trace trap"
299 .It Dv SIGABRT No " create core image" Ta Xr abort 3
302 .It Dv SIGEMT No " create core image" " emulate instruction executed"
303 .It Dv SIGFPE No " create core image" " floating-point exception"
304 .It Dv SIGKILL No " terminate process" " kill program"
305 .It Dv SIGBUS No " create core image" " bus error"
306 .It Dv SIGSEGV No " create core image" " segmentation violation"
307 .It Dv SIGSYS No " create core image" " non-existent system call invoked"
308 .It Dv SIGPIPE No " terminate process" " write on a pipe with no reader"
309 .It Dv SIGALRM No " terminate process" " real-time timer expired"
310 .It Dv SIGTERM No " terminate process" " software termination signal"
311 .It Dv SIGURG No " discard signal" " urgent condition present on socket"
312 .It Dv SIGSTOP No " stop process" " stop (cannot be caught or ignored)"
313 .It Dv SIGTSTP No " stop process" " stop signal generated from keyboard"
314 .It Dv SIGCONT No " discard signal" " continue after stop"
315 .It Dv SIGCHLD No " discard signal" " child status has changed"
316 .It Dv SIGTTIN No " stop process" " background read attempted from control terminal"
317 .It Dv SIGTTOU No " stop process" " background write attempted to control terminal"
318 .It Dv SIGIO No " discard signal" Tn " I/O"
319 is possible on a descriptor (see
321 .It Dv SIGXCPU No " terminate process" " cpu time limit exceeded (see"
323 .It Dv SIGXFSZ No " terminate process" " file size limit exceeded (see"
325 .It Dv SIGVTALRM No " terminate process" " virtual time alarm (see"
327 .It Dv SIGPROF No " terminate process" " profiling timer alarm (see"
329 .It Dv SIGWINCH No " discard signal" " Window size change"
330 .It Dv SIGINFO No " discard signal" " status request from keyboard"
331 .It Dv SIGUSR1 No " terminate process" " User defined signal 1"
332 .It Dv SIGUSR2 No " terminate process" " User defined signal 2"
339 is not allowed to block
343 Any attempt to do so will be silently ignored.
345 The following functions are either reentrant or not interruptible
346 by signals and are async-signal safe.
347 Therefore applications may
348 invoke them, without restriction, from signal-catching functions:
434 .Fn timer_getoverrun ,
451 and perhaps some others.
453 Extension Interfaces:
458 All functions not in the above lists are considered to be unsafe
459 with respect to signals.
460 That is to say, the behaviour of such
461 functions when called from a signal handler is undefined.
462 In general though, signal handlers should do little more than set a
463 flag; most other actions are not safe.
465 Also, it is good practice to make a copy of the global variable
467 and restore it before returning from the signal handler.
468 This protects against the side effect of
470 being set by functions called from inside the signal handler.
474 There are three possible prototypes the handler may match:
475 .Bl -tag -offset indent -width short
479 .It Traditional BSD style:
481 .Fn handler int "int code" "struct sigcontext *scp" ;
482 .It Tn POSIX Dv SA_SIGINFO :
484 .Fn handler int "siginfo_t *info" "ucontext_t *uap" ;
487 The handler function should match the
493 It then should be pointed to by the
496 .Vt "struct sigaction" .
497 Note that you should not assign
505 flag is not set, the handler function should match
510 prototype and be pointed to by
514 .Vt "struct sigaction" .
517 always sends the three arguments of the latter and since the
519 prototype is a subset, both will work.
522 member declaration in
524 include files is that of
528 so a function pointer of a
530 function needs to be casted to
531 compile without warning.
534 style is not portable and since its capabilities
535 are a full subset of a
538 its use is deprecated.
542 argument is the signal number, one of the
557 handler contain a numeric code explaining the
558 cause of the signal, usually one of the
562 or codes specific to a signal, i.e., one of the
571 handler points to an instance of
572 .Vt "struct sigcontext" .
579 handler points to an instance of
585 will fail and no new signal handler will be installed if one
586 of the following occurs:
593 points to memory that is not a valid part of the process
599 is not a valid signal number.
601 An attempt is made to ignore or supply a handler for
629 system call is expected to conform to
635 flags are Berkeley extensions,
650 Those signals are available on most
657 flags are intended for backwards compatibility with other operating
665 flags are featuring options commonly found in other operating systems.
666 The flags are approved by
668 along with the option to avoid zombie creation by ignoring