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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 $
34 .\" $DragonFly: src/lib/libc/sys/sigaction.2,v 1.8 2007/06/30 19:03:52 swildner Exp $
41 .Nd software signal facilities
49 * Signal handler function if flag SA_SIGINFO is not used and for
50 * SIG_DFL and SIG_IGN.
52 void (*sa_handler)(int);
54 /* Signal handler function if flag SA_SIGINFO is used */
55 void (*sa_sigaction)(int, siginfo_t *, void *);
57 sigset_t sa_mask; /* signal mask to apply */
58 int sa_flags; /* see signal options below */
62 .Fn sigaction "int sig" "const struct sigaction *act" "struct sigaction *oact"
64 The system defines a set of signals that may be delivered to a process.
65 Signal delivery resembles the occurrence of a hardware interrupt:
66 the signal is normally blocked from further occurrence, the current process
67 context is saved, and a new one is built. 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
80 of the process. This may be changed, on a per-handler basis,
81 so that signals are taken on a special
84 Signal routines normally execute with the signal that caused their
87 but other signals may yet occur.
90 defines the set of signals currently blocked from delivery
91 to a process. The signal mask for a process is initialized
92 from that of its parent (normally empty). It
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. The call to the handler
117 is arranged so that if the signal handling routine returns
118 normally the process will resume execution in the context
119 from before the signal's delivery.
120 If the process wishes to resume in a different context, then it
121 must arrange to restore the previous context itself.
123 When a signal is delivered to a process a new signal mask is
124 installed for the duration of the process' signal handler
128 This mask is formed by taking the union of the current signal mask set,
129 the signal to be delivered, and
130 the signal mask associated with the handler to be invoked.
133 assigns an action for a signal specified by
141 or a handler routine) and mask
142 to be used when delivering the specified signal.
145 is non-zero, the previous handling information for the signal
146 is returned to the user.
148 Once a signal handler is installed, it normally remains installed
154 A signal-specific default action may be reset by
159 The defaults are process termination, possibly with core dump;
160 no action; stopping the process; or continuing the process.
161 See the signal list below for each signal's default action.
166 the default action for the signal is to discard the signal,
167 and if a signal is pending,
168 the pending signal is discarded even if the signal is masked.
173 current and pending instances
174 of the signal are ignored and discarded.
176 Options may be specified by setting
178 The meaning of the various bits is as follows:
179 .Bl -tag -offset indent -width SA_RESETHANDXX
181 If this bit is set when installing a catching function
187 signal will be generated only when a child process exits,
188 not when a child process stops.
190 If this bit is set when calling
194 signal, the system will not create zombie processes when children of
195 the calling process exit. If the calling process subsequently issues
198 (or equivalent), it blocks until all of the calling process's child
199 processes terminate, and then returns a value of -1 with
204 If this bit is set, the system will deliver the signal to the process
210 If this bit is set, further occurrences of the delivered signal are
211 not masked during the execution of the handler.
213 If this bit is set, the handler is reset back to
215 at the moment the signal is delivered.
217 If this bit is set, the handler function is assumed to be pointed to by the
219 member of struct sigaction and should match the prototype shown above or as
222 This bit should not be set when assigning
228 If a signal is caught during the system calls listed below,
229 the call may be forced to terminate
232 the call may return with a data transfer shorter than requested,
233 or the call may be restarted.
234 Restart of pending calls is requested
239 The affected system calls include
248 on a communications channel or a slow device (such as a terminal,
249 but not a regular file)
254 However, calls that have already committed are not restarted,
255 but instead return a partial success (for example, a short read count).
261 all signals, the signal mask, the signal stack,
262 and the restart/interrupt flags are inherited by the child.
265 reinstates the default
266 action for all signals which were caught and
267 resets all signals to be caught on the user stack.
268 Ignored signals remain ignored;
269 the signal mask remains the same;
270 signals that restart pending system calls continue to do so.
272 The following is a list of all signals
273 with names as in the include file
275 .Bl -column SIGVTALARMXX "create core imagexxx"
276 .It Sy "NAME Default Action Description"
277 .It Dv SIGHUP No " terminate process" " terminal line hangup"
278 .It Dv SIGINT No " terminate process" " interrupt program"
279 .It Dv SIGQUIT No " create core image" " quit program"
280 .It Dv SIGILL No " create core image" " illegal instruction"
281 .It Dv SIGTRAP No " create core image" " trace trap"
282 .It Dv SIGABRT No " create core image" Ta Xr abort 3
285 .It Dv SIGEMT No " create core image" " emulate instruction executed"
286 .It Dv SIGFPE No " create core image" " floating-point exception"
287 .It Dv SIGKILL No " terminate process" " kill program"
288 .It Dv SIGBUS No " create core image" " bus error"
289 .It Dv SIGSEGV No " create core image" " segmentation violation"
290 .It Dv SIGSYS No " create core image" " non-existent system call invoked"
291 .It Dv SIGPIPE No " terminate process" " write on a pipe with no reader"
292 .It Dv SIGALRM No " terminate process" " real-time timer expired"
293 .It Dv SIGTERM No " terminate process" " software termination signal"
294 .It Dv SIGURG No " discard signal" " urgent condition present on socket"
295 .It Dv SIGSTOP No " stop process" " stop (cannot be caught or ignored)"
296 .It Dv SIGTSTP No " stop process" " stop signal generated from keyboard"
297 .It Dv SIGCONT No " discard signal" " continue after stop"
298 .It Dv SIGCHLD No " discard signal" " child status has changed"
299 .It Dv SIGTTIN No " stop process" " background read attempted from control terminal"
300 .It Dv SIGTTOU No " stop process" " background write attempted to control terminal"
301 .It Dv SIGIO No " discard signal" Tn " I/O"
302 is possible on a descriptor (see
304 .It Dv SIGXCPU No " terminate process" " cpu time limit exceeded (see"
306 .It Dv SIGXFSZ No " terminate process" " file size limit exceeded (see"
308 .It Dv SIGVTALRM No " terminate process" " virtual time alarm (see"
310 .It Dv SIGPROF No " terminate process" " profiling timer alarm (see"
312 .It Dv SIGWINCH No " discard signal" " Window size change"
313 .It Dv SIGINFO No " discard signal" " status request from keyboard"
314 .It Dv SIGUSR1 No " terminate process" " User defined signal 1"
315 .It Dv SIGUSR2 No " terminate process" " User defined signal 2"
322 is not allowed to block
326 Any attempt to do so will be silently ignored.
328 The following functions are either reentrant or not interruptible
329 by signals and are async-signal safe.
330 Therefore applications may
331 invoke them, without restriction, from signal-catching functions:
417 .Fn timer_getoverrun ,
427 All functions not in the above lists are considered to be unsafe
428 with respect to signals. That is to say, the behaviour of such
429 functions when called from a signal handler is undefined.
433 There are three possible prototypes the handler may match:
434 .Bl -tag -offset indent -width short
438 .It Traditional BSD style:
440 .Fn handler int "int code" "struct sigcontext *scp" ;
441 .It POSIX SA_SIGINFO:
443 .Fn handler int "siginfo_t *info" "ucontext_t *uap" ;
446 The handler function should match the SA_SIGINFO prototype if the
447 SA_SIGINFO bit is set in flags.
448 It then should be pointed to by the
451 .Dv struct sigaction .
452 Note that you should not assign SIG_DFL or SIG_IGN this way.
454 If the SA_SIGINFO flag is not set, the handler function should match
455 either the ANSI C or traditional
457 prototype and be pointed to by
461 .Dv struct sigaction .
464 always sends the three arguments of the latter and since the ANSI C
465 prototype is a subset, both will work.
468 member declaration in
470 include files is that of ANSI C (as required by POSIX),
471 so a function pointer of a
473 function needs to be casted to
474 compile without warning.
477 style is not portable and since its capabilities
478 are a full subset of a SA_SIGINFO handler,
479 its use is deprecated.
483 argument is the signal number, one of the
495 argument to a SA_SIGINFO handler contain a numeric code explaining the
496 cause of the signal, usually one of the
500 or codes specific to a signal, i.e. one of the
508 handler points to an instance of struct
513 argument to a POSIX SA_SIGINFO handler points to an instance of
517 will fail and no new signal handler will be installed if one
518 of the following occurs:
525 points to memory that is not a valid part of the process
529 is not a valid signal number.
531 An attempt is made to ignore or supply a handler for
558 function call is expected to conform to
564 flags are Berkeley extensions,
579 Those signals are available on most
586 flags are intended for backwards compatibility with other operating
593 flags are featuring options commonly found in other operating systems.