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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/libcr/sys/Attic/sigaction.2,v 1.3 2004/03/11 12:28:52 hmp 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 errno set to
202 If this bit is set, the system will deliver the signal to the process
208 If this bit is set, further occurrences of the delivered signal are
209 not masked during the execution of the handler.
211 If this bit is set, the handler is reset back to
213 at the moment the signal is delivered.
215 If this bit is set, the handler function is assumed to be pointed to by the
217 member of struct sigaction and should match the prototype shown above or as
220 This bit should not be set when assigning
226 If a signal is caught during the system calls listed below,
227 the call may be forced to terminate
230 the call may return with a data transfer shorter than requested,
231 or the call may be restarted.
232 Restart of pending calls is requested
237 The affected system calls include
246 on a communications channel or a slow device (such as a terminal,
247 but not a regular file)
252 However, calls that have already committed are not restarted,
253 but instead return a partial success (for example, a short read count).
259 all signals, the signal mask, the signal stack,
260 and the restart/interrupt flags are inherited by the child.
263 reinstates the default
264 action for all signals which were caught and
265 resets all signals to be caught on the user stack.
266 Ignored signals remain ignored;
267 the signal mask remains the same;
268 signals that restart pending system calls continue to do so.
270 The following is a list of all signals
271 with names as in the include file
273 .Bl -column SIGVTALARMXX "create core imagexxx"
274 .It Sy "NAME Default Action Description"
275 .It Dv SIGHUP No " terminate process" " terminal line hangup"
276 .It Dv SIGINT No " terminate process" " interrupt program"
277 .It Dv SIGQUIT No " create core image" " quit program"
278 .It Dv SIGILL No " create core image" " illegal instruction"
279 .It Dv SIGTRAP No " create core image" " trace trap"
280 .It Dv SIGABRT No " create core image" Ta Xr abort 3
283 .It Dv SIGEMT No " create core image" " emulate instruction executed"
284 .It Dv SIGFPE No " create core image" " floating-point exception"
285 .It Dv SIGKILL No " terminate process" " kill program"
286 .It Dv SIGBUS No " create core image" " bus error"
287 .It Dv SIGSEGV No " create core image" " segmentation violation"
288 .It Dv SIGSYS No " create core image" " non-existent system call invoked"
289 .It Dv SIGPIPE No " terminate process" " write on a pipe with no reader"
290 .It Dv SIGALRM No " terminate process" " real-time timer expired"
291 .It Dv SIGTERM No " terminate process" " software termination signal"
292 .It Dv SIGURG No " discard signal" " urgent condition present on socket"
293 .It Dv SIGSTOP No " stop process" " stop (cannot be caught or ignored)"
294 .It Dv SIGTSTP No " stop process" " stop signal generated from keyboard"
295 .It Dv SIGCONT No " discard signal" " continue after stop"
296 .It Dv SIGCHLD No " discard signal" " child status has changed"
297 .It Dv SIGTTIN No " stop process" " background read attempted from control terminal"
298 .It Dv SIGTTOU No " stop process" " background write attempted to control terminal"
299 .It Dv SIGIO No " discard signal" Tn " I/O"
300 is possible on a descriptor (see
302 .It Dv SIGXCPU No " terminate process" " cpu time limit exceeded (see"
304 .It Dv SIGXFSZ No " terminate process" " file size limit exceeded (see"
306 .It Dv SIGVTALRM No " terminate process" " virtual time alarm (see"
308 .It Dv SIGPROF No " terminate process" " profiling timer alarm (see"
310 .It Dv SIGWINCH No " discard signal" " Window size change"
311 .It Dv SIGINFO No " discard signal" " status request from keyboard"
312 .It Dv SIGUSR1 No " terminate process" " User defined signal 1"
313 .It Dv SIGUSR2 No " terminate process" " User defined signal 2"
320 is not allowed to block
324 Any attempt to do so will be silently ignored.
326 The following functions are either reentrant or not interruptible
327 by signals and are async-signal safe.
328 Therefore applications may
329 invoke them, without restriction, from signal-catching functions:
415 .Fn timer_getoverrun ,
425 All functions not in the above lists are considered to be unsafe
426 with respect to signals. That is to say, the behaviour of such
427 functions when called from a signal handler is undefined.
431 There are three possible prototypes the handler may match:
432 .Bl -tag -offset indent -width short
436 .It Traditional BSD style:
438 .Fn handler int "int code" "struct sigcontext *scp" ;
439 .It POSIX SA_SIGINFO:
441 .Fn handler int "siginfo_t *info" "ucontext_t *uap" ;
444 The handler function should match the SA_SIGINFO prototype if the
445 SA_SIGINFO bit is set in flags.
446 It then should be pointed to by the
449 .Dv struct sigaction .
450 Note that you should not assign SIG_DFL or SIG_IGN this way.
452 If the SA_SIGINFO flag is not set, the handler function should match
453 either the ANSI C or traditional
455 prototype and be pointed to by
459 .Dv struct sigaction .
462 always sends the three arguments of the latter and since the ANSI C
463 prototype is a subset, both will work.
466 member declaration in
468 include files is that of ANSI C (as required by POSIX),
469 so a function pointer of a
471 function needs to be casted to
472 compile without warning.
475 style is not portable and since its capabilities
476 are a full subset of a SA_SIGINFO handler,
477 its use is deprecated.
481 argument is the signal number, one of the
483 values from <signal.h>.
493 argument to a SA_SIGINFO handler contain a numeric code explaining the
494 cause of the signal, usually one of the
497 <sys/signal.h> or codes specific to a signal, i.e. one of the
505 handler points to an instance of struct
510 argument to a POSIX SA_SIGINFO handler points to an instance of
514 will fail and no new signal handler will be installed if one
515 of the following occurs:
522 points to memory that is not a valid part of the process
526 is not a valid signal number.
528 An attempt is made to ignore or supply a handler for
536 function call is expected to conform to
542 flags are Berkeley extensions,
557 Those signals are available on most
564 flags are intended for backwards compatibility with other operating
571 flags are featuring options commonly found in other operating systems.