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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 $
40 .Nd software signal facilities
48 * Signal handler function if flag SA_SIGINFO is not used and for
49 * SIG_DFL and SIG_IGN.
51 void (*sa_handler)(int);
53 /* Signal handler function if flag SA_SIGINFO is used */
54 void (*sa_sigaction)(int, siginfo_t *, void *);
56 sigset_t sa_mask; /* signal mask to apply */
57 int sa_flags; /* see signal options below */
61 .Fn sigaction "int sig" "const struct sigaction *act" "struct sigaction *oact"
63 The system defines a set of signals that may be delivered to a process.
64 Signal delivery resembles the occurrence of a hardware interrupt:
65 the signal is normally blocked from further occurrence, the current process
66 context is saved, and a new one is built. A process may specify a
68 to which a signal is delivered, or specify that a signal is to be
70 A process may also specify that a default action is to be taken
71 by the system when a signal occurs.
74 in which case its delivery is postponed until it is
76 The action to be taken on delivery is determined at the time
78 Normally, signal handlers execute on the current stack
79 of the process. 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
90 to a process. The signal mask for a process is initialized
91 from that of its parent (normally empty). It
94 call, or when a signal is delivered to the process.
97 condition arises for a process, the signal is added to a set of
98 signals pending for the process.
99 If the signal is not currently
101 by the process then it is delivered to the process.
102 Signals may be delivered any time a process enters the operating system
103 (e.g., during a system call, page fault or trap, or clock interrupt).
104 If multiple signals are ready to be delivered at the same time,
105 any signals that could be caused by traps are delivered first.
106 Additional signals may be processed at the same time, with each
107 appearing to interrupt the handlers for the previous signals
108 before their first instructions.
109 The set of pending signals is returned by the
113 is delivered, the current state of the process is saved,
114 a new signal mask is calculated (as described below),
115 and the signal handler is invoked. The call to the handler
116 is arranged so that if the signal handling routine returns
117 normally the process will resume execution in the context
118 from before the signal's delivery.
119 If the process wishes to resume in a different context, then it
120 must arrange to restore the previous context itself.
122 When a signal is delivered to a process a new signal mask is
123 installed for the duration of the process' signal handler
127 This mask is formed by taking the union of the current signal mask set,
128 the signal to be delivered, and
129 the signal mask associated with the handler to be invoked.
132 assigns an action for a signal specified by
140 or a handler routine) and mask
141 to be used when delivering the specified signal.
144 is non-zero, the previous handling information for the signal
145 is returned to the user.
147 Once a signal handler is installed, it normally remains installed
153 A signal-specific default action may be reset by
158 The defaults are process termination, possibly with core dump;
159 no action; stopping the process; or continuing the process.
160 See the signal list below for each signal's default action.
165 the default action for the signal is to discard the signal,
166 and if a signal is pending,
167 the pending signal is discarded even if the signal is masked.
172 current and pending instances
173 of the signal are ignored and discarded.
175 Options may be specified by setting
177 The meaning of the various bits is as follows:
178 .Bl -tag -offset indent -width SA_RESETHANDXX
180 If this bit is set when installing a catching function
186 signal will be generated only when a child process exits,
187 not when a child process stops.
189 If this bit is set when calling
193 signal, the system will not create zombie processes when children of
194 the calling process exit. If the calling process subsequently issues
197 (or equivalent), it blocks until all of the calling process's child
198 processes terminate, and then returns a value of -1 with errno set to
201 If this bit is set, the system will deliver the signal to the process
207 If this bit is set, further occurrences of the delivered signal are
208 not masked during the execution of the handler.
210 If this bit is set, the handler is reset back to
212 at the moment the signal is delivered.
214 If this bit is set, the handler function is assumed to be pointed to by the
216 member of struct sigaction and should match the prototype shown above or as
219 This bit should not be set when assigning
225 If a signal is caught during the system calls listed below,
226 the call may be forced to terminate
229 the call may return with a data transfer shorter than requested,
230 or the call may be restarted.
231 Restart of pending calls is requested
236 The affected system calls include
245 on a communications channel or a slow device (such as a terminal,
246 but not a regular file)
251 However, calls that have already committed are not restarted,
252 but instead return a partial success (for example, a short read count).
258 all signals, the signal mask, the signal stack,
259 and the restart/interrupt flags are inherited by the child.
262 reinstates the default
263 action for all signals which were caught and
264 resets all signals to be caught on the user stack.
265 Ignored signals remain ignored;
266 the signal mask remains the same;
267 signals that restart pending system calls continue to do so.
269 The following is a list of all signals
270 with names as in the include file
272 .Bl -column SIGVTALARMXX "create core imagexxx"
273 .It Sy "NAME Default Action Description"
274 .It Dv SIGHUP No " terminate process" " terminal line hangup"
275 .It Dv SIGINT No " terminate process" " interrupt program"
276 .It Dv SIGQUIT No " create core image" " quit program"
277 .It Dv SIGILL No " create core image" " illegal instruction"
278 .It Dv SIGTRAP No " create core image" " trace trap"
279 .It Dv SIGABRT No " create core image" Ta Xr abort 3
282 .It Dv SIGEMT No " create core image" " emulate instruction executed"
283 .It Dv SIGFPE No " create core image" " floating-point exception"
284 .It Dv SIGKILL No " terminate process" " kill program"
285 .It Dv SIGBUS No " create core image" " bus error"
286 .It Dv SIGSEGV No " create core image" " segmentation violation"
287 .It Dv SIGSYS No " create core image" " non-existent system call invoked"
288 .It Dv SIGPIPE No " terminate process" " write on a pipe with no reader"
289 .It Dv SIGALRM No " terminate process" " real-time timer expired"
290 .It Dv SIGTERM No " terminate process" " software termination signal"
291 .It Dv SIGURG No " discard signal" " urgent condition present on socket"
292 .It Dv SIGSTOP No " stop process" " stop (cannot be caught or ignored)"
293 .It Dv SIGTSTP No " stop process" " stop signal generated from keyboard"
294 .It Dv SIGCONT No " discard signal" " continue after stop"
295 .It Dv SIGCHLD No " discard signal" " child status has changed"
296 .It Dv SIGTTIN No " stop process" " background read attempted from control terminal"
297 .It Dv SIGTTOU No " stop process" " background write attempted to control terminal"
298 .It Dv SIGIO No " discard signal" Tn " I/O"
299 is possible on a descriptor (see
301 .It Dv SIGXCPU No " terminate process" " cpu time limit exceeded (see"
303 .It Dv SIGXFSZ No " terminate process" " file size limit exceeded (see"
305 .It Dv SIGVTALRM No " terminate process" " virtual time alarm (see"
307 .It Dv SIGPROF No " terminate process" " profiling timer alarm (see"
309 .It Dv SIGWINCH No " discard signal" " Window size change"
310 .It Dv SIGINFO No " discard signal" " status request from keyboard"
311 .It Dv SIGUSR1 No " terminate process" " User defined signal 1"
312 .It Dv SIGUSR2 No " terminate process" " User defined signal 2"
319 is not allowed to block
323 Any attempt to do so will be silently ignored.
325 The following functions are either reentrant or not interruptible
326 by signals and are async-signal safe.
327 Therefore applications may
328 invoke them, without restriction, from signal-catching functions:
414 .Fn timer_getoverrun ,
424 All functions not in the above lists are considered to be unsafe
425 with respect to signals. That is to say, the behaviour of such
426 functions when called from a signal handler is undefined.
430 There are three possible prototypes the handler may match:
431 .Bl -tag -offset indent -width short
435 .It Traditional BSD style:
437 .Fn handler int "int code" "struct sigcontext *scp" ;
438 .It POSIX SA_SIGINFO:
440 .Fn handler int "siginfo_t *info" "ucontext_t *uap" ;
443 The handler function should match the SA_SIGINFO prototype if the
444 SA_SIGINFO bit is set in flags.
445 It then should be pointed to by the
448 .Dv struct sigaction .
449 Note that you should not assign SIG_DFL or SIG_IGN this way.
451 If the SA_SIGINFO flag is not set, the handler function should match
452 either the ANSI C or traditional
454 prototype and be pointed to by
458 .Dv struct sigaction .
461 always sends the three arguments of the latter and since the ANSI C
462 prototype is a subset, both will work.
465 member declaration in
467 include files is that of ANSI C (as required by POSIX),
468 so a function pointer of a
470 function needs to be casted to
471 compile without warning.
474 style is not portable and since its capabilities
475 are a full subset of a SA_SIGINFO handler,
476 its use is deprecated.
480 argument is the signal number, one of the
482 values from <signal.h>.
492 argument to a SA_SIGINFO handler contain a numeric code explaining the
493 cause of the signal, usually one of the
496 <sys/signal.h> or codes specific to a signal, i.e. one of the
504 handler points to an instance of struct
509 argument to a POSIX SA_SIGINFO handler points to an instance of
513 will fail and no new signal handler will be installed if one
514 of the following occurs:
521 points to memory that is not a valid part of the process
525 is not a valid signal number.
527 An attempt is made to ignore or supply a handler for
535 function call is expected to conform to
541 flags are Berkeley extensions,
556 Those signals are available on most
563 flags are intended for backwards compatibility with other operating
570 flags are featuring options commonly found in other operating systems.