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28 .\" From: @(#)sigaction.2 8.2 (Berkeley) 4/3/94
35 .Nd software signal facilities
42 void (*sa_handler)(int);
43 void (*sa_sigaction)(int, siginfo_t *, void *);
44 int sa_flags; /* see signal options below */
45 sigset_t sa_mask; /* signal mask to apply */
52 .Fa "const struct sigaction * restrict act"
53 .Fa "struct sigaction * restrict oact"
56 The system defines a set of signals that may be delivered to a process.
57 Signal delivery resembles the occurrence of a hardware interrupt:
58 the signal is normally blocked from further occurrence, the current thread
59 context is saved, and a new one is built.
60 A process may specify a
62 to which a signal is delivered, or specify that a signal is to be
64 A process may also specify that a default action is to be taken
65 by the system when a signal occurs.
69 in which case it will not be delivered to that thread until it is
71 The action to be taken on delivery is determined at the time
73 Normally, signal handlers execute on the current stack
75 This may be changed, on a per-handler basis,
76 so that signals are taken on a special
79 Signal routines normally execute with the signal that caused their
82 but other signals may yet occur.
85 defines the set of signals currently blocked from delivery
87 The signal mask for a thread is initialized
88 from that of its parent (normally empty).
89 It may be changed with a
93 call, or when a signal is delivered to the thread.
96 condition arises for a process or thread, the signal is added to a set of
97 signals pending for the process or thread.
98 Whether the signal is directed at the process in general or at a specific
99 thread depends on how it is generated.
100 For signals directed at a specific thread,
101 if the signal is not currently
103 by the thread then it is delivered to the thread.
104 For signals directed at the process,
105 if the signal is not currently
107 by all threads then it is delivered to one thread that does not have it blocked
108 (the selection of which is unspecified).
109 Signals may be delivered any time a thread enters the operating system
110 (e.g., during a system call, page fault or trap, or clock interrupt).
111 If multiple signals are ready to be delivered at the same time,
112 any signals that could be caused by traps are delivered first.
113 Additional signals may be processed at the same time, with each
114 appearing to interrupt the handlers for the previous signals
115 before their first instructions.
116 The set of pending signals is returned by the
120 is delivered, the current state of the thread is saved,
121 a new signal mask is calculated (as described below),
122 and the signal handler is invoked.
123 The call to the handler
124 is arranged so that if the signal handling routine returns
125 normally the thread will resume execution in the context
126 from before the signal's delivery.
127 If the thread wishes to resume in a different context, then it
128 must arrange to restore the previous context itself.
130 When a signal is delivered to a thread a new signal mask is
131 installed for the duration of the process' signal handler
134 system call is made).
135 This mask is formed by taking the union of the current signal mask set,
136 the signal to be delivered, and
137 the signal mask associated with the handler to be invoked.
142 assigns an action for a signal specified by
146 is non-NULL, it specifies an action
149 or a handler routine) and mask to be used when delivering the specified signal.
152 is non-NULL, the previous handling information for the signal
153 is returned to the user.
155 The above declaration of
156 .Vt "struct sigaction"
158 It is provided only to list the accessible members.
161 for the actual definition.
162 In particular, the storage occupied by
166 overlaps, and it is nonsensical for an application to attempt to use both
169 Once a signal handler is installed, it normally remains installed
172 system call is made, or an
175 A signal-specific default action may be reset by
180 The defaults are process termination, possibly with core dump;
181 no action; stopping the process; or continuing the process.
182 See the signal list below for each signal's default action.
187 the default action for the signal is to discard the signal,
188 and if a signal is pending,
189 the pending signal is discarded even if the signal is masked.
194 current and pending instances
195 of the signal are ignored and discarded.
197 Options may be specified by setting
199 The meaning of the various bits is as follows:
200 .Bl -tag -offset indent -width SA_RESETHANDXX
202 If this bit is set when installing a catching function
208 signal will be generated only when a child process exits,
209 not when a child process stops.
211 If this bit is set when calling
215 signal, the system will not create zombie processes when children of
216 the calling process exit.
217 If the calling process subsequently issues a
219 (or equivalent), it blocks until all of the calling process's child
220 processes terminate, and then returns a value of \-1 with
224 The same effect of avoiding zombie creation can also be achieved by setting
231 If this bit is set, the system will deliver the signal to the process
234 specified by each thread with
237 If this bit is set, further occurrences of the delivered signal are
238 not masked during the execution of the handler.
240 If this bit is set, the handler is reset back to
242 at the moment the signal is delivered.
246 If this bit is set, the handler function is assumed to be pointed to by the
249 .Vt "struct sigaction"
250 and should match the prototype shown above or as below in
252 This bit should not be set when assigning
258 If a signal is caught during the system calls listed below,
259 the call may be forced to terminate
262 the call may return with a data transfer shorter than requested,
263 or the call may be restarted.
264 Restart of pending calls is requested
269 The affected system calls include
278 on a communications channel or a slow device (such as a terminal,
279 but not a regular file)
284 However, calls that have already committed are not restarted,
285 but instead return a partial success (for example, a short read count).
289 the signal mask is inherited by the new thread and
290 the set of pending signals and the signal stack for the new thread are empty.
296 all signals, the signal mask, the signal stack,
297 and the restart/interrupt flags are inherited by the child.
301 system call reinstates the default
302 action for all signals which were caught and
303 resets all signals to be caught on the user stack.
304 Ignored signals remain ignored;
305 the signal mask remains the same;
306 signals that restart pending system calls continue to do so.
308 The following is a list of all signals
309 with names as in the include file
311 .Bl -column SIGVTALARMXX "create core imagexxx"
312 .It Sy NAME Ta Sy Default Action Ta Sy Description
313 .It Dv SIGHUP Ta terminate process Ta terminal line hangup
314 .It Dv SIGINT Ta terminate process Ta interrupt program
315 .It Dv SIGQUIT Ta create core image Ta quit program
316 .It Dv SIGILL Ta create core image Ta illegal instruction
317 .It Dv SIGTRAP Ta create core image Ta trace trap
318 .It Dv SIGABRT Ta create core image Ta Xr abort 3 call (formerly Dv SIGIOT )
319 .It Dv SIGEMT Ta create core image Ta emulate instruction executed
320 .It Dv SIGFPE Ta create core image Ta floating-point exception
321 .It Dv SIGKILL Ta terminate process Ta kill program
322 .It Dv SIGBUS Ta create core image Ta bus error
323 .It Dv SIGSEGV Ta create core image Ta segmentation violation
324 .It Dv SIGSYS Ta create core image Ta non-existent system call invoked
325 .It Dv SIGPIPE Ta terminate process Ta write on a pipe with no reader
326 .It Dv SIGALRM Ta terminate process Ta real-time timer expired
327 .It Dv SIGTERM Ta terminate process Ta software termination signal
328 .It Dv SIGURG Ta discard signal Ta urgent condition present on socket
329 .It Dv SIGSTOP Ta stop process Ta stop (cannot be caught or ignored)
330 .It Dv SIGTSTP Ta stop process Ta stop signal generated from keyboard
331 .It Dv SIGCONT Ta discard signal Ta continue after stop
332 .It Dv SIGCHLD Ta discard signal Ta child status has changed
333 .It Dv SIGTTIN Ta stop process Ta background read attempted from control terminal
334 .It Dv SIGTTOU Ta stop process Ta background write attempted to control terminal
335 .It Dv SIGIO Ta discard signal Ta I/O is possible on a descriptor (see Xr fcntl 2 )
336 .It Dv SIGXCPU Ta terminate process Ta cpu time limit exceeded (see Xr setrlimit 2 )
337 .It Dv SIGXFSZ Ta terminate process Ta file size limit exceeded (see Xr setrlimit 2 )
338 .It Dv SIGVTALRM Ta terminate process Ta virtual time alarm (see Xr setitimer 2 )
339 .It Dv SIGPROF Ta terminate process Ta profiling timer alarm (see Xr setitimer 2 )
340 .It Dv SIGWINCH Ta discard signal Ta window size change
341 .It Dv SIGINFO Ta discard signal Ta status request from keyboard
342 .It Dv SIGUSR1 Ta terminate process Ta user defined signal 1
343 .It Dv SIGUSR2 Ta terminate process Ta user defined signal 2
350 is not allowed to block
354 Any attempt to do so will be silently ignored.
356 The following functions are either reentrant or not interruptible
357 by signals and are async-signal safe.
358 Therefore applications may
359 invoke them, without restriction, from signal-catching functions
360 or from a child process after calling
362 in a multi-threaded process:
429 .Fn pthread_sigmask ,
486 X/Open Systems Interfaces:
496 .Fn timer_getoverrun ,
505 Base Interfaces not specified as async-signal safe by
512 Base Interfaces not specified as async-signal safe by
567 Extension Interfaces:
587 In addition, reading or writing
589 is async-signal safe.
591 All functions not in the above lists are considered to be unsafe
592 with respect to signals.
593 That is to say, the behaviour of such
594 functions is undefined when they are called from a signal handler
595 that interrupted an unsafe function.
596 In general though, signal handlers should do little more than set a
597 flag; most other actions are not safe.
599 Also, it is good practice to make a copy of the global variable
601 and restore it before returning from the signal handler.
602 This protects against the side effect of
604 being set by functions called from inside the signal handler.
608 There are three possible prototypes the handler may match:
609 .Bl -tag -offset indent -width short
613 .It Traditional BSD style:
615 .Fn handler int "int code" "struct sigcontext *scp" ;
616 .It Tn POSIX Dv SA_SIGINFO :
618 .Fn handler int "siginfo_t *info" "ucontext_t *uap" ;
621 The handler function should match the
627 It then should be pointed to by the
630 .Vt "struct sigaction" .
631 Note that you should not assign
639 flag is not set, the handler function should match
644 prototype and be pointed to by
648 .Vt "struct sigaction" .
651 always sends the three arguments of the latter and since the
653 prototype is a subset, both will work.
656 member declaration in
658 include files is that of
662 so a function pointer of a
664 function needs to be casted to
665 compile without warning.
668 style is not portable and since its capabilities
669 are a full subset of a
672 its use is deprecated.
676 argument is the signal number, one of the
691 handler contain a numeric code explaining the
692 cause of the signal, usually one of the
696 or codes specific to a signal, i.e., one of the
705 handler points to an instance of
706 .Vt "struct sigcontext" .
713 handler points to an instance of
719 will fail and no new signal handler will be installed if one
720 of the following occurs:
726 is not a valid signal number.
728 An attempt is made to ignore or supply a handler for
754 system call is expected to conform to
760 flags are Berkeley extensions,
775 Those signals are available on most
782 flags are intended for backwards compatibility with other operating
790 flags are featuring options commonly found in other operating systems.
791 The flags are approved by
793 along with the option to avoid zombie creation by ignoring