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32 .\" From: @(#)sigaction.2 8.2 (Berkeley) 4/3/94
40 .Nd software signal facilities
44 .Fd #include <signal.h>
48 void (*__sa_handler) __P((int));
49 void (*__sa_sigaction) __P((int, struct __siginfo *,
51 } __sigaction_u; /* signal handler */
52 int sa_flags; /* see signal options below */
53 sigset_t sa_mask; /* signal mask to apply */
56 #define sa_handler __sigaction_u.__sa_handler
57 #define sa_sigaction __sigaction_u.__sa_sigaction
60 .Fn sigaction "int sig" "const struct sigaction *act" "struct sigaction *oact"
62 The system defines a set of signals that may be delivered to a process.
63 Signal delivery resembles the occurrence of a hardware interrupt:
64 the signal is normally blocked from further occurrence, the current process
65 context is saved, and a new one is built. A process may specify a
67 to which a signal is delivered, or specify that a signal is to be
69 A process may also specify that a default action is to be taken
70 by the system when a signal occurs.
73 in which case its delivery is postponed until it is
75 The action to be taken on delivery is determined at the time
77 Normally, signal handlers execute on the current stack
78 of the process. This may be changed, on a per-handler basis,
79 so that signals are taken on a special
82 Signal routines normally execute with the signal that caused their
85 but other signals may yet occur.
88 defines the set of signals currently blocked from delivery
89 to a process. The signal mask for a process is initialized
90 from that of its parent (normally empty). It
93 call, or when a signal is delivered to the process.
96 condition arises for a process, the signal is added to a set of
97 signals pending for the process.
98 If the signal is not currently
100 by the process then it is delivered to the process.
101 Signals may be delivered any time a process enters the operating system
102 (e.g., during a system call, page fault or trap, or clock interrupt).
103 If multiple signals are ready to be delivered at the same time,
104 any signals that could be caused by traps are delivered first.
105 Additional signals may be processed at the same time, with each
106 appearing to interrupt the handlers for the previous signals
107 before their first instructions.
108 The set of pending signals is returned by the
112 is delivered, the current state of the process is saved,
113 a new signal mask is calculated (as described below),
114 and the signal handler is invoked. The call to the handler
115 is arranged so that if the signal handling routine returns
116 normally the process will resume execution in the context
117 from before the signal's delivery.
118 If the process wishes to resume in a different context, then it
119 must arrange to restore the previous context itself.
121 When a signal is delivered to a process a new signal mask is
122 installed for the duration of the process' signal handler
126 This mask is formed by taking the union of the current signal mask set,
127 the signal to be delivered, and
128 the signal mask associated with the handler to be invoked.
131 assigns an action for a signal specified by
139 or a handler routine) and mask
140 to be used when delivering the specified signal.
143 is non-zero, the previous handling information for the signal
144 is returned to the user.
146 Once a signal handler is installed, it normally remains installed
152 A signal-specific default action may be reset by
157 The defaults are process termination, possibly with core dump;
158 no action; stopping the process; or continuing the process.
159 See the signal list below for each signal's default action.
164 the default action for the signal is to discard the signal,
165 and if a signal is pending,
166 the pending signal is discarded even if the signal is masked.
171 current and pending instances
172 of the signal are ignored and discarded.
174 Options may be specified by setting
176 The meaning of the various bits is as follows:
177 .Bl -tag -offset indent -width SA_RESETHANDXX
179 If this bit is set when installing a catching function
185 signal will be generated only when a child process exits,
186 not when a child process stops.
188 If this bit is set when calling
192 signal, the system will not create zombie processes when children of
193 the calling process exit. If the calling process subsequently issues
196 (or equivalent), it blocks until all of the calling process's child
197 processes terminate, and then returns a value of -1 with errno set to
200 If this bit is set, the system will deliver the signal to the process
206 If this bit is set, further occurrences of the delivered signal are
207 not masked during the execution of the handler.
209 If this bit is set, the handler is reset back to
211 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 ,
431 and perhaps some others.
433 Extension Interfaces:
438 All functions not in the above lists are considered to be unsafe
439 with respect to signals. That is to say, the behaviour of such
440 functions when called from a signal handler is undefined.
441 In general though, signal handlers should do little more than set a
442 flag; most other actions are not safe.
444 Also, it is good practice to make a copy of the global variable
446 and restore it before returning from the signal handler.
447 This protects against the side effect of
449 being set by functions called from inside the signal handler.
453 There are three possible prototypes the handler may match:
454 .Bl -tag -offset indent -width short
458 .It Traditional BSD style:
460 .Fn handler int "int code" "struct sigcontext *scp" ;
461 .It POSIX SA_SIGINFO:
463 .Fn handler int "siginfo_t *info" "void *context" ;
466 The handler function should match the SA_SIGINFO prototype if the
467 SA_SIGINFO bit is set in flags.
468 It then should be pointed to by the
471 .Dv struct sigaction .
472 Note that you should not assign SIG_DFL or SIG_IGN this way.
474 If the SA_SIGINFO flag is not set, the handler function should match
475 either the ANSI C or traditional BSD prototype and be pointed to by
479 .Dv struct sigaction .
482 always sends the three arguments of the latter and since the ANSI C
483 prototype is a subset, both will work.
486 member declaration in
488 include files is that of ANSI C (as required by POSIX),
489 so a function pointer of a BSD-style function needs to be casted to
490 compile without warning.
491 The traditional BSD style is not portable and since its capabilities
492 are a full subset of a SA_SIGINFO handler,
493 its use is deprecated.
497 argument is the signal number, one of the
499 values from <signal.h>.
503 argument of the BSD-style handler and the
507 argument to a SA_SIGINFO handler contain a numeric code explaning the
508 cause of the signal, usually one of the
511 <sys/signal.h> or codes specific to a signal, i.e. one of the
517 argument to a BSD-style handler points to an instance of struct
522 argument to a POSIX SA_SIGINFO handler points to an instance of
526 will fail and no new signal handler will be installed if one
527 of the following occurs:
534 points to memory that is not a valid part of the process
538 is not a valid signal number.
540 An attempt is made to ignore or supply a handler for
548 function call is expected to conform to
554 flags are Berkeley extensions,
569 Those signals are available on most
576 flags are intended for backwards compatibility with other operating
583 flags are featuring options commonly found in other operating systems.