1SIGACTION(3P) POSIX Programmer's Manual SIGACTION(3P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
12 sigaction — examine and change a signal action
13
15 #include <signal.h>
16 int sigaction(int sig, const struct sigaction *restrict act,
18 struct sigaction *restrict oact);
19
21 The sigaction() function allows the calling process to examine and/or
22 specify the action to be associated with a specific signal. The argu‐
23 ment sig specifies the signal; acceptable values are defined in <sig‐
24 nal.h>.
25 The structure sigaction, used to describe an action to be taken, is
27 defined in the <signal.h> header to include at least the following mem‐
28 bers:
29 ┌────────────────┬───────────────┬───────────────────────────────────────┐
31 │ Member Type │ Member Name │ Description │
32 ├────────────────┼───────────────┼───────────────────────────────────────┤
33 │void(*) (int) │ sa_handler │Pointer to a signal-catching function │
34 │ │ │or one of the macros SIG_IGN or │
35 │ │ │SIG_DFL. │
36 │sigset_t │ sa_mask │Additional set of signals to be │
37 │ │ │blocked during execution of signal- │
38 │ │ │catching function. │
39 │int │ sa_flags │Special flags to affect behavior of │
40 │ │ │signal. │
41 │void(*) (int, │ sa_sigaction │Pointer to a signal-catching function. │
42 │ siginfo_t *, │ │ │
43 │void *) │ │ │
44 └────────────────┴───────────────┴───────────────────────────────────────┘
45 The storage occupied by sa_handler and sa_sigaction may overlap, and a
46 conforming application shall not use both simultaneously.
47 If the argument act is not a null pointer, it points to a structure
49 specifying the action to be associated with the specified signal. If
50 the argument oact is not a null pointer, the action previously associ‐
51 ated with the signal is stored in the location pointed to by the argu‐
52 ment oact. If the argument act is a null pointer, signal handling is
53 unchanged; thus, the call can be used to enquire about the current han‐
54 dling of a given signal. The SIGKILL and SIGSTOP signals shall not be
55 added to the signal mask using this mechanism; this restriction shall
56 be enforced by the system without causing an error to be indicated.
57 If the SA_SIGINFO flag (see below) is cleared in the sa_flags field of
59 the sigaction structure, the sa_handler field identifies the action to
60 be associated with the specified signal. If the SA_SIGINFO flag is set
61 in the sa_flags field, the sa_sigaction field specifies a signal-catch‐
62 ing function.
63 The sa_flags field can be used to modify the behavior of the specified
65 signal.
66 The following flags, defined in the <signal.h> header, can be set in
68 sa_flags:
69 SA_NOCLDSTOP Do not generate SIGCHLD when children stop or stopped
71 children continue.
72 If sig is SIGCHLD and the SA_NOCLDSTOP flag is not set in
74 sa_flags, and the implementation supports the SIGCHLD
75 signal, then a SIGCHLD signal shall be generated for the
76 calling process whenever any of its child processes stop
77 and a SIGCHLD signal may be generated for the calling
78 process whenever any of its stopped child processes are
79 continued. If sig is SIGCHLD and the SA_NOCLDSTOP flag
80 is set in sa_flags, then the implementation shall not
81 generate a SIGCHLD signal in this way.
82 SA_ONSTACK If set and an alternate signal stack has been declared
84 with sigaltstack(), the signal shall be delivered to the
85 calling process on that stack. Otherwise, the signal
86 shall be delivered on the current stack.
87 SA_RESETHAND If set, the disposition of the signal shall be reset to
89 SIG_DFL and the SA_SIGINFO flag shall be cleared on entry
90 to the signal handler.
91 Note: SIGILL and SIGTRAP cannot be automatically
93 reset when delivered; the system silently
94 enforces this restriction.
95 Otherwise, the disposition of the signal shall not be
97 modified on entry to the signal handler.
98 In addition, if this flag is set, sigaction() may behave
100 as if the SA_NODEFER flag were also set.
101 SA_RESTART This flag affects the behavior of interruptible func‐
103 tions; that is, those specified to fail with errno set to
104 [EINTR]. If set, and a function specified as interrupt‐
105 ible is interrupted by this signal, the function shall
106 restart and shall not fail with [EINTR] unless otherwise
107 specified. If an interruptible function which uses a
108 timeout is restarted, the duration of the timeout follow‐
109 ing the restart is set to an unspecified value that does
110 not exceed the original timeout value. If the flag is not
111 set, interruptible functions interrupted by this signal
112 shall fail with errno set to [EINTR].
113 SA_SIGINFO If cleared and the signal is caught, the signal-catching
115 function shall be entered as:
116 void func(int signo);
119 where signo is the only argument to the signal-catching
121 function. In this case, the application shall use the
122 sa_handler member to describe the signal-catching func‐
123 tion and the application shall not modify the sa_sigac‐
124 tion member.
125 If SA_SIGINFO is set and the signal is caught, the sig‐
127 nal-catching function shall be entered as:
128 void func(int signo, siginfo_t *info, void *context);
131 where two additional arguments are passed to the signal-
133 catching function. The second argument shall point to an
134 object of type siginfo_t explaining the reason why the
135 signal was generated; the third argument can be cast to a
136 pointer to an object of type ucontext_t to refer to the
137 receiving thread's context that was interrupted when the
138 signal was delivered. In this case, the application shall
139 use the sa_sigaction member to describe the signal-catch‐
140 ing function and the application shall not modify the
141 sa_handler member.
142 The si_signo member contains the system-generated signal
144 number.
145 The si_errno member may contain implementation-defined
147 additional error information; if non-zero, it contains an
148 error number identifying the condition that caused the
149 signal to be generated.
150 The si_code member contains a code identifying the cause
152 of the signal, as described in Section 2.4.3, Signal
153 Actions.
154 SA_NOCLDWAIT If sig does not equal SIGCHLD, the behavior is unspeci‐
156 fied. Otherwise, the behavior of the SA_NOCLDWAIT flag is
157 as specified in Consequences of Process Termination.
158 SA_NODEFER If set and sig is caught, sig shall not be added to the
160 thread's signal mask on entry to the signal handler
161 unless it is included in sa_mask. Otherwise, sig shall
162 always be added to the thread's signal mask on entry to
163 the signal handler.
164 When a signal is caught by a signal-catching function installed by
166 sigaction(), a new signal mask is calculated and installed for the
167 duration of the signal-catching function (or until a call to either
168 sigprocmask() or sigsuspend() is made). This mask is formed by taking
169 the union of the current signal mask and the value of the sa_mask for
170 the signal being delivered, and unless SA_NODEFER or SA_RESETHAND is
171 set, then including the signal being delivered. If and when the user's
172 signal handler returns normally, the original signal mask is restored.
173 Once an action is installed for a specific signal, it shall remain
175 installed until another action is explicitly requested (by another call
176 to sigaction()), until the SA_RESETHAND flag causes resetting of the
177 handler, or until one of the exec functions is called.
178 If the previous action for sig had been established by signal(), the
180 values of the fields returned in the structure pointed to by oact are
181 unspecified, and in particular oact->sa_handler is not necessarily the
182 same value passed to signal(). However, if a pointer to the same
183 structure or a copy thereof is passed to a subsequent call to sigac‐
184 tion() via the act argument, handling of the signal shall be as if the
185 original call to signal() were repeated.
186 If sigaction() fails, no new signal handler is installed.
188 It is unspecified whether an attempt to set the action for a signal
190 that cannot be caught or ignored to SIG_DFL is ignored or causes an
191 error to be returned with errno set to [EINVAL].
192 If SA_SIGINFO is not set in sa_flags, then the disposition of subse‐
194 quent occurrences of sig when it is already pending is implementation-
195 defined; the signal-catching function shall be invoked with a single
196 argument. If SA_SIGINFO is set in sa_flags, then subsequent occur‐
197 rences of sig generated by sigqueue() or as a result of any signal-gen‐
198 erating function that supports the specification of an application-
199 defined value (when sig is already pending) shall be queued in FIFO
200 order until delivered or accepted; the signal-catching function shall
201 be invoked with three arguments. The application specified value is
202 passed to the signal-catching function as the si_value member of the
203 siginfo_t structure.
204 The result of the use of sigaction() and a sigwait() function concur‐
206 rently within a process on the same signal is unspecified.
207
209 Upon successful completion, sigaction() shall return 0; otherwise, -1
210 shall be returned, errno shall be set to indicate the error, and no new
211 signal-catching function shall be installed.
212
214 The sigaction() function shall fail if:
215 EINVAL The sig argument is not a valid signal number or an attempt is
217 made to catch a signal that cannot be caught or ignore a signal
218 that cannot be ignored.
219 The sigaction() function may fail if:
221 EINVAL An attempt was made to set the action to SIG_DFL for a signal
223 that cannot be caught or ignored (or both).
224 In addition, on systems that do not support the XSI option, the sigac‐
226 tion() function may fail if the SA_SIGINFO flag is set in the sa_flags
227 field of the sigaction structure for a signal not in the range SIGRTMIN
228 to SIGRTMAX.
229 The following sections are informative.
231
233 Establishing a Signal Handler
234 The following example demonstrates the use of sigaction() to establish
235 a handler for the SIGINT signal.
236 #include <signal.h>
239 static void handler(int signum)
241 {
242 /* Take appropriate actions for signal delivery */
243 }
244 int main()
246 {
247 struct sigaction sa;
248 sa.sa_handler = handler;
250 sigemptyset(&sa.sa_mask);
251 sa.sa_flags = SA_RESTART; /* Restart functions if
252 interrupted by handler */
253 if (sigaction(SIGINT, &sa, NULL) == -1)
254 /* Handle error */;
255 /* Further code */
257 }
258
260 The sigaction() function supersedes the signal() function, and should
261 be used in preference. In particular, sigaction() and signal() should
262 not be used in the same process to control the same signal. The behav‐
263 ior of async-signal-safe functions, as defined in their respective
264 DESCRIPTION sections, is as specified by this volume of POSIX.1‐2017,
265 regardless of invocation from a signal-catching function. This is the
266 only intended meaning of the statement that async-signal-safe functions
267 may be used in signal-catching functions without restrictions. Applica‐
268 tions must still consider all effects of such functions on such things
269 as data structures, files, and process state. In particular, applica‐
270 tion developers need to consider the restrictions on interactions when
271 interrupting sleep() and interactions among multiple handles for a file
272 description. The fact that any specific function is listed as async-
273 signal-safe does not necessarily mean that invocation of that function
274 from a signal-catching function is recommended.
275 In order to prevent errors arising from interrupting non-async-signal-
277 safe function calls, applications should protect calls to these func‐
278 tions either by blocking the appropriate signals or through the use of
279 some programmatic semaphore (see semget(), sem_init(), sem_open(), and
280 so on). Note in particular that even the ``safe'' functions may modify
281 errno; the signal-catching function, if not executing as an independent
282 thread, should save and restore its value in order to avoid the possi‐
283 bility that delivery of a signal in between an error return from a
284 function that sets errno and the subsequent examination of errno could
285 result in the signal-catching function changing the value of errno.
286 Naturally, the same principles apply to the async-signal-safety of
287 application routines and asynchronous data access. Note that longjmp()
288 and siglongjmp() are not in the list of async-signal-safe functions.
289 This is because the code executing after longjmp() and siglongjmp() can
290 call any unsafe functions with the same danger as calling those unsafe
291 functions directly from the signal handler. Applications that use
292 longjmp() and siglongjmp() from within signal handlers require rigorous
293 protection in order to be portable. Many of the other functions that
294 are excluded from the list are traditionally implemented using either
295 malloc() or free() functions or the standard I/O library, both of which
296 traditionally use data structures in a non-async-signal-safe manner.
297 Since any combination of different functions using a common data struc‐
298 ture can cause async-signal-safety problems, this volume of
299 POSIX.1‐2017 does not define the behavior when any unsafe function is
300 called in a signal handler that interrupts an unsafe function.
301 Usually, the signal is executed on the stack that was in effect before
303 the signal was delivered. An alternate stack may be specified to
304 receive a subset of the signals being caught.
305 When the signal handler returns, the receiving thread resumes execution
307 at the point it was interrupted unless the signal handler makes other
308 arrangements. If longjmp() or _longjmp() is used to leave the signal
309 handler, then the signal mask must be explicitly restored.
310 This volume of POSIX.1‐2017 defines the third argument of a signal han‐
312 dling function when SA_SIGINFO is set as a void * instead of a ucon‐
313 text_t *, but without requiring type checking. New applications should
314 explicitly cast the third argument of the signal handling function to
315 ucontext_t *.
316 The BSD optional four argument signal handling function is not sup‐
318 ported by this volume of POSIX.1‐2017. The BSD declaration would be:
319 void handler(int sig, int code, struct sigcontext *scp,
322 char *addr);
323 where sig is the signal number, code is additional information on cer‐
325 tain signals, scp is a pointer to the sigcontext structure, and addr is
326 additional address information. Much the same information is available
327 in the objects pointed to by the second argument of the signal handler
328 specified when SA_SIGINFO is set.
329 Since the sigaction() function is allowed but not required to set
331 SA_NODEFER when the application sets the SA_RESETHAND flag, applica‐
332 tions which depend on the SA_RESETHAND functionality for the newly
333 installed signal handler must always explicitly set SA_NODEFER when
334 they set SA_RESETHAND in order to be portable.
335 See also the rationale for Realtime Signal Generation and Delivery in
337 the Rationale (Informative) volume of POSIX.1‐2017, Section B.2.4.2,
338 Signal Generation and Delivery.
339
341 Although this volume of POSIX.1‐2017 requires that signals that cannot
342 be ignored shall not be added to the signal mask when a signal-catching
343 function is entered, there is no explicit requirement that subsequent
344 calls to sigaction() reflect this in the information returned in the
345 oact argument. In other words, if SIGKILL is included in the sa_mask
346 field of act, it is unspecified whether or not a subsequent call to
347 sigaction() returns with SIGKILL included in the sa_mask field of oact.
348 The SA_NOCLDSTOP flag, when supplied in the act->sa_flags parameter,
350 allows overloading SIGCHLD with the System V semantics that each SIGCLD
351 signal indicates a single terminated child. Most conforming applica‐
352 tions that catch SIGCHLD are expected to install signal-catching func‐
353 tions that repeatedly call the waitpid() function with the WNOHANG flag
354 set, acting on each child for which status is returned, until waitpid()
355 returns zero. If stopped children are not of interest, the use of the
356 SA_NOCLDSTOP flag can prevent the overhead from invoking the signal-
357 catching routine when they stop.
358 Some historical implementations also define other mechanisms for stop‐
360 ping processes, such as the ptrace() function. These implementations
361 usually do not generate a SIGCHLD signal when processes stop due to
362 this mechanism; however, that is beyond the scope of this volume of
363 POSIX.1‐2017.
364 This volume of POSIX.1‐2017 requires that calls to sigaction() that
366 supply a NULL act argument succeed, even in the case of signals that
367 cannot be caught or ignored (that is, SIGKILL or SIGSTOP). The System
368 V signal() and BSD sigvec() functions return [EINVAL] in these cases
369 and, in this respect, their behavior varies from sigaction().
370 This volume of POSIX.1‐2017 requires that sigaction() properly save and
372 restore a signal action set up by the ISO C standard signal() function.
373 However, there is no guarantee that the reverse is true, nor could
374 there be given the greater amount of information conveyed by the sigac‐
375 tion structure. Because of this, applications should avoid using both
376 functions for the same signal in the same process. Since this cannot
377 always be avoided in case of general-purpose library routines, they
378 should always be implemented with sigaction().
379 It was intended that the signal() function should be implementable as a
381 library routine using sigaction().
382 The POSIX Realtime Extension extends the sigaction() function as speci‐
384 fied by the POSIX.1‐1990 standard to allow the application to request
385 on a per-signal basis via an additional signal action flag that the
386 extra parameters, including the application-defined signal value, if
387 any, be passed to the signal-catching function.
388
390 None.
391
393 Section 2.4, Signal Concepts, exec, _Exit(), kill(), _longjmp(),
394 longjmp(), pthread_sigmask(), raise(), semget(), sem_init(),
395 sem_open(), sigaddset(), sigaltstack(), sigdelset(), sigemptyset(),
396 sigfillset(), sigismember(), signal(), sigsuspend(), wait(), waitid()
397 The Base Definitions volume of POSIX.1‐2017, <signal.h>
399
401 Portions of this text are reprinted and reproduced in electronic form
402 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
403 table Operating System Interface (POSIX), The Open Group Base Specifi‐
404 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
405 Electrical and Electronics Engineers, Inc and The Open Group. In the
406 event of any discrepancy between this version and the original IEEE and
407 The Open Group Standard, the original IEEE and The Open Group Standard
408 is the referee document. The original Standard can be obtained online
409 at http://www.opengroup.org/unix/online.html .
410 Any typographical or formatting errors that appear in this page are
412 most likely to have been introduced during the conversion of the source
413 files to man page format. To report such errors, see https://www.ker‐
414 nel.org/doc/man-pages/reporting_bugs.html .
415IEEE/The Open Group 2017 SIGACTION(3P)