1SIGNAL(7) Linux Programmer's Manual SIGNAL(7)
2
6 signal - overview of signals
7
9 Linux supports both POSIX reliable signals (hereinafter "standard sig‐
10 nals") and POSIX real-time signals.
11 Signal dispositions
13 Each signal has a current disposition, which determines how the process
14 behaves when it is delivered the signal.
15 The entries in the "Action" column of the table below specify the de‐
17 fault disposition for each signal, as follows:
18 Term Default action is to terminate the process.
20 Ign Default action is to ignore the signal.
22 Core Default action is to terminate the process and dump core (see
24 core(5)).
25 Stop Default action is to stop the process.
27 Cont Default action is to continue the process if it is currently
29 stopped.
30 A process can change the disposition of a signal using sigaction(2) or
32 signal(2). (The latter is less portable when establishing a signal
33 handler; see signal(2) for details.) Using these system calls, a
34 process can elect one of the following behaviors to occur on delivery
35 of the signal: perform the default action; ignore the signal; or catch
36 the signal with a signal handler, a programmer-defined function that is
37 automatically invoked when the signal is delivered.
38 By default, a signal handler is invoked on the normal process stack.
40 It is possible to arrange that the signal handler uses an alternate
41 stack; see sigaltstack(2) for a discussion of how to do this and when
42 it might be useful.
43 The signal disposition is a per-process attribute: in a multithreaded
45 application, the disposition of a particular signal is the same for all
46 threads.
47 A child created via fork(2) inherits a copy of its parent's signal dis‐
49 positions. During an execve(2), the dispositions of handled signals
50 are reset to the default; the dispositions of ignored signals are left
51 unchanged.
52 Sending a signal
54 The following system calls and library functions allow the caller to
55 send a signal:
56 raise(3)
58 Sends a signal to the calling thread.
59 kill(2)
61 Sends a signal to a specified process, to all members of a spec‐
62 ified process group, or to all processes on the system.
63 pidfd_send_signal(2)
65 Sends a signal to a process identified by a PID file descriptor.
66 killpg(3)
68 Sends a signal to all of the members of a specified process
69 group.
70 pthread_kill(3)
72 Sends a signal to a specified POSIX thread in the same process
73 as the caller.
74 tgkill(2)
76 Sends a signal to a specified thread within a specific process.
77 (This is the system call used to implement pthread_kill(3).)
78 sigqueue(3)
80 Sends a real-time signal with accompanying data to a specified
81 process.
82 Waiting for a signal to be caught
84 The following system calls suspend execution of the calling thread un‐
85 til a signal is caught (or an unhandled signal terminates the process):
86 pause(2)
88 Suspends execution until any signal is caught.
89 sigsuspend(2)
91 Temporarily changes the signal mask (see below) and suspends ex‐
92 ecution until one of the unmasked signals is caught.
93 Synchronously accepting a signal
95 Rather than asynchronously catching a signal via a signal handler, it
96 is possible to synchronously accept the signal, that is, to block exe‐
97 cution until the signal is delivered, at which point the kernel returns
98 information about the signal to the caller. There are two general ways
99 to do this:
100 * sigwaitinfo(2), sigtimedwait(2), and sigwait(3) suspend execution un‐
102 til one of the signals in a specified set is delivered. Each of
103 these calls returns information about the delivered signal.
104 * signalfd(2) returns a file descriptor that can be used to read infor‐
106 mation about signals that are delivered to the caller. Each read(2)
107 from this file descriptor blocks until one of the signals in the set
108 specified in the signalfd(2) call is delivered to the caller. The
109 buffer returned by read(2) contains a structure describing the sig‐
110 nal.
111 Signal mask and pending signals
113 A signal may be blocked, which means that it will not be delivered un‐
114 til it is later unblocked. Between the time when it is generated and
115 when it is delivered a signal is said to be pending.
116 Each thread in a process has an independent signal mask, which indi‐
118 cates the set of signals that the thread is currently blocking. A
119 thread can manipulate its signal mask using pthread_sigmask(3). In a
120 traditional single-threaded application, sigprocmask(2) can be used to
121 manipulate the signal mask.
122 A child created via fork(2) inherits a copy of its parent's signal
124 mask; the signal mask is preserved across execve(2).
125 A signal may be process-directed or thread-directed. A process-di‐
127 rected signal is one that is targeted at (and thus pending for) the
128 process as a whole. A signal may be process-directed because it was
129 generated by the kernel for reasons other than a hardware exception, or
130 because it was sent using kill(2) or sigqueue(3). A thread-directed
131 signal is one that is targeted at a specific thread. A signal may be
132 thread-directed because it was generated as a consequence of executing
133 a specific machine-language instruction that triggered a hardware ex‐
134 ception (e.g., SIGSEGV for an invalid memory access, or SIGFPE for a
135 math error), or because it was targeted at a specific thread using in‐
136 terfaces such as tgkill(2) or pthread_kill(3).
137 A process-directed signal may be delivered to any one of the threads
139 that does not currently have the signal blocked. If more than one of
140 the threads has the signal unblocked, then the kernel chooses an arbi‐
141 trary thread to which to deliver the signal.
142 A thread can obtain the set of signals that it currently has pending
144 using sigpending(2). This set will consist of the union of the set of
145 pending process-directed signals and the set of signals pending for the
146 calling thread.
147 A child created via fork(2) initially has an empty pending signal set;
149 the pending signal set is preserved across an execve(2).
150 Execution of signal handlers
152 Whenever there is a transition from kernel-mode to user-mode execution
153 (e.g., on return from a system call or scheduling of a thread onto the
154 CPU), the kernel checks whether there is a pending unblocked signal for
155 which the process has established a signal handler. If there is such a
156 pending signal, the following steps occur:
157 1. The kernel performs the necessary preparatory steps for execution of
159 the signal handler:
160 a) The signal is removed from the set of pending signals.
162 b) If the signal handler was installed by a call to sigaction(2)
164 that specified the SA_ONSTACK flag and the thread has defined an
165 alternate signal stack (using sigaltstack(2)), then that stack is
166 installed.
167 c) Various pieces of signal-related context are saved into a special
169 frame that is created on the stack. The saved information in‐
170 cludes:
171 + the program counter register (i.e., the address of the next in‐
173 struction in the main program that should be executed when the
174 signal handler returns);
175 + architecture-specific register state required for resuming the
177 interrupted program;
178 + the thread's current signal mask;
180 + the thread's alternate signal stack settings.
182 (If the signal handler was installed using the sigaction(2)
184 SA_SIGINFO flag, then the above information is accessible via the
185 ucontext_t object that is pointed to by the third argument of the
186 signal handler.)
187 d) Any signals specified in act->sa_mask when registering the han‐
189 dler with sigprocmask(2) are added to the thread's signal mask.
190 The signal being delivered is also added to the signal mask, un‐
191 less SA_NODEFER was specified when registering the handler.
192 These signals are thus blocked while the handler executes.
193 2. The kernel constructs a frame for the signal handler on the stack.
195 The kernel sets the program counter for the thread to point to the
196 first instruction of the signal handler function, and configures the
197 return address for that function to point to a piece of user-space
198 code known as the signal trampoline (described in sigreturn(2)).
199 3. The kernel passes control back to user-space, where execution com‐
201 mences at the start of the signal handler function.
202 4. When the signal handler returns, control passes to the signal tram‐
204 poline code.
205 5. The signal trampoline calls sigreturn(2), a system call that uses
207 the information in the stack frame created in step 1 to restore the
208 thread to its state before the signal handler was called. The
209 thread's signal mask and alternate signal stack settings are re‐
210 stored as part of this procedure. Upon completion of the call to
211 sigreturn(2), the kernel transfers control back to user space, and
212 the thread recommences execution at the point where it was inter‐
213 rupted by the signal handler.
214 Note that if the signal handler does not return (e.g., control is
216 transferred out of the handler using siglongjmp(3), or the handler exe‐
217 cutes a new program with execve(2)), then the final step is not per‐
218 formed. In particular, in such scenarios it is the programmer's re‐
219 sponsibility to restore the state of the signal mask (using sigproc‐
220 mask(2)), if it is desired to unblock the signals that were blocked on
221 entry to the signal handler. (Note that siglongjmp(3) may or may not
222 restore the signal mask, depending on the savesigs value that was spec‐
223 ified in the corresponding call to sigsetjmp(3).)
224 From the kernel's point of view, execution of the signal handler code
226 is exactly the same as the execution of any other user-space code.
227 That is to say, the kernel does not record any special state informa‐
228 tion indicating that the thread is currently executing inside a signal
229 handler. All necessary state information is maintained in user-space
230 registers and the user-space stack. The depth to which nested signal
231 handlers may be invoked is thus limited only by the user-space stack
232 (and sensible software design!).
233 Standard signals
235 Linux supports the standard signals listed below. The second column of
236 the table indicates which standard (if any) specified the signal:
237 "P1990" indicates that the signal is described in the original
238 POSIX.1-1990 standard; "P2001" indicates that the signal was added in
239 SUSv2 and POSIX.1-2001.
240 Signal Standard Action Comment
242 ────────────────────────────────────────────────────────────────────────
243 SIGABRT P1990 Core Abort signal from abort(3)
244 SIGALRM P1990 Term Timer signal from alarm(2)
245 SIGBUS P2001 Core Bus error (bad memory access)
246 SIGCHLD P1990 Ign Child stopped or terminated
247 SIGCLD - Ign A synonym for SIGCHLD
248 SIGCONT P1990 Cont Continue if stopped
249 SIGEMT - Term Emulator trap
250 SIGFPE P1990 Core Floating-point exception
251 SIGHUP P1990 Term Hangup detected on controlling terminal
252 or death of controlling process
253 SIGILL P1990 Core Illegal Instruction
254 SIGINFO - A synonym for SIGPWR
255 SIGINT P1990 Term Interrupt from keyboard
256 SIGIO - Term I/O now possible (4.2BSD)
257 SIGIOT - Core IOT trap. A synonym for SIGABRT
258 SIGKILL P1990 Term Kill signal
259 SIGLOST - Term File lock lost (unused)
260 SIGPIPE P1990 Term Broken pipe: write to pipe with no
261 readers; see pipe(7)
262 SIGPOLL P2001 Term Pollable event (Sys V);
263 synonym for SIGIO
264 SIGPROF P2001 Term Profiling timer expired
266 SIGPWR - Term Power failure (System V)
267 SIGQUIT P1990 Core Quit from keyboard
268 SIGSEGV P1990 Core Invalid memory reference
269 SIGSTKFLT - Term Stack fault on coprocessor (unused)
270 SIGSTOP P1990 Stop Stop process
271 SIGTSTP P1990 Stop Stop typed at terminal
272 SIGSYS P2001 Core Bad system call (SVr4);
273 see also seccomp(2)
274 SIGTERM P1990 Term Termination signal
275 SIGTRAP P2001 Core Trace/breakpoint trap
276 SIGTTIN P1990 Stop Terminal input for background process
277 SIGTTOU P1990 Stop Terminal output for background process
278 SIGUNUSED - Core Synonymous with SIGSYS
279 SIGURG P2001 Ign Urgent condition on socket (4.2BSD)
280 SIGUSR1 P1990 Term User-defined signal 1
281 SIGUSR2 P1990 Term User-defined signal 2
282 SIGVTALRM P2001 Term Virtual alarm clock (4.2BSD)
283 SIGXCPU P2001 Core CPU time limit exceeded (4.2BSD);
284 see setrlimit(2)
285 SIGXFSZ P2001 Core File size limit exceeded (4.2BSD);
286 see setrlimit(2)
287 SIGWINCH - Ign Window resize signal (4.3BSD, Sun)
288 The signals SIGKILL and SIGSTOP cannot be caught, blocked, or ignored.
290 Up to and including Linux 2.2, the default behavior for SIGSYS, SIGX‐
292 CPU, SIGXFSZ, and (on architectures other than SPARC and MIPS) SIGBUS
293 was to terminate the process (without a core dump). (On some other
294 UNIX systems the default action for SIGXCPU and SIGXFSZ is to terminate
295 the process without a core dump.) Linux 2.4 conforms to the
296 POSIX.1-2001 requirements for these signals, terminating the process
297 with a core dump.
298 SIGEMT is not specified in POSIX.1-2001, but nevertheless appears on
300 most other UNIX systems, where its default action is typically to ter‐
301 minate the process with a core dump.
302 SIGPWR (which is not specified in POSIX.1-2001) is typically ignored by
304 default on those other UNIX systems where it appears.
305 SIGIO (which is not specified in POSIX.1-2001) is ignored by default on
307 several other UNIX systems.
308 Queueing and delivery semantics for standard signals
310 If multiple standard signals are pending for a process, the order in
311 which the signals are delivered is unspecified.
312 Standard signals do not queue. If multiple instances of a standard
314 signal are generated while that signal is blocked, then only one in‐
315 stance of the signal is marked as pending (and the signal will be de‐
316 livered just once when it is unblocked). In the case where a standard
317 signal is already pending, the siginfo_t structure (see sigaction(2))
318 associated with that signal is not overwritten on arrival of subsequent
319 instances of the same signal. Thus, the process will receive the in‐
320 formation associated with the first instance of the signal.
321 Signal numbering for standard signals
323 The numeric value for each signal is given in the table below. As
324 shown in the table, many signals have different numeric values on dif‐
325 ferent architectures. The first numeric value in each table row shows
326 the signal number on x86, ARM, and most other architectures; the second
327 value is for Alpha and SPARC; the third is for MIPS; and the last is
328 for PARISC. A dash (-) denotes that a signal is absent on the corre‐
329 sponding architecture.
330 Signal x86/ARM Alpha/ MIPS PARISC Notes
332 most others SPARC
333 ─────────────────────────────────────────────────────────────────
334 SIGHUP 1 1 1 1
335 SIGINT 2 2 2 2
336 SIGQUIT 3 3 3 3
337 SIGILL 4 4 4 4
338 SIGTRAP 5 5 5 5
339 SIGABRT 6 6 6 6
340 SIGIOT 6 6 6 6
341 SIGBUS 7 10 10 10
342 SIGEMT - 7 7 -
343 SIGFPE 8 8 8 8
344 SIGKILL 9 9 9 9
345 SIGUSR1 10 30 16 16
346 SIGSEGV 11 11 11 11
347 SIGUSR2 12 31 17 17
348 SIGPIPE 13 13 13 13
349 SIGALRM 14 14 14 14
350 SIGTERM 15 15 15 15
351 SIGSTKFLT 16 - - 7
352 SIGCHLD 17 20 18 18
353 SIGCLD - - 18 -
354 SIGCONT 18 19 25 26
355 SIGSTOP 19 17 23 24
356 SIGTSTP 20 18 24 25
357 SIGTTIN 21 21 26 27
358 SIGTTOU 22 22 27 28
359 SIGURG 23 16 21 29
360 SIGXCPU 24 24 30 12
361 SIGXFSZ 25 25 31 30
362 SIGVTALRM 26 26 28 20
363 SIGPROF 27 27 29 21
364 SIGWINCH 28 28 20 23
365 SIGIO 29 23 22 22
366 SIGPOLL Same as SIGIO
367 SIGPWR 30 29/- 19 19
368 SIGINFO - 29/- - -
369 SIGLOST - -/29 - -
370 SIGSYS 31 12 12 31
371 SIGUNUSED 31 - - 31
372 Note the following:
374 * Where defined, SIGUNUSED is synonymous with SIGSYS. Since glibc
376 2.26, SIGUNUSED is no longer defined on any architecture.
377 * Signal 29 is SIGINFO/SIGPWR (synonyms for the same value) on Alpha
379 but SIGLOST on SPARC.
380 Real-time signals
382 Starting with version 2.2, Linux supports real-time signals as origi‐
383 nally defined in the POSIX.1b real-time extensions (and now included in
384 POSIX.1-2001). The range of supported real-time signals is defined by
385 the macros SIGRTMIN and SIGRTMAX. POSIX.1-2001 requires that an imple‐
386 mentation support at least _POSIX_RTSIG_MAX (8) real-time signals.
387 The Linux kernel supports a range of 33 different real-time signals,
389 numbered 32 to 64. However, the glibc POSIX threads implementation in‐
390 ternally uses two (for NPTL) or three (for LinuxThreads) real-time sig‐
391 nals (see pthreads(7)), and adjusts the value of SIGRTMIN suitably (to
392 34 or 35). Because the range of available real-time signals varies ac‐
393 cording to the glibc threading implementation (and this variation can
394 occur at run time according to the available kernel and glibc), and in‐
395 deed the range of real-time signals varies across UNIX systems, pro‐
396 grams should never refer to real-time signals using hard-coded numbers,
397 but instead should always refer to real-time signals using the notation
398 SIGRTMIN+n, and include suitable (run-time) checks that SIGRTMIN+n does
399 not exceed SIGRTMAX.
400 Unlike standard signals, real-time signals have no predefined meanings:
402 the entire set of real-time signals can be used for application-defined
403 purposes.
404 The default action for an unhandled real-time signal is to terminate
406 the receiving process.
407 Real-time signals are distinguished by the following:
409 1. Multiple instances of real-time signals can be queued. By con‐
411 trast, if multiple instances of a standard signal are delivered
412 while that signal is currently blocked, then only one instance is
413 queued.
414 2. If the signal is sent using sigqueue(3), an accompanying value (ei‐
416 ther an integer or a pointer) can be sent with the signal. If the
417 receiving process establishes a handler for this signal using the
418 SA_SIGINFO flag to sigaction(2), then it can obtain this data via
419 the si_value field of the siginfo_t structure passed as the second
420 argument to the handler. Furthermore, the si_pid and si_uid fields
421 of this structure can be used to obtain the PID and real user ID of
422 the process sending the signal.
423 3. Real-time signals are delivered in a guaranteed order. Multiple
425 real-time signals of the same type are delivered in the order they
426 were sent. If different real-time signals are sent to a process,
427 they are delivered starting with the lowest-numbered signal.
428 (I.e., low-numbered signals have highest priority.) By contrast,
429 if multiple standard signals are pending for a process, the order
430 in which they are delivered is unspecified.
431 If both standard and real-time signals are pending for a process, POSIX
433 leaves it unspecified which is delivered first. Linux, like many other
434 implementations, gives priority to standard signals in this case.
435 According to POSIX, an implementation should permit at least
437 _POSIX_SIGQUEUE_MAX (32) real-time signals to be queued to a process.
438 However, Linux does things differently. In kernels up to and including
439 2.6.7, Linux imposes a system-wide limit on the number of queued real-
440 time signals for all processes. This limit can be viewed and (with
441 privilege) changed via the /proc/sys/kernel/rtsig-max file. A related
442 file, /proc/sys/kernel/rtsig-nr, can be used to find out how many real-
443 time signals are currently queued. In Linux 2.6.8, these /proc inter‐
444 faces were replaced by the RLIMIT_SIGPENDING resource limit, which
445 specifies a per-user limit for queued signals; see setrlimit(2) for
446 further details.
447 The addition of real-time signals required the widening of the signal
449 set structure (sigset_t) from 32 to 64 bits. Consequently, various
450 system calls were superseded by new system calls that supported the
451 larger signal sets. The old and new system calls are as follows:
452 Linux 2.0 and earlier Linux 2.2 and later
454 sigaction(2) rt_sigaction(2)
455 sigpending(2) rt_sigpending(2)
456 sigprocmask(2) rt_sigprocmask(2)
457 sigreturn(2) rt_sigreturn(2)
458 sigsuspend(2) rt_sigsuspend(2)
459 sigtimedwait(2) rt_sigtimedwait(2)
460 Interruption of system calls and library functions by signal handlers
462 If a signal handler is invoked while a system call or library function
463 call is blocked, then either:
464 * the call is automatically restarted after the signal handler returns;
466 or
467 * the call fails with the error EINTR.
469 Which of these two behaviors occurs depends on the interface and
471 whether or not the signal handler was established using the SA_RESTART
472 flag (see sigaction(2)). The details vary across UNIX systems; below,
473 the details for Linux.
474 If a blocked call to one of the following interfaces is interrupted by
476 a signal handler, then the call is automatically restarted after the
477 signal handler returns if the SA_RESTART flag was used; otherwise the
478 call fails with the error EINTR:
479 * read(2), readv(2), write(2), writev(2), and ioctl(2) calls on "slow"
481 devices. A "slow" device is one where the I/O call may block for an
482 indefinite time, for example, a terminal, pipe, or socket. If an I/O
483 call on a slow device has already transferred some data by the time
484 it is interrupted by a signal handler, then the call will return a
485 success status (normally, the number of bytes transferred). Note
486 that a (local) disk is not a slow device according to this defini‐
487 tion; I/O operations on disk devices are not interrupted by signals.
488 * open(2), if it can block (e.g., when opening a FIFO; see fifo(7)).
490 * wait(2), wait3(2), wait4(2), waitid(2), and waitpid(2).
492 * Socket interfaces: accept(2), connect(2), recv(2), recvfrom(2),
494 recvmmsg(2), recvmsg(2), send(2), sendto(2), and sendmsg(2), unless a
495 timeout has been set on the socket (see below).
496 * File locking interfaces: flock(2) and the F_SETLKW and F_OFD_SETLKW
498 operations of fcntl(2)
499 * POSIX message queue interfaces: mq_receive(3), mq_timedreceive(3),
501 mq_send(3), and mq_timedsend(3).
502 * futex(2) FUTEX_WAIT (since Linux 2.6.22; beforehand, always failed
504 with EINTR).
505 * getrandom(2).
507 * pthread_mutex_lock(3), pthread_cond_wait(3), and related APIs.
509 * futex(2) FUTEX_WAIT_BITSET.
511 * POSIX semaphore interfaces: sem_wait(3) and sem_timedwait(3) (since
513 Linux 2.6.22; beforehand, always failed with EINTR).
514 * read(2) from an inotify(7) file descriptor (since Linux 3.8; before‐
516 hand, always failed with EINTR).
517 The following interfaces are never restarted after being interrupted by
519 a signal handler, regardless of the use of SA_RESTART; they always fail
520 with the error EINTR when interrupted by a signal handler:
521 * "Input" socket interfaces, when a timeout (SO_RCVTIMEO) has been set
523 on the socket using setsockopt(2): accept(2), recv(2), recvfrom(2),
524 recvmmsg(2) (also with a non-NULL timeout argument), and recvmsg(2).
525 * "Output" socket interfaces, when a timeout (SO_RCVTIMEO) has been set
527 on the socket using setsockopt(2): connect(2), send(2), sendto(2),
528 and sendmsg(2).
529 * Interfaces used to wait for signals: pause(2), sigsuspend(2), sig‐
531 timedwait(2), and sigwaitinfo(2).
532 * File descriptor multiplexing interfaces: epoll_wait(2),
534 epoll_pwait(2), poll(2), ppoll(2), select(2), and pselect(2).
535 * System V IPC interfaces: msgrcv(2), msgsnd(2), semop(2), and semtime‐
537 dop(2).
538 * Sleep interfaces: clock_nanosleep(2), nanosleep(2), and usleep(3).
540 * io_getevents(2).
542 The sleep(3) function is also never restarted if interrupted by a han‐
544 dler, but gives a success return: the number of seconds remaining to
545 sleep.
546 In certain circumstances, the seccomp(2) user-space notification fea‐
548 ture can lead to restarting of system calls that would otherwise never
549 be restarted by SA_RESTART; for details, see seccomp_unotify(2).
550 Interruption of system calls and library functions by stop signals
552 On Linux, even in the absence of signal handlers, certain blocking in‐
553 terfaces can fail with the error EINTR after the process is stopped by
554 one of the stop signals and then resumed via SIGCONT. This behavior is
555 not sanctioned by POSIX.1, and doesn't occur on other systems.
556 The Linux interfaces that display this behavior are:
558 * "Input" socket interfaces, when a timeout (SO_RCVTIMEO) has been set
560 on the socket using setsockopt(2): accept(2), recv(2), recvfrom(2),
561 recvmmsg(2) (also with a non-NULL timeout argument), and recvmsg(2).
562 * "Output" socket interfaces, when a timeout (SO_RCVTIMEO) has been set
564 on the socket using setsockopt(2): connect(2), send(2), sendto(2),
565 and sendmsg(2), if a send timeout (SO_SNDTIMEO) has been set.
566 * epoll_wait(2), epoll_pwait(2).
568 * semop(2), semtimedop(2).
570 * sigtimedwait(2), sigwaitinfo(2).
572 * Linux 3.7 and earlier: read(2) from an inotify(7) file descriptor
574 * Linux 2.6.21 and earlier: futex(2) FUTEX_WAIT, sem_timedwait(3),
576 sem_wait(3).
577 * Linux 2.6.8 and earlier: msgrcv(2), msgsnd(2).
579 * Linux 2.4 and earlier: nanosleep(2).
581
583 POSIX.1, except as noted.
584
586 For a discussion of async-signal-safe functions, see signal-safety(7).
587 The /proc/[pid]/task/[tid]/status file contains various fields that
589 show the signals that a thread is blocking (SigBlk), catching (SigCgt),
590 or ignoring (SigIgn). (The set of signals that are caught or ignored
591 will be the same across all threads in a process.) Other fields show
592 the set of pending signals that are directed to the thread (SigPnd) as
593 well as the set of pending signals that are directed to the process as
594 a whole (ShdPnd). The corresponding fields in /proc/[pid]/status show
595 the information for the main thread. See proc(5) for further details.
596
598 There are six signals that can be delivered as a consequence of a hard‐
599 ware exception: SIGBUS, SIGEMT, SIGFPE, SIGILL, SIGSEGV, and SIGTRAP.
600 Which of these signals is delivered, for any given hardware exception,
601 is not documented and does not always make sense.
602 For example, an invalid memory access that causes delivery of SIGSEGV
604 on one CPU architecture may cause delivery of SIGBUS on another archi‐
605 tecture, or vice versa.
606 For another example, using the x86 int instruction with a forbidden ar‐
608 gument (any number other than 3 or 128) causes delivery of SIGSEGV,
609 even though SIGILL would make more sense, because of how the CPU re‐
610 ports the forbidden operation to the kernel.
611
613 kill(1), clone(2), getrlimit(2), kill(2), pidfd_send_signal(2),
614 restart_syscall(2), rt_sigqueueinfo(2), setitimer(2), setrlimit(2),
615 sgetmask(2), sigaction(2), sigaltstack(2), signal(2), signalfd(2), sig‐
616 pending(2), sigprocmask(2), sigreturn(2), sigsuspend(2), sigwait‐
617 info(2), abort(3), bsd_signal(3), killpg(3), longjmp(3),
618 pthread_sigqueue(3), raise(3), sigqueue(3), sigset(3), sigsetops(3),
619 sigvec(3), sigwait(3), strsignal(3), swapcontext(3), sysv_signal(3),
620 core(5), proc(5), nptl(7), pthreads(7), sigevent(7)
621
623 This page is part of release 5.13 of the Linux man-pages project. A
624 description of the project, information about reporting bugs, and the
625 latest version of this page, can be found at
626 https://www.kernel.org/doc/man-pages/.
627Linux 2021-03-22 SIGNAL(7)