2 .\" Copyright (c) 1993 by Thomas Koenig (ig25@rz.uni-karlsruhe.de)
3 .\" and Copyright (c) 2002, 2006 by Michael Kerrisk <mtk.manpages@gmail.com>
4 .\" and Copyright (c) 2008 Linux Foundation, written by Michael Kerrisk
5 .\" <mtk.manpages@gmail.com>
7 .\" %%%LICENSE_START(VERBATIM)
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12 .\" Permission is granted to copy and distribute modified versions of this
13 .\" manual under the conditions for verbatim copying, provided that the
14 .\" entire resulting derived work is distributed under the terms of a
15 .\" permission notice identical to this one.
17 .\" Since the Linux kernel and libraries are constantly changing, this
18 .\" manual page may be incorrect or out-of-date. The author(s) assume no
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20 .\" the use of the information contained herein. The author(s) may not
21 .\" have taken the same level of care in the production of this manual,
22 .\" which is licensed free of charge, as they might when working
25 .\" Formatted or processed versions of this manual, if unaccompanied by
26 .\" the source, must acknowledge the copyright and authors of this work.
29 .\" Modified Sat Jul 24 17:34:08 1993 by Rik Faith (faith@cs.unc.edu)
30 .\" Modified Sun Jan 7 01:41:27 1996 by Andries Brouwer (aeb@cwi.nl)
31 .\" Modified Sun Apr 14 12:02:29 1996 by Andries Brouwer (aeb@cwi.nl)
32 .\" Modified Sat Nov 13 16:28:23 1999 by Andries Brouwer (aeb@cwi.nl)
33 .\" Modified 10 Apr 2002, by Michael Kerrisk <mtk.manpages@gmail.com>
34 .\" Modified 7 Jun 2002, by Michael Kerrisk <mtk.manpages@gmail.com>
35 .\" Added information on real-time signals
36 .\" Modified 13 Jun 2002, by Michael Kerrisk <mtk.manpages@gmail.com>
37 .\" Noted that SIGSTKFLT is in fact unused
38 .\" 2004-12-03, Modified mtk, added notes on RLIMIT_SIGPENDING
39 .\" 2006-04-24, mtk, Added text on changing signal dispositions,
40 .\" signal mask, and pending signals.
42 .\" Added section on system call restarting (SA_RESTART)
43 .\" Added section on stop/cont signals interrupting syscalls.
44 .\" 2008-10-05, mtk: various additions
46 .TH SIGNAL 7 2014-06-13 "Linux" "Linux Programmer's Manual"
48 signal \- overview of signals
50 Linux supports both POSIX reliable signals (hereinafter
51 "standard signals") and POSIX real-time signals.
52 .SS Signal dispositions
53 Each signal has a current
55 which determines how the process behaves when it is delivered
58 The entries in the "Action" column of the tables below specify
59 the default disposition for each signal, as follows:
61 Default action is to terminate the process.
63 Default action is to ignore the signal.
65 Default action is to terminate the process and dump core (see
68 Default action is to stop the process.
70 Default action is to continue the process if it is currently stopped.
72 A process can change the disposition of a signal using
76 (The latter is less portable when establishing a signal handler;
80 Using these system calls, a process can elect one of the
81 following behaviors to occur on delivery of the signal:
82 perform the default action; ignore the signal;
83 or catch the signal with a
84 .IR "signal handler" ,
85 a programmer-defined function that is automatically invoked
86 when the signal is delivered.
87 (By default, the signal handler is invoked on the
89 It is possible to arrange that the signal handler
90 uses an alternate stack; see
92 for a discussion of how to do this and when it might be useful.)
94 The signal disposition is a per-process attribute:
95 in a multithreaded application, the disposition of a
96 particular signal is the same for all threads.
100 inherits a copy of its parent's signal dispositions.
103 the dispositions of handled signals are reset to the default;
104 the dispositions of ignored signals are left unchanged.
106 The following system calls and library functions allow
107 the caller to send a signal:
110 Sends a signal to the calling thread.
113 Sends a signal to a specified process,
114 to all members of a specified process group,
115 or to all processes on the system.
118 Sends a signal to all of the members of a specified process group.
121 Sends a signal to a specified POSIX thread in the same process as
125 Sends a signal to a specified thread within a specific process.
126 (This is the system call used to implement
127 .BR pthread_kill (3).)
130 Sends a real-time signal with accompanying data to a specified process.
131 .SS Waiting for a signal to be caught
132 The following system calls suspend execution of the calling process
133 or thread until a signal is caught
134 (or an unhandled signal terminates the process):
137 Suspends execution until any signal is caught.
140 Temporarily changes the signal mask (see below) and suspends
141 execution until one of the unmasked signals is caught.
142 .SS Synchronously accepting a signal
143 Rather than asynchronously catching a signal via a signal handler,
144 it is possible to synchronously accept the signal, that is,
145 to block execution until the signal is delivered,
146 at which point the kernel returns information about the
147 signal to the caller.
148 There are two general ways to do this:
151 .BR sigtimedwait (2),
154 suspend execution until one of the signals in a specified
156 Each of these calls returns information about the delivered signal.
159 returns a file descriptor that can be used to read information
160 about signals that are delivered to the caller.
163 from this file descriptor blocks until one of the signals
164 in the set specified in the
166 call is delivered to the caller.
167 The buffer returned by
169 contains a structure describing the signal.
170 .SS Signal mask and pending signals
173 which means that it will not be delivered until it is later unblocked.
174 Between the time when it is generated and when it is delivered
175 a signal is said to be
178 Each thread in a process has an independent
180 which indicates the set of signals that the thread is currently blocking.
181 A thread can manipulate its signal mask using
182 .BR pthread_sigmask (3).
183 In a traditional single-threaded application,
185 can be used to manipulate the signal mask.
189 inherits a copy of its parent's signal mask;
190 the signal mask is preserved across
193 A signal may be generated (and thus pending)
194 for a process as a whole (e.g., when sent using
196 or for a specific thread (e.g., certain signals,
202 consequence of executing a specific machine-language instruction
203 are thread directed, as are signals targeted at a specific thread using
204 .BR pthread_kill (3)).
205 A process-directed signal may be delivered to any one of the
206 threads that does not currently have the signal blocked.
207 If more than one of the threads has the signal unblocked, then the
208 kernel chooses an arbitrary thread to which to deliver the signal.
210 A thread can obtain the set of signals that it currently has pending
213 This set will consist of the union of the set of pending
214 process-directed signals and the set of signals pending for
219 initially has an empty pending signal set;
220 the pending signal set is preserved across an
223 Linux supports the standard signals listed below.
224 Several signal numbers
225 are architecture-dependent, as indicated in the "Value" column.
226 (Where three values are given, the first one is usually valid for
228 the middle one for x86, arm, and most other architectures,
229 and the last one for mips.
230 (Values for parisc are
232 shown; see the Linux kernel source for signal numbering on that architecture.)
233 A \- denotes that a signal is absent on the corresponding architecture.)
235 First the signals described in the original POSIX.1-1990 standard.
240 Signal Value Action Comment
241 SIGHUP \01 Term Hangup detected on controlling terminal
242 or death of controlling process
243 SIGINT \02 Term Interrupt from keyboard
244 SIGQUIT \03 Core Quit from keyboard
245 SIGILL \04 Core Illegal Instruction
246 SIGABRT \06 Core Abort signal from \fBabort\fP(3)
247 SIGFPE \08 Core Floating point exception
248 SIGKILL \09 Term Kill signal
249 SIGSEGV 11 Core Invalid memory reference
250 SIGPIPE 13 Term Broken pipe: write to pipe with no
252 SIGALRM 14 Term Timer signal from \fBalarm\fP(2)
253 SIGTERM 15 Term Termination signal
254 SIGUSR1 30,10,16 Term User-defined signal 1
255 SIGUSR2 31,12,17 Term User-defined signal 2
256 SIGCHLD 20,17,18 Ign Child stopped or terminated
257 SIGCONT 19,18,25 Cont Continue if stopped
258 SIGSTOP 17,19,23 Stop Stop process
259 SIGTSTP 18,20,24 Stop Stop typed at terminal
260 SIGTTIN 21,21,26 Stop Terminal input for background process
261 SIGTTOU 22,22,27 Stop Terminal output for background process
268 cannot be caught, blocked, or ignored.
270 Next the signals not in the POSIX.1-1990 standard but described in
271 SUSv2 and POSIX.1-2001.
276 Signal Value Action Comment
277 SIGBUS 10,7,10 Core Bus error (bad memory access)
278 SIGPOLL Term Pollable event (Sys V).
279 Synonym for \fBSIGIO\fP
280 SIGPROF 27,27,29 Term Profiling timer expired
281 SIGSYS 12,31,12 Core Bad argument to routine (SVr4)
282 SIGTRAP 5 Core Trace/breakpoint trap
283 SIGURG 16,23,21 Ign Urgent condition on socket (4.2BSD)
284 SIGVTALRM 26,26,28 Term Virtual alarm clock (4.2BSD)
285 SIGXCPU 24,24,30 Core CPU time limit exceeded (4.2BSD)
286 SIGXFSZ 25,25,31 Core File size limit exceeded (4.2BSD)
289 Up to and including Linux 2.2, the default behavior for
290 .BR SIGSYS ", " SIGXCPU ", " SIGXFSZ ", "
291 and (on architectures other than SPARC and MIPS)
293 was to terminate the process (without a core dump).
294 (On some other UNIX systems the default action for
295 .BR SIGXCPU " and " SIGXFSZ
296 is to terminate the process without a core dump.)
297 Linux 2.4 conforms to the POSIX.1-2001 requirements for these signals,
298 terminating the process with a core dump.
300 Next various other signals.
305 Signal Value Action Comment
306 SIGIOT 6 Core IOT trap. A synonym for \fBSIGABRT\fP
308 SIGSTKFLT \-,16,\- Term Stack fault on coprocessor (unused)
309 SIGIO 23,29,22 Term I/O now possible (4.2BSD)
310 SIGCLD \-,\-,18 Ign A synonym for \fBSIGCHLD\fP
311 SIGPWR 29,30,19 Term Power failure (System V)
312 SIGINFO 29,\-,\- A synonym for \fBSIGPWR\fP
313 SIGLOST \-,\-,\- Term File lock lost (unused)
314 SIGWINCH 28,28,20 Ign Window resize signal (4.3BSD, Sun)
315 SIGUNUSED \-,31,\- Core Synonymous with \fBSIGSYS\fP
327 is not specified in POSIX.1-2001, but nevertheless appears
328 on most other UNIX systems,
329 where its default action is typically to terminate
330 the process with a core dump.
333 (which is not specified in POSIX.1-2001) is typically ignored
334 by default on those other UNIX systems where it appears.
337 (which is not specified in POSIX.1-2001) is ignored by default
338 on several other UNIX systems.
343 .\" parisc is the only exception: SIGSYS is 12, SIGUNUSED is 31
345 on most architectures.
346 .SS Real-time signals
347 Linux supports real-time signals as originally defined in the POSIX.1b
348 real-time extensions (and now included in POSIX.1-2001).
349 The range of supported real-time signals is defined by the macros
353 POSIX.1-2001 requires that an implementation support at least
355 (8) real-time signals.
357 The Linux kernel supports a range of 32 different real-time
358 signals, numbered 33 to 64.
359 However, the glibc POSIX threads implementation internally uses
360 two (for NPTL) or three (for LinuxThreads) real-time signals
363 and adjusts the value of
365 suitably (to 34 or 35).
366 Because the range of available real-time signals varies according
367 to the glibc threading implementation (and this variation can occur
368 at run time according to the available kernel and glibc),
369 and indeed the range of real-time signals varies across UNIX systems,
371 .IR "never refer to real-time signals using hard-coded numbers" ,
372 but instead should always refer to real-time signals using the notation
374 and include suitable (run-time) checks that
379 Unlike standard signals, real-time signals have no predefined meanings:
380 the entire set of real-time signals can be used for application-defined
383 The default action for an unhandled real-time signal is to terminate the
386 Real-time signals are distinguished by the following:
388 Multiple instances of real-time signals can be queued.
389 By contrast, if multiple instances of a standard signal are delivered
390 while that signal is currently blocked, then only one instance is queued.
392 If the signal is sent using
394 an accompanying value (either an integer or a pointer) can be sent
396 If the receiving process establishes a handler for this signal using the
400 then it can obtain this data via the
404 structure passed as the second argument to the handler.
409 fields of this structure can be used to obtain the PID
410 and real user ID of the process sending the signal.
412 Real-time signals are delivered in a guaranteed order.
413 Multiple real-time signals of the same type are delivered in the order
415 If different real-time signals are sent to a process, they are delivered
416 starting with the lowest-numbered signal.
417 (I.e., low-numbered signals have highest priority.)
418 By contrast, if multiple standard signals are pending for a process,
419 the order in which they are delivered is unspecified.
421 If both standard and real-time signals are pending for a process,
422 POSIX leaves it unspecified which is delivered first.
423 Linux, like many other implementations, gives priority
424 to standard signals in this case.
426 According to POSIX, an implementation should permit at least
427 .B _POSIX_SIGQUEUE_MAX
428 (32) real-time signals to be queued to
430 However, Linux does things differently.
431 In kernels up to and including 2.6.7, Linux imposes
432 a system-wide limit on the number of queued real-time signals
434 This limit can be viewed and (with privilege) changed via the
435 .I /proc/sys/kernel/rtsig-max
438 .IR /proc/sys/kernel/rtsig-nr ,
439 can be used to find out how many real-time signals are currently queued.
440 In Linux 2.6.8, these
442 interfaces were replaced by the
444 resource limit, which specifies a per-user limit for queued
448 .SS Async-signal-safe functions
450 A signal handler function must be very careful,
451 since processing elsewhere may be interrupted
452 at some arbitrary point in the execution of the program.
453 POSIX has the concept of "safe function".
454 If a signal interrupts the execution of an unsafe function, and
456 calls an unsafe function, then the behavior of the program is undefined.
458 POSIX.1-2004 (also known as POSIX.1-2001 Technical Corrigendum 2)
459 requires an implementation to guarantee that the following
460 functions can be safely called inside a signal handler:
585 POSIX.1-2008 removes fpathconf(), pathconf(), and sysconf()
586 from the above list, and adds the following functions:
612 .SS Interruption of system calls and library functions by signal handlers
613 If a signal handler is invoked while a system call or library
614 function call is blocked, then either:
616 the call is automatically restarted after the signal handler returns; or
618 the call fails with the error
621 Which of these two behaviors occurs depends on the interface and
622 whether or not the signal handler was established using the
626 The details vary across UNIX systems;
627 below, the details for Linux.
629 If a blocked call to one of the following interfaces is interrupted
630 by a signal handler, then the call will be automatically restarted
631 after the signal handler returns if the
633 flag was used; otherwise the call will fail with the error
635 .\" The following system calls use ERESTARTSYS,
636 .\" so that they are restartable
645 calls on "slow" devices.
646 A "slow" device is one where the I/O call may block for an
647 indefinite time, for example, a terminal, pipe, or socket.
648 (A disk is not a slow device according to this definition.)
649 If an I/O call on a slow device has already transferred some
650 data by the time it is interrupted by a signal handler,
651 then the call will return a success status
652 (normally, the number of bytes transferred).
655 if it can block (e.g., when opening a FIFO; see
666 .\" If a timeout (setsockopt()) is in effect on the socket, then these
667 .\" system calls switch to using EINTR. Consequently, they and are not
668 .\" automatically restarted, and they show the stop/cont behavior
669 .\" described below. (Verified from 2.6.26 source, and by experiment; mtk)
679 .\" FIXME . What about sendmmsg()?
681 unless a timeout has been set on the socket (see below).
683 File locking interfaces:
689 POSIX message queue interfaces:
691 .BR mq_timedreceive (3),
694 .BR mq_timedsend (3).
698 (since Linux 2.6.22; beforehand, always failed with
701 POSIX semaphore interfaces:
704 .BR sem_timedwait (3)
705 (since Linux 2.6.22; beforehand, always failed with
709 The following interfaces are never restarted after
710 being interrupted by a signal handler,
711 regardless of the use of
713 they always fail with the error
715 when interrupted by a signal handler:
716 .\" These are the system calls that give EINTR or ERESTARTNOHAND
717 .\" on interruption by a signal handler.
720 "Input" socket interfaces, when a timeout
722 has been set on the socket using
728 (also with a non-NULL
734 "Output" socket interfaces, when a timeout
736 has been set on the socket using
742 .\" FIXME . What about sendmmsg()?
745 Interfaces used to wait for signals:
748 .BR sigtimedwait (2),
752 File descriptor multiplexing interfaces:
761 System V IPC interfaces:
762 .\" On some other systems, SA_RESTART does restart these system calls
770 .BR clock_nanosleep (2),
780 .BR io_getevents (2).
785 function is also never restarted if interrupted by a handler,
786 but gives a success return: the number of seconds remaining to sleep.
787 .SS Interruption of system calls and library functions by stop signals
788 On Linux, even in the absence of signal handlers,
789 certain blocking interfaces can fail with the error
791 after the process is stopped by one of the stop signals
794 This behavior is not sanctioned by POSIX.1, and doesn't occur
797 The Linux interfaces that display this behavior are:
800 "Input" socket interfaces, when a timeout
802 has been set on the socket using
808 (also with a non-NULL
814 "Output" socket interfaces, when a timeout
816 has been set on the socket using
822 .\" FIXME . What about sendmmsg()?
834 .BR sigtimedwait (2),
842 Linux 2.6.21 and earlier:
845 .BR sem_timedwait (3),
848 Linux 2.6.8 and earlier:
852 Linux 2.4 and earlier:
856 POSIX.1, except as noted.
857 .\" It must be a *very* long time since this was true:
862 .\" have the same value.
863 .\" The latter is commented out in the kernel source, but
864 .\" the build process of some software still thinks that
872 .BR restart_syscall (2),
873 .BR rt_sigqueueinfo (2),
889 .BR pthread_sigqueue (3),