'\" t .\" Copyright (c) 1993 by Thomas Koenig (ig25@rz.uni-karlsruhe.de) .\" and Copyright (c) 2002, 2006 by Michael Kerrisk .\" and Copyright (c) 2008 Linux Foundation, written by Michael Kerrisk .\" .\" .\" %%%LICENSE_START(VERBATIM) .\" Permission is granted to make and distribute verbatim copies of this .\" manual provided the copyright notice and this permission notice are .\" preserved on all copies. .\" .\" Permission is granted to copy and distribute modified versions of this .\" manual under the conditions for verbatim copying, provided that the .\" entire resulting derived work is distributed under the terms of a .\" permission notice identical to this one. .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date. The author(s) assume no .\" responsibility for errors or omissions, or for damages resulting from .\" the use of the information contained herein. The author(s) may not .\" have taken the same level of care in the production of this manual, .\" which is licensed free of charge, as they might when working .\" professionally. .\" .\" Formatted or processed versions of this manual, if unaccompanied by .\" the source, must acknowledge the copyright and authors of this work. .\" %%%LICENSE_END .\" .\" Modified Sat Jul 24 17:34:08 1993 by Rik Faith (faith@cs.unc.edu) .\" Modified Sun Jan 7 01:41:27 1996 by Andries Brouwer (aeb@cwi.nl) .\" Modified Sun Apr 14 12:02:29 1996 by Andries Brouwer (aeb@cwi.nl) .\" Modified Sat Nov 13 16:28:23 1999 by Andries Brouwer (aeb@cwi.nl) .\" Modified 10 Apr 2002, by Michael Kerrisk .\" Modified 7 Jun 2002, by Michael Kerrisk .\" Added information on real-time signals .\" Modified 13 Jun 2002, by Michael Kerrisk .\" Noted that SIGSTKFLT is in fact unused .\" 2004-12-03, Modified mtk, added notes on RLIMIT_SIGPENDING .\" 2006-04-24, mtk, Added text on changing signal dispositions, .\" signal mask, and pending signals. .\" 2008-07-04, mtk: .\" Added section on system call restarting (SA_RESTART) .\" Added section on stop/cont signals interrupting syscalls. .\" 2008-10-05, mtk: various additions .\" .TH SIGNAL 7 2017-09-15 "Linux" "Linux Programmer's Manual" .SH NAME signal \- overview of signals .SH DESCRIPTION Linux supports both POSIX reliable signals (hereinafter "standard signals") and POSIX real-time signals. .SS Signal dispositions Each signal has a current .IR disposition , which determines how the process behaves when it is delivered the signal. .PP The entries in the "Action" column of the tables below specify the default disposition for each signal, as follows: .IP Term Default action is to terminate the process. .IP Ign Default action is to ignore the signal. .IP Core Default action is to terminate the process and dump core (see .BR core (5)). .IP Stop Default action is to stop the process. .IP Cont Default action is to continue the process if it is currently stopped. .PP A process can change the disposition of a signal using .BR sigaction (2) or .BR signal (2). (The latter is less portable when establishing a signal handler; see .BR signal (2) for details.) Using these system calls, a process can elect one of the following behaviors to occur on delivery of the signal: perform the default action; ignore the signal; or catch the signal with a .IR "signal handler" , a programmer-defined function that is automatically invoked when the signal is delivered. (By default, the signal handler is invoked on the normal process stack. It is possible to arrange that the signal handler uses an alternate stack; see .BR sigaltstack (2) for a discussion of how to do this and when it might be useful.) .PP The signal disposition is a per-process attribute: in a multithreaded application, the disposition of a particular signal is the same for all threads. .PP A child created via .BR fork (2) inherits a copy of its parent's signal dispositions. During an .BR execve (2), the dispositions of handled signals are reset to the default; the dispositions of ignored signals are left unchanged. .SS Sending a signal The following system calls and library functions allow the caller to send a signal: .TP 16 .BR raise (3) Sends a signal to the calling thread. .TP .BR kill (2) Sends a signal to a specified process, to all members of a specified process group, or to all processes on the system. .TP .BR killpg (3) Sends a signal to all of the members of a specified process group. .TP .BR pthread_kill (3) Sends a signal to a specified POSIX thread in the same process as the caller. .TP .BR tgkill (2) Sends a signal to a specified thread within a specific process. (This is the system call used to implement .BR pthread_kill (3).) .TP .BR sigqueue (3) Sends a real-time signal with accompanying data to a specified process. .SS Waiting for a signal to be caught The following system calls suspend execution of the calling process or thread until a signal is caught (or an unhandled signal terminates the process): .TP 16 .BR pause (2) Suspends execution until any signal is caught. .TP .BR sigsuspend (2) Temporarily changes the signal mask (see below) and suspends execution until one of the unmasked signals is caught. .SS Synchronously accepting a signal Rather than asynchronously catching a signal via a signal handler, it is possible to synchronously accept the signal, that is, to block execution until the signal is delivered, at which point the kernel returns information about the signal to the caller. There are two general ways to do this: .IP * 2 .BR sigwaitinfo (2), .BR sigtimedwait (2), and .BR sigwait (3) suspend execution until one of the signals in a specified set is delivered. Each of these calls returns information about the delivered signal. .IP * .BR signalfd (2) returns a file descriptor that can be used to read information about signals that are delivered to the caller. Each .BR read (2) from this file descriptor blocks until one of the signals in the set specified in the .BR signalfd (2) call is delivered to the caller. The buffer returned by .BR read (2) contains a structure describing the signal. .SS Signal mask and pending signals A signal may be .IR blocked , which means that it will not be delivered until it is later unblocked. Between the time when it is generated and when it is delivered a signal is said to be .IR pending . .PP Each thread in a process has an independent .IR "signal mask" , which indicates the set of signals that the thread is currently blocking. A thread can manipulate its signal mask using .BR pthread_sigmask (3). In a traditional single-threaded application, .BR sigprocmask (2) can be used to manipulate the signal mask. .PP A child created via .BR fork (2) inherits a copy of its parent's signal mask; the signal mask is preserved across .BR execve (2). .PP A signal may be generated (and thus pending) for a process as a whole (e.g., when sent using .BR kill (2)) or for a specific thread (e.g., certain signals, such as .B SIGSEGV and .BR SIGFPE , generated as a consequence of executing a specific machine-language instruction are thread directed, as are signals targeted at a specific thread using .BR pthread_kill (3)). A process-directed signal may be delivered to any one of the threads that does not currently have the signal blocked. If more than one of the threads has the signal unblocked, then the kernel chooses an arbitrary thread to which to deliver the signal. .PP A thread can obtain the set of signals that it currently has pending using .BR sigpending (2). This set will consist of the union of the set of pending process-directed signals and the set of signals pending for the calling thread. .PP A child created via .BR fork (2) initially has an empty pending signal set; the pending signal set is preserved across an .BR execve (2). .SS Standard signals Linux supports the standard signals listed below. Several signal numbers are architecture-dependent, as indicated in the "Value" column. (Where three values are given, the first one is usually valid for alpha and sparc, the middle one for x86, arm, and most other architectures, and the last one for mips. (Values for parisc are .I not shown; see the Linux kernel source for signal numbering on that architecture.) A dash (\-) denotes that a signal is absent on the corresponding architecture. .PP First the signals described in the original POSIX.1-1990 standard. .TS l c c l ____ lB c c l. Signal Value Action Comment SIGHUP \01 Term Hangup detected on controlling terminal or death of controlling process SIGINT \02 Term Interrupt from keyboard SIGQUIT \03 Core Quit from keyboard SIGILL \04 Core Illegal Instruction SIGABRT \06 Core Abort signal from \fBabort\fP(3) SIGFPE \08 Core Floating-point exception SIGKILL \09 Term Kill signal SIGSEGV 11 Core Invalid memory reference SIGPIPE 13 Term Broken pipe: write to pipe with no readers; see \fBpipe\fP(7) SIGALRM 14 Term Timer signal from \fBalarm\fP(2) SIGTERM 15 Term Termination signal SIGUSR1 30,10,16 Term User-defined signal 1 SIGUSR2 31,12,17 Term User-defined signal 2 SIGCHLD 20,17,18 Ign Child stopped or terminated SIGCONT 19,18,25 Cont Continue if stopped SIGSTOP 17,19,23 Stop Stop process SIGTSTP 18,20,24 Stop Stop typed at terminal SIGTTIN 21,21,26 Stop Terminal input for background process SIGTTOU 22,22,27 Stop Terminal output for background process .TE .sp 1 The signals .B SIGKILL and .B SIGSTOP cannot be caught, blocked, or ignored. .PP Next the signals not in the POSIX.1-1990 standard but described in SUSv2 and POSIX.1-2001. .TS l c c l ____ lB c c l. Signal Value Action Comment SIGBUS 10,7,10 Core Bus error (bad memory access) SIGPOLL Term Pollable event (Sys V). Synonym for \fBSIGIO\fP SIGPROF 27,27,29 Term Profiling timer expired SIGSYS 12,31,12 Core Bad system call (SVr4); see also \fBseccomp\fP(2) SIGTRAP 5 Core Trace/breakpoint trap SIGURG 16,23,21 Ign Urgent condition on socket (4.2BSD) SIGVTALRM 26,26,28 Term Virtual alarm clock (4.2BSD) SIGXCPU 24,24,30 Core CPU time limit exceeded (4.2BSD); see \fBsetrlimit\fP(2) SIGXFSZ 25,25,31 Core File size limit exceeded (4.2BSD); see \fBsetrlimit\fP(2) .TE .sp 1 Up to and including Linux 2.2, the default behavior for .BR SIGSYS ", " SIGXCPU ", " SIGXFSZ ", " and (on architectures other than SPARC and MIPS) .B SIGBUS was to terminate the process (without a core dump). (On some other UNIX systems the default action for .BR SIGXCPU " and " SIGXFSZ is to terminate the process without a core dump.) Linux 2.4 conforms to the POSIX.1-2001 requirements for these signals, terminating the process with a core dump. .PP Next various other signals. .TS l c c l ____ lB c c l. Signal Value Action Comment SIGIOT 6 Core IOT trap. A synonym for \fBSIGABRT\fP SIGEMT 7,\-,7 Term Emulator trap SIGSTKFLT \-,16,\- Term Stack fault on coprocessor (unused) SIGIO 23,29,22 Term I/O now possible (4.2BSD) SIGCLD \-,\-,18 Ign A synonym for \fBSIGCHLD\fP SIGPWR 29,30,19 Term Power failure (System V) SIGINFO 29,\-,\- A synonym for \fBSIGPWR\fP SIGLOST \-,\-,\- Term File lock lost (unused) SIGWINCH 28,28,20 Ign Window resize signal (4.3BSD, Sun) SIGUNUSED \-,31,\- Core Synonymous with \fBSIGSYS\fP .TE .sp 1 (Signal 29 is .B SIGINFO / .B SIGPWR on an alpha but .B SIGLOST on a sparc.) .PP .B SIGEMT is not specified in POSIX.1-2001, but nevertheless appears on most other UNIX systems, where its default action is typically to terminate the process with a core dump. .PP .B SIGPWR (which is not specified in POSIX.1-2001) is typically ignored by default on those other UNIX systems where it appears. .PP .B SIGIO (which is not specified in POSIX.1-2001) is ignored by default on several other UNIX systems. .PP Where defined, .B SIGUNUSED is synonymous with .\" parisc is the only exception: SIGSYS is 12, SIGUNUSED is 31 .B SIGSYS on most architectures. Since glibc 2.26, .B SIGUNUSED is no longer defined on any architecture. .SS Real-time signals Starting with version 2.2, Linux supports real-time signals as originally defined in the POSIX.1b real-time extensions (and now included in POSIX.1-2001). The range of supported real-time signals is defined by the macros .B SIGRTMIN and .BR SIGRTMAX . POSIX.1-2001 requires that an implementation support at least .B _POSIX_RTSIG_MAX (8) real-time signals. .PP The Linux kernel supports a range of 33 different real-time signals, numbered 32 to 64. However, the glibc POSIX threads implementation internally uses two (for NPTL) or three (for LinuxThreads) real-time signals (see .BR pthreads (7)), and adjusts the value of .B SIGRTMIN suitably (to 34 or 35). Because the range of available real-time signals varies according to the glibc threading implementation (and this variation can occur at run time according to the available kernel and glibc), and indeed the range of real-time signals varies across UNIX systems, programs should .IR "never refer to real-time signals using hard-coded numbers" , but instead should always refer to real-time signals using the notation .BR SIGRTMIN +n, and include suitable (run-time) checks that .BR SIGRTMIN +n does not exceed .BR SIGRTMAX . .PP Unlike standard signals, real-time signals have no predefined meanings: the entire set of real-time signals can be used for application-defined purposes. .PP The default action for an unhandled real-time signal is to terminate the receiving process. .PP Real-time signals are distinguished by the following: .IP 1. 4 Multiple instances of real-time signals can be queued. By contrast, if multiple instances of a standard signal are delivered while that signal is currently blocked, then only one instance is queued. .IP 2. 4 If the signal is sent using .BR sigqueue (3), an accompanying value (either an integer or a pointer) can be sent with the signal. If the receiving process establishes a handler for this signal using the .B SA_SIGINFO flag to .BR sigaction (2), then it can obtain this data via the .I si_value field of the .I siginfo_t structure passed as the second argument to the handler. Furthermore, the .I si_pid and .I si_uid fields of this structure can be used to obtain the PID and real user ID of the process sending the signal. .IP 3. 4 Real-time signals are delivered in a guaranteed order. Multiple real-time signals of the same type are delivered in the order they were sent. If different real-time signals are sent to a process, they are delivered starting with the lowest-numbered signal. (I.e., low-numbered signals have highest priority.) By contrast, if multiple standard signals are pending for a process, the order in which they are delivered is unspecified. .PP If both standard and real-time signals are pending for a process, POSIX leaves it unspecified which is delivered first. Linux, like many other implementations, gives priority to standard signals in this case. .PP According to POSIX, an implementation should permit at least .B _POSIX_SIGQUEUE_MAX (32) real-time signals to be queued to a process. However, Linux does things differently. In kernels up to and including 2.6.7, Linux imposes a system-wide limit on the number of queued real-time signals for all processes. This limit can be viewed and (with privilege) changed via the .I /proc/sys/kernel/rtsig-max file. A related file, .IR /proc/sys/kernel/rtsig-nr , can be used to find out how many real-time signals are currently queued. In Linux 2.6.8, these .I /proc interfaces were replaced by the .B RLIMIT_SIGPENDING resource limit, which specifies a per-user limit for queued signals; see .BR setrlimit (2) for further details. .PP The addition of real-time signals required the widening of the signal set structure .RI ( sigset_t ) from 32 to 64 bits. Consequently, various system calls were superseded by new system calls that supported the larger signal sets. The old and new system calls are as follows: .TS lb lb l l. Linux 2.0 and earlier Linux 2.2 and later \fBsigaction\fP(2) \fBrt_sigaction\fP(2) \fBsigpending\fP(2) \fBrt_sigpending\fP(2) \fBsigprocmask\fP(2) \fBrt_sigprocmask\fP(2) \fBsigreturn\fP(2) \fBrt_sigreturn\fP(2) \fBsigsuspend\fP(2) \fBrt_sigsuspend\fP(2) \fBsigtimedwait\fP(2) \fBrt_sigtimedwait\fP(2) .TE .\" .SS Interruption of system calls and library functions by signal handlers If a signal handler is invoked while a system call or library function call is blocked, then either: .IP * 2 the call is automatically restarted after the signal handler returns; or .IP * the call fails with the error .BR EINTR . .PP Which of these two behaviors occurs depends on the interface and whether or not the signal handler was established using the .BR SA_RESTART flag (see .BR sigaction (2)). The details vary across UNIX systems; below, the details for Linux. .PP If a blocked call to one of the following interfaces is interrupted by a signal handler, then the call is automatically restarted after the signal handler returns if the .BR SA_RESTART flag was used; otherwise the call fails with the error .BR EINTR : .\" The following system calls use ERESTARTSYS, .\" so that they are restartable .IP * 2 .BR read (2), .BR readv (2), .BR write (2), .BR writev (2), and .BR ioctl (2) calls on "slow" devices. A "slow" device is one where the I/O call may block for an indefinite time, for example, a terminal, pipe, or socket. If an I/O call on a slow device has already transferred some data by the time it is interrupted by a signal handler, then the call will return a success status (normally, the number of bytes transferred). Note that a (local) disk is not a slow device according to this definition; I/O operations on disk devices are not interrupted by signals. .IP * .BR open (2), if it can block (e.g., when opening a FIFO; see .BR fifo (7)). .IP * .BR wait (2), .BR wait3 (2), .BR wait4 (2), .BR waitid (2), and .BR waitpid (2). .IP * Socket interfaces: .\" If a timeout (setsockopt()) is in effect on the socket, then these .\" system calls switch to using EINTR. Consequently, they and are not .\" automatically restarted, and they show the stop/cont behavior .\" described below. (Verified from 2.6.26 source, and by experiment; mtk) .BR accept (2), .BR connect (2), .BR recv (2), .BR recvfrom (2), .BR recvmmsg (2), .BR recvmsg (2), .BR send (2), .BR sendto (2), and .BR sendmsg (2), .\" FIXME What about sendmmsg()? unless a timeout has been set on the socket (see below). .IP * File locking interfaces: .BR flock (2) and the .BR F_SETLKW and .BR F_OFD_SETLKW operations of .BR fcntl (2) .IP * POSIX message queue interfaces: .BR mq_receive (3), .BR mq_timedreceive (3), .BR mq_send (3), and .BR mq_timedsend (3). .IP * .BR futex (2) .B FUTEX_WAIT (since Linux 2.6.22; .\" commit 72c1bbf308c75a136803d2d76d0e18258be14c7a beforehand, always failed with .BR EINTR ). .IP * .BR getrandom (2). .IP * .BR pthread_mutex_lock (3), .BR pthread_cond_wait (3), and related APIs. .IP * .BR futex (2) .BR FUTEX_WAIT_BITSET . .IP * POSIX semaphore interfaces: .BR sem_wait (3) and .BR sem_timedwait (3) (since Linux 2.6.22; .\" as a consequence of the 2.6.22 changes in the futex() implementation beforehand, always failed with .BR EINTR ). .IP * .BR read (2) from an .BR inotify (7) file descriptor (since Linux 3.8; .\" commit 1ca39ab9d21ac93f94b9e3eb364ea9a5cf2aba06 beforehand, always failed with .BR EINTR ). .PP The following interfaces are never restarted after being interrupted by a signal handler, regardless of the use of .BR SA_RESTART ; they always fail with the error .B EINTR when interrupted by a signal handler: .\" These are the system calls that give EINTR or ERESTARTNOHAND .\" on interruption by a signal handler. .IP * 2 "Input" socket interfaces, when a timeout .RB ( SO_RCVTIMEO ) has been set on the socket using .BR setsockopt (2): .BR accept (2), .BR recv (2), .BR recvfrom (2), .BR recvmmsg (2) (also with a non-NULL .IR timeout argument), and .BR recvmsg (2). .IP * "Output" socket interfaces, when a timeout .RB ( SO_RCVTIMEO ) has been set on the socket using .BR setsockopt (2): .BR connect (2), .BR send (2), .BR sendto (2), and .BR sendmsg (2). .\" FIXME What about sendmmsg()? .IP * Interfaces used to wait for signals: .BR pause (2), .BR sigsuspend (2), .BR sigtimedwait (2), and .BR sigwaitinfo (2). .IP * File descriptor multiplexing interfaces: .BR epoll_wait (2), .BR epoll_pwait (2), .BR poll (2), .BR ppoll (2), .BR select (2), and .BR pselect (2). .IP * System V IPC interfaces: .\" On some other systems, SA_RESTART does restart these system calls .BR msgrcv (2), .BR msgsnd (2), .BR semop (2), and .BR semtimedop (2). .IP * Sleep interfaces: .BR clock_nanosleep (2), .BR nanosleep (2), and .BR usleep (3). .IP * .BR io_getevents (2). .PP The .BR sleep (3) function is also never restarted if interrupted by a handler, but gives a success return: the number of seconds remaining to sleep. .SS Interruption of system calls and library functions by stop signals On Linux, even in the absence of signal handlers, certain blocking interfaces can fail with the error .BR EINTR after the process is stopped by one of the stop signals and then resumed via .BR SIGCONT . This behavior is not sanctioned by POSIX.1, and doesn't occur on other systems. .PP The Linux interfaces that display this behavior are: .IP * 2 "Input" socket interfaces, when a timeout .RB ( SO_RCVTIMEO ) has been set on the socket using .BR setsockopt (2): .BR accept (2), .BR recv (2), .BR recvfrom (2), .BR recvmmsg (2) (also with a non-NULL .IR timeout argument), and .BR recvmsg (2). .IP * "Output" socket interfaces, when a timeout .RB ( SO_RCVTIMEO ) has been set on the socket using .BR setsockopt (2): .BR connect (2), .BR send (2), .BR sendto (2), and .\" FIXME What about sendmmsg()? .BR sendmsg (2), if a send timeout .RB ( SO_SNDTIMEO ) has been set. .IP * 2 .BR epoll_wait (2), .BR epoll_pwait (2). .IP * .BR semop (2), .BR semtimedop (2). .IP * .BR sigtimedwait (2), .BR sigwaitinfo (2). .IP * Linux 3.7 and earlier: .BR read (2) from an .BR inotify (7) file descriptor .\" commit 1ca39ab9d21ac93f94b9e3eb364ea9a5cf2aba06 .IP * Linux 2.6.21 and earlier: .BR futex (2) .BR FUTEX_WAIT , .BR sem_timedwait (3), .BR sem_wait (3). .IP * Linux 2.6.8 and earlier: .BR msgrcv (2), .BR msgsnd (2). .IP * Linux 2.4 and earlier: .BR nanosleep (2). .SH CONFORMING TO POSIX.1, except as noted. .\" It must be a *very* long time since this was true: .\" .SH BUGS .\" .B SIGIO .\" and .\" .B SIGLOST .\" have the same value. .\" The latter is commented out in the kernel source, but .\" the build process of some software still thinks that .\" signal 29 is .\" .BR SIGLOST . .SH NOTES For a discussion of async-signal-safe functions, see .BR signal-safety (7). .SH SEE ALSO .BR kill (1), .BR getrlimit (2), .BR kill (2), .BR restart_syscall (2), .BR rt_sigqueueinfo (2), .BR setitimer (2), .BR setrlimit (2), .BR sgetmask (2), .BR sigaction (2), .BR sigaltstack (2), .BR signal (2), .BR signalfd (2), .BR sigpending (2), .BR sigprocmask (2), .BR sigreturn (2), .BR sigsuspend (2), .BR sigwaitinfo (2), .BR abort (3), .BR bsd_signal (3), .BR killpg (3), .BR longjmp (3), .BR pthread_sigqueue (3), .BR raise (3), .BR sigqueue (3), .BR sigset (3), .BR sigsetops (3), .BR sigvec (3), .BR sigwait (3), .BR strsignal (3), .BR sysv_signal (3), .BR core (5), .BR proc (5), .BR nptl (7), .BR pthreads (7), .BR sigevent (7) .SH COLOPHON This page is part of release 4.16 of the Linux .I man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at \%https://www.kernel.org/doc/man\-pages/.