.\" Added section on stop/cont signals interrupting syscalls.
.\" 2008-10-05, mtk: various additions
.\"
-.TH SIGNAL 7 2017-05-03 "Linux" "Linux Programmer's Manual"
+.TH SIGNAL 7 2019-03-06 "Linux" "Linux Programmer's Manual"
.SH NAME
signal \- overview of signals
.SH DESCRIPTION
.IR disposition ,
which determines how the process behaves when it is delivered
the signal.
-
-The entries in the "Action" column of the tables below specify
+.PP
+The entries in the "Action" column of the table below specify
the default disposition for each signal, as follows:
.IP Term
Default action is to terminate the process.
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.
.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
+The following system calls suspend execution of the calling
+thread until a signal is caught
(or an unhandled signal terminates the process):
.TP 16
.BR pause (2)
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.
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))
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;
.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.
-
-First the signals described in the original POSIX.1-1990 standard.
+The second column of the table indicates which standard (if any)
+specified the signal: "P1990" indicates that the signal is described
+in the original POSIX.1-1990 standard;
+"P2001" indicates that the signal was added in SUSv2 and POSIX.1-2001.
.TS
l c c l
____
lB c c l.
-Signal Value Action Comment
-SIGHUP \01 Term Hangup detected on controlling terminal
+Signal Standard Action Comment
+SIGABRT P1990 Core Abort signal from \fBabort\fP(3)
+SIGALRM P1990 Term Timer signal from \fBalarm\fP(2)
+SIGBUS P2001 Core Bus error (bad memory access)
+SIGCHLD P1990 Ign Child stopped or terminated
+SIGCLD \- Ign A synonym for \fBSIGCHLD\fP
+SIGCONT P1990 Cont Continue if stopped
+SIGEMT \- Term Emulator trap
+SIGFPE P1990 Core Floating-point exception
+SIGHUP P1990 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
+SIGILL P1990 Core Illegal Instruction
+SIGINFO \- A synonym for \fBSIGPWR\fP
+SIGINT P1990 Term Interrupt from keyboard
+SIGIO \- Term I/O now possible (4.2BSD)
+SIGIOT \- Core IOT trap. A synonym for \fBSIGABRT\fP
+SIGKILL P1990 Term Kill signal
+SIGLOST \- Term File lock lost (unused)
+SIGPIPE P1990 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
+SIGPOLL P2001 Term Pollable event (Sys V).
+ Synonym for \fBSIGIO\fP
+SIGPROF P2001 Term Profiling timer expired
+SIGPWR \- Term Power failure (System V)
+SIGQUIT P1990 Core Quit from keyboard
+SIGSEGV P1990 Core Invalid memory reference
+SIGSTKFLT \- Term Stack fault on coprocessor (unused)
+SIGSTOP P1990 Stop Stop process
+SIGTSTP P1990 Stop Stop typed at terminal
+SIGSYS P2001 Core Bad system call (SVr4);
+ see also \fBseccomp\fP(2)
+SIGTERM P1990 Term Termination signal
+SIGTRAP P2001 Core Trace/breakpoint trap
+SIGTTIN P1990 Stop Terminal input for background process
+SIGTTOU P1990 Stop Terminal output for background process
+SIGUNUSED \- Core Synonymous with \fBSIGSYS\fP
+SIGURG P2001 Ign Urgent condition on socket (4.2BSD)
+SIGUSR1 P1990 Term User-defined signal 1
+SIGUSR2 P1990 Term User-defined signal 2
+SIGVTALRM P2001 Term Virtual alarm clock (4.2BSD)
+SIGXCPU P2001 Core CPU time limit exceeded (4.2BSD);
+ see \fBsetrlimit\fP(2)
+SIGXFSZ P2001 Core File size limit exceeded (4.2BSD);
+ see \fBsetrlimit\fP(2)
+SIGWINCH \- Ign Window resize signal (4.3BSD, Sun)
.TE
-
+.PP
The signals
.B SIGKILL
and
.B SIGSTOP
cannot be caught, blocked, or ignored.
-
-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
-
+.PP
Up to and including Linux 2.2, the default behavior for
.BR SIGSYS ", " SIGXCPU ", " SIGXFSZ ", "
and (on architectures other than SPARC and MIPS)
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.
-
-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
-
-(Signal 29 is
-.B SIGINFO
-/
-.B SIGPWR
-on an alpha but
-.B SIGLOST
-on a sparc.)
-
+.PP
+.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.
-
+.\"
+.SS Signal numbering for standard signals
+The numeric value for each signal is given in the table below.
+As shown in the table, many signals have different numeric values
+on different architectures.
+The first numeric value in each table row shows the signal number
+on x86, ARM, and most other architectures;
+the second value is for Alpha and SPARC; the third is for MIPS;
+and the last is for PARISC.
+A dash (\-) denotes that a signal is absent on the corresponding architecture.
+.TS
+l c c c c l
+l c c c c l
+______
+lB c c c c l.
+Signal x86/ARM Alpha/ MIPS PARISC Notes
+ most others SPARC
+SIGHUP \01 \01 \01 \01
+SIGINT \02 \02 \02 \02
+SIGQUIT \03 \03 \03 \03
+SIGILL \04 \04 \04 \04
+SIGTRAP \05 \05 \05 \05
+SIGABRT \06 \06 \06 \06
+SIGIOT \06 \06 \06 \06
+SIGBUS \07 10 10 10
+SIGEMT \- \07 \07 -
+SIGFPE \08 \08 \08 \08
+SIGKILL \09 \09 \09 \09
+SIGUSR1 10 30 16 16
+SIGSEGV 11 11 11 11
+SIGUSR2 12 31 17 17
+SIGPIPE 13 13 13 13
+SIGALRM 14 14 14 14
+SIGTERM 15 15 15 15
+SIGSTKFLT 16 \- \- \07
+SIGCHLD 17 20 18 18
+SIGCLD \- \- 18 \-
+SIGCONT 18 19 25 26
+SIGSTOP 19 17 23 24
+SIGTSTP 20 18 24 25
+SIGTTIN 21 21 26 27
+SIGTTOU 22 22 27 28
+SIGURG 23 16 21 29
+SIGXCPU 24 24 30 12
+SIGXFSZ 25 25 31 30
+SIGVTALRM 26 26 28 20
+SIGPROF 27 27 29 21
+SIGWINCH 28 28 20 23
+SIGIO 29 23 22 22
+SIGPOLL Same as SIGIO
+SIGPWR 30 29/\- 19 19
+SIGINFO \- 29/\- \- \-
+SIGLOST \- \-/29 \- \-
+SIGSYS 31 12 12 31
+SIGUNUSED 31 \- \- 31
+.TE
+.PP
+Note the following:
+.IP * 3
Where defined,
.B SIGUNUSED
is synonymous with
-.\" parisc is the only exception: SIGSYS is 12, SIGUNUSED is 31
-.B SIGSYS
-on most architectures.
+.BR SIGSYS .
+Since glibc 2.26,
+.B SIGUNUSED
+is no longer defined on any architecture.
+.IP *
+Signal 29 is
+.BR SIGINFO / SIGPWR
+(synonyms for the same value) on Alpha but
+.B SIGLOST
+on SPARC.
+.\"
.SS Real-time signals
Starting with version 2.2,
Linux supports real-time signals as originally defined in the POSIX.1b
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 )
.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 will be automatically restarted
+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 will fail with the error
+flag was used; otherwise the call fails with the error
.BR EINTR :
.\" The following system calls use ERESTARTSYS,
.\" so that they are restartable
.\" 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,
and
.BR usleep (3).
.IP *
-.BR read (2)
-from an
-.BR inotify (7)
-file descriptor.
-.IP *
.BR io_getevents (2).
.PP
The
.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
.BR sigtimedwait (2),
.BR sigwaitinfo (2).
.IP *
+Linux 3.7 and earlier:
.BR read (2)
from an
.BR inotify (7)
-file descriptor.
+file descriptor
+.\" commit 1ca39ab9d21ac93f94b9e3eb364ea9a5cf2aba06
.IP *
Linux 2.6.21 and earlier:
.BR futex (2)
.BR signal-safety (7).
.SH SEE ALSO
.BR kill (1),
+.BR clone (2),
.BR getrlimit (2),
.BR kill (2),
.BR restart_syscall (2),
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