man-pages/man7/signal.7

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.\" Copyright (c) 2002 by Michael Kerrisk <mtk-manpages@gmx.net>
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.\" 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 <mtk-manpages@gmx.net>
.\" Modified  7 Jun 2002, by Michael Kerrisk <mtk-manpages@gmx.net>
.\"	Added information on real-time signals
.\" Modified 13 Jun 2002, by Michael Kerrisk <mtk-manpages@gmx.net>
.\"	Noted that SIGSTKFLT is in fact unused
.\" 2004-12-03, Modified mtk, added notes on RLIMIT_SIGPENDING
.\"
.TH SIGNAL 7  2002-06-13 "Linux 2.4.18" "Linux Programmer's Manual"
.SH NAME
signal \- list of available signals
.SH DESCRIPTION
Linux supports both POSIX reliable signals (hereinafter
"standard signals") and POSIX real-time signals.
.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 i386, ppc and sh, and
the last one for mips.
A \- denotes that a signal is absent on the corresponding architecture.)

The entries in the "Action" column of the table specify
the default action for the 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.
.IP Stop
Default action is to stop the process.
.PP
First the signals described in the original POSIX.1 standard.
.sp
.PP
.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 \fIabort\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
SIGALRM	14	Term	Timer signal from \fIalarm\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		Continue if stopped
SIGSTOP	17,19,23	Stop	Stop process
SIGTSTP	18,20,24	Stop	Stop typed at tty
SIGTTIN	21,21,26	Stop	tty input for background process
SIGTTOU	22,22,27	Stop	tty output for background process
.TE

The signals
.B SIGKILL
and
.B SIGSTOP
cannot be caught, blocked, or ignored.

Next the signals not in the POSIX.1 standard but described in SUSv2 and
SUSv3 / POSIX 1003.1-2001.
.sp
.PP
.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 of SIGIO
SIGPROF	27,27,29	Term	Profiling timer expired
SIGSYS	12,\-,12	Core	Bad argument to routine (SVID)
SIGTRAP	5	Core	Trace/breakpoint trap
SIGURG	16,23,21	Ign	Urgent condition on socket (4.2 BSD)
SIGVTALRM	26,26,28	Term	Virtual alarm clock (4.2 BSD)
SIGXCPU	24,24,30	Core	CPU time limit exceeded (4.2 BSD)
SIGXFSZ	25,25,31	Core	File size limit exceeded (4.2 BSD)
.TE

Up to and including Linux 2.2, the default behaviour 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 Unices the default action for
.BR SIGXCPU " and " SIGXFSZ
is to terminate the process without a core dump.)
Linux 2.4 conforms to the POSIX 1003.1-2001 requirements for these signals,
terminating the process with a core dump.

Next various other signals.
.sp
.PP
.TS
l c c l
____
lB c c l.
Signal	Value	Action	Comment
SIGIOT	6	Core	IOT trap. A synonym for SIGABRT
SIGEMT	7,\-,7	Term
SIGSTKFLT	\-,16,\-	Term	Stack fault on coprocessor (unused)
SIGIO	23,29,22	Term	I/O now possible (4.2 BSD)
SIGCLD	\-,\-,18	Ign	A synonym for SIGCHLD
SIGPWR	29,30,19	Term	Power failure (System V)
SIGINFO	29,\-,\-		A synonym for SIGPWR
SIGLOST	\-,\-,\-	Term	File lock lost
SIGWINCH	28,28,20	Ign	Window resize signal (4.3 BSD, Sun)
SIGUNUSED	\-,31,\-	Term	Unused signal (will be SIGSYS)
.TE

(Signal 29 is
.B SIGINFO
/
.B SIGPWR
on an alpha but
.B SIGLOST
on a sparc.)

.B SIGEMT
is not specified in POSIX 1003.1-2001, but neverthless appears
on most other Unices, where its default action is typically to terminate
the process with a core dump.

.B SIGPWR
(which is not specified in POSIX 1003.1-2001) is typically ignored
by default on those other Unices where it appears.

.B SIGIO
(which is not specified in POSIX 1003.1-2001) is ignored by default
on several other Unices.
.SS "Real-time Signals"
Linux supports real-time signals as originally defined in the POSIX.4
real-time extensions (and now included in POSIX 1003.1-2001).
Linux supports 32 real-time signals, numbered from 32
.RB ( SIGRTMIN )
to 63
.RB ( SIGRTMAX ).
(Programs should always refer to real-time signals using notation
.BR SIGRTMIN +n,
since the range of real-time signal numbers varies across Unices.)
.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.
(Note, however, that the LinuxThreads implementation uses the first
three real-time signals.)
.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 (2),
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_SIGACTION
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.)
.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
_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
.B RLIMIT_SIGPENDING
resource limit, which specifies a per-user limit for queued
real-time signals; see
.BR setrlimit (2)
for further details.
.SH "CONFORMING TO"
POSIX.1
.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 "SEE ALSO"
.BR kill (1),
.BR kill (2),
.BR setitimer (2),
.BR setrlimit (2),
.BR sigaction (2),
.BR signal (2),
.BR sigprocmask (2),
.BR sigqueue (2),
.BR proc (5)