signal
SIGNAL(7) Linux Programmer’s Manual SIGNAL(7)
NAME
signal - list of available signals
DESCRIPTION
Linux supports both POSIX reliable signals (hereinafter "standard sig-
nals") and POSIX real-time signals.
Standard Signals
Linux supports the standard signals listed below. Several signal num-
bers 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:
Term Default action is to terminate the process.
Ign Default action is to ignore the signal.
Core Default action is to terminate the process and dump core.
Stop Default action is to stop the process.
First the signals described in the original POSIX.1 standard.
Signal Value Action Comment
-------------------------------------------------------------------------
SIGHUP 1 Term Hangup detected on controlling terminal
or death of controlling process
SIGINT 2 Term Interrupt from keyboard
SIGQUIT 3 Core Quit from keyboard
SIGILL 4 Core Illegal Instruction
SIGABRT 6 Core Abort signal from abort(3)
SIGFPE 8 Core Floating point exception
SIGKILL 9 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 alarm(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
The signals SIGKILL and 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.
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)
Up to and including Linux 2.2, the default behaviour for SIGSYS, SIGX-
CPU, SIGXFSZ, and (on architectures other than SPARC and MIPS) SIGBUS
was to terminate the process (without a core dump). (On some other
Unices the default action for 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.
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)
(Signal 29 is SIGINFO / SIGPWR on an alpha but SIGLOST on a sparc.)
SIGEMT is not specified in POSIX 1003.1-2001, but neverthless appears
on most other Unices, where its default action is typically to termi-
nate the process with a core dump.
SIGPWR (which is not specified in POSIX 1003.1-2001) is typically
ignored by default on those other Unices where it appears.
SIGIO (which is not specified in POSIX 1003.1-2001) is ignored by
default on several other Unices.
Real-time Signals
Linux supports real-time signals as originally defined in the POSIX.1b
real-time extensions (and now included in POSIX.1-2001). Linux sup-
ports real-time signals numbered from SIGRTMIN to SIGRTMAX. Programs
should always refer to real-time signals using notation SIGRTMIN+n,
since the range of real-time signal numbers depends on Linux kernel
version and on libc build. And, of course, it varies across other
Unix systems.
Unlike standard signals, real-time signals have no predefined mean-
ings: the entire set of real-time signals can be used for application-
defined purposes. (Note, however, that the LinuxThreads implementa-
tion uses the first three real-time signals.)
The default action for an unhandled real-time signal is to terminate
the receiving process.
Real-time signals are distinguished by the following:
1. Multiple instances of real-time signals can be queued. By con-
trast, if multiple instances of a standard signal are delivered
while that signal is currently blocked, then only one instance is
queued.
2. If the signal is sent using 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 SA_SIGACTION flag to sigaction(2) then it can obtain this data
via the si_value field of the siginfo_t structure passed as the
second argument to the handler. Furthermore, the si_pid and
si_uid fields of this structure can be used to obtain the PID and
real user ID of the process sending the signal.
3. 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.)
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.
According to POSIX, an implementation should permit at least
_POSIX_SIGQUEUE_MAX (32) real-time signals to be queued to a process.
However, rather than placing a per-process limit, Linux imposes a sys-
tem-wide limit on the number of queued real-time signals for all pro-
cesses. This limit can be viewed and (with privilege) changed via the
/proc/sys/kernel/rtsig-max file. A related file, /proc/sys/ker-
nel/rtsig-nr, can be used to find out how many real-time signals are
currently queued.
CONFORMING TO
POSIX.1
BUGS
SIGIO and 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 SIGLOST.
SEE ALSO
kill(1), kill(2), setitimer(2), sigaction(2), signal(2), sigproc-
mask(2), sigqueue(2)
Linux 2.4.18 2002-06-13 SIGNAL(7)
