1. Technology
Linux / Unix Command: signal
Command Library

NAME

signal - list of available signals

DESCRIPTION

Linux supports both POSIX reliable signals (hereinafter "standard signals") and POSIX real-time signals.

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:

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.

SignalValueActionComment




or death of controlling process
SIGINT 2TermInterrupt from keyboard
SIGQUIT 3CoreQuit from keyboard
SIGILL 4CoreIllegal Instruction
SIGABRT 6CoreAbort signal from abort(3)
SIGFPE 8CoreFloating point exception
SIGKILL 9TermKill signal
SIGSEGV11CoreInvalid memory reference
SIGPIPE13TermBroken pipe: write to pipe with no readers
SIGALRM14TermTimer signal from alarm(2)
SIGTERM15TermTermination signal
SIGUSR130,10,16TermUser-defined signal 1
SIGUSR231,12,17TermUser-defined signal 2
SIGCHLD20,17,18IgnChild stopped or terminated
SIGCONT19,18,25Continue if stopped
SIGSTOP17,19,23StopStop process
SIGTSTP18,20,24StopStop typed at tty
SIGTTIN21,21,26Stoptty input for background process
SIGTTOU22,22,27Stoptty 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.

SignalValueActionComment




SIGPOLLTermPollable event (Sys V). Synonym of SIGIO
SIGPROF27,27,29TermProfiling timer expired
SIGSYS12,-,12CoreBad argument to routine (SVID)
SIGTRAP5CoreTrace/breakpoint trap
SIGURG16,23,21IgnUrgent condition on socket (4.2 BSD)
SIGVTALRM26,26,28TermVirtual alarm clock (4.2 BSD)
SIGXCPU24,24,30CoreCPU time limit exceeded (4.2 BSD)
SIGXFSZ25,25,31CoreFile size limit exceeded (4.2 BSD)

Up to and including Linux 2.2, the default behaviour for SIGSYS, SIGXCPU, 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.

SignalValueActionComment




SIGEMT7,-,7Term
SIGSTKFLT-,16,-TermStack fault on coprocessor (unused)
SIGIO23,29,22TermI/O now possible (4.2 BSD)
SIGCLD-,-,18IgnA synonym for SIGCHLD
SIGPWR29,30,19TermPower failure (System V)
SIGINFO29,-,-A synonym for SIGPWR
SIGLOST-,-,-TermFile lock lost
SIGWINCH28,28,20IgnWindow resize signal (4.3 BSD, Sun)
SIGUNUSED-,31,-TermUnused 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 terminate 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.4 real-time extensions (and now included in POSIX 1003.1-2001). Linux supports 32 real-time signals, numbered from 32 (SIGRTMIN) to 63 (SIGRTMAX). (Programs should always refer to real-time signals using notation SIGRTMIN+n, since the range of real-time signal numbers varies across Unices.)

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.)

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 contrast, 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 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 /proc/sys/kernel/rtsig-max file. A related file, /proc/sys/kernel/rtsig-max, can be used to find out how many real-time signals are currently queued.

CONFORMING TO

POSIX.1

SEE ALSO

kill(1), kill(2), setitimer(2), sigaction(2), signal(2), sigprocmask(2), sigqueue(2)


Important: Use the man command (% man) to see how a command is used on your particular computer.

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