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SIGNAL(7)		   Linux Programmer's Manual		     SIGNAL(7)

       signal -	list of	available signals

       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	archi-

       The entries in the "Action" column of the table specify the default ac-
       tion 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 ig-
       nored by	default	on those other Unices where it appears.

       SIGIO (which is not specified in	POSIX 1003.1-2001) is ignored  by  de-
       fault 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  con-
	   trast,  if  multiple	 instances  of a standard signal are delivered
	   while that signal is	currently blocked, then	only one  instance  is

       2.  If the signal is sent using sigqueue(2), an accompanying value (ei-
	   ther	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/kernel/rt-
       sig-max,	can be used to find out	how many real-time  signals  are  cur-
       rently queued.


       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.

       kill(1),	  kill(2),  setitimer(2),  sigaction(2),  signal(2),  sigproc-
       mask(2),	sigqueue(2)

Linux 2.4.18			  2002-06-13			     SIGNAL(7)


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