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

NAME
       signal -	overview of signals

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

   Signal Dispositions
       Each signal has a current disposition, which determines how the process
       behaves when it is delivered the	signal.

       The  entries  in	 the  "Action"	column of the tables below specify the
       default disposition for each 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  (see
	      core(5)).

       Stop   Default action is	to stop	the process.

       Cont   Default  action  is  to  continue	the process if it is currently
	      stopped.

       A process can change the	disposition of a signal	using sigaction(2)  or
       signal(2).   (The  latter  is  less portable when establishing a	signal
       handler;	see signal(2) for  details.)   Using  these  system  calls,  a
       process	can  elect one of the following	behaviors to occur on delivery
       of the signal: perform the default action; ignore the signal; or	 catch
       the signal with a signal	handler, a programmer-defined function that is
       automatically invoked when the signal is	delivered.  (By	 default,  the
       signal  handler is invoked on the normal	process	stack.	It is possible
       to arrange that the signal handler uses an alternate stack; see sigalt-
       stack(2)	 for  a	discussion of how to do	this and when it might be use-
       ful.)

       The signal disposition is a per-process attribute: in  a	 multithreaded
       application, the	disposition of a particular signal is the same for all
       threads.

       A child created via fork(2) inherits a copy of its parent's signal dis-
       positions.   During  an	execve(2), the dispositions of handled signals
       are reset to the	default; the dispositions of ignored signals are  left
       unchanged.

   Sending a Signal
       The  following  system  calls and library functions allow the caller to
       send a signal:

       raise(3)	       Sends a signal to the calling thread.

       kill(2)	       Sends a signal to a specified process, to  all  members
		       of  a  specified	 process group,	or to all processes on
		       the system.

       killpg(2)       Sends a signal to all of	the  members  of  a  specified
		       process group.

       pthread_kill(3) Sends  a	signal to a specified POSIX thread in the same
		       process as the caller.

       tgkill(2)       Sends a signal to a specified thread within a  specific
		       process.	  (This	 is  the system	call used to implement
		       pthread_kill(3).)

       sigqueue(2)     Sends a real-time signal	with accompanying  data	 to  a
		       specified process.

   Waiting for a Signal	to be Caught
       The  following system calls suspend execution of	the calling process or
       thread until a signal is	caught (or an unhandled	signal terminates  the
       process):

       pause(2)	       Suspends	execution until	any signal is caught.

       sigsuspend(2)   Temporarily  changes  the  signal  mask (see below) and
		       suspends	execution until	one of the unmasked signals is
		       caught.

   Synchronously Accepting a Signal
       Rather  than  asynchronously catching a signal via a signal handler, it
       is possible to synchronously accept the signal, that is,	to block  exe-
       cution until the	signal is delivered, at	which point the	kernel returns
       information about the signal to the caller.  There are two general ways
       to do this:

       * sigwaitinfo(2),  sigtimedwait(2),  and	 sigwait(3)  suspend execution
	 until one of the signals in a specified set is	 delivered.   Each  of
	 these calls returns information about the delivered signal.

       * signalfd(2) returns a file descriptor that can	be used	to read	infor-
	 mation	about signals that are delivered to the	caller.	 Each  read(2)
	 from  this file descriptor blocks until one of	the signals in the set
	 specified in the signalfd(2) call is delivered	to  the	 caller.   The
	 buffer	 returned  by read(2) contains a structure describing the sig-
	 nal.

   Signal Mask and Pending Signals
       A signal	may be blocked,	which means that  it  will  not	 be  delivered
       until it	is later unblocked.  Between the time when it is generated and
       when it is delivered a signal is	said to	be pending.

       Each thread in a	process	has an independent signal  mask,  which	 indi-
       cates  the  set	of  signals  that the thread is	currently blocking.  A
       thread can manipulate its signal	mask using pthread_sigmask(3).	 In  a
       traditional  single-threaded application, sigprocmask(2)	can be used to
       manipulate the signal mask.

       A child created via fork(2) inherits a  copy  of	 its  parent's	signal
       mask; the signal	mask is	preserved across execve(2).

       A  signal  may be generated (and	thus pending) for a process as a whole
       (e.g., when sent	using kill(2)) or for a	specific thread	(e.g., certain
       signals,	such as	SIGSEGV	and SIGFPE, generated as a consequence of exe-
       cuting a	specific machine-language instruction are thread directed,  as
       are  signals  targeted  at a specific thread using pthread_kill(3)).  A
       process-directed	signal may be delivered	to any one of the 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.

       A  thread  can  obtain the set of signals that it currently has pending
       using 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.

       A child created via fork(2) initially has an empty pending signal  set;
       the pending signal set is preserved across an execve(2).

   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 ix86, ia64, ppc, s390, arm	 and  sh,  and
       the last	one for	mips.  A - denotes that	a signal is absent on the cor-
       responding architecture.)

       First the signals described in the original POSIX.1-1990	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    Cont    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.1-1990 standard but described in
       SUSv2 and POSIX.1-2001.

       Signal	    Value     Action   Comment
       --------------------------------------------------------------------
       SIGBUS	   10,7,10     Core    Bus error (bad memory access)
       SIGPOLL		       Term    Pollable	event (Sys V).
				       Synonym for SIGIO
       SIGPROF	   27,27,29    Term    Profiling timer expired
       SIGSYS	   12,31,12    Core    Bad argument to routine (SVr4)
       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)
       SIGXFSZ	   25,25,31    Core    File size limit exceeded	(4.2BSD)

       Up to and including Linux 2.2, the default behavior 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
       Unix systems the	default	action for SIGXCPU and SIGXFSZ 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.

       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	(4.2BSD)
       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 (4.3BSD, Sun)
       SIGUNUSED    -,31,-     Core    Synonymous with SIGSYS

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

       SIGEMT is not specified in POSIX.1-2001,	but  nevertheless  appears  on
       most  other Unix	systems, where its default action is typically to ter-
       minate the process with a core dump.

       SIGPWR (which is	not specified in POSIX.1-2001) is typically ignored by
       default on those	other Unix systems where it appears.

       SIGIO (which is not specified in	POSIX.1-2001) is ignored by default on
       several other Unix systems.

       Where defined, SIGUNUSED	is synonymous with SIGSYS  on  most  architec-
       tures.

   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).	 The range  of
       supported  real-time  signals  is  defined  by  the macros SIGRTMIN and
       SIGRTMAX.  POSIX.1-2001 requires	 that  an  implementation  support  at
       least _POSIX_RTSIG_MAX (8) real-time signals.

       The  Linux  kernel  supports a range of 32 different real-time signals,
       numbered	33 to 64.  However, the	 glibc	POSIX  threads	implementation
       internally  uses	 two  (for NPTL) or three (for LinuxThreads) real-time
       signals (see pthreads(7)), and adjusts the value	of  SIGRTMIN  suitably
       (to 34 or 35).  Because the range of available real-time	signals	varies
       according to the	glibc threading	implementation (and this variation can
       occur  at  run  time  according to the available	kernel and glibc), and
       indeed the range	of real-time signals varies across Unix	systems,  pro-
       grams should never refer	to real-time signals using hard-coded numbers,
       but instead should always refer to real-time signals using the notation
       SIGRTMIN+n, and include suitable	(run-time) checks that SIGRTMIN+n does
       not exceed SIGRTMAX.

       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
	   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_SIGINFO 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.)	 By  contrast,
	   if  multiple	 standard signals are pending for a process, the order
	   in which they are delivered is unspecified.

       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,	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 /proc/sys/kernel/rtsig-max file.  A related
       file, /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 /proc	inter-
       faces were replaced by  the  RLIMIT_SIGPENDING  resource	 limit,	 which
       specifies  a  per-user  limit  for queued signals; see setrlimit(2) for
       further details.

   Async-signal-safe functions
       A signal	handler	function must be very careful, since processing	 else-
       where  may  be  interrupted at some arbitrary point in the execution of
       the program.  POSIX has the concept of "safe function".	 If  a	signal
       interrupts  the	execution  of an unsafe	function, and handler calls an
       unsafe function,	then the behavior of the program is undefined.

       POSIX.1-2004 (also  known  as  POSIX.1-2001  Technical  Corrigendum  2)
       requires	 an  implementation  to	guarantee that the following functions
       can be safely called inside a signal handler:

	   _Exit()
	   _exit()
	   abort()
	   accept()
	   access()
	   aio_error()
	   aio_return()
	   aio_suspend()
	   alarm()
	   bind()
	   cfgetispeed()
	   cfgetospeed()
	   cfsetispeed()
	   cfsetospeed()
	   chdir()
	   chmod()
	   chown()
	   clock_gettime()
	   close()
	   connect()
	   creat()
	   dup()
	   dup2()
	   execle()
	   execve()
	   fchmod()
	   fchown()
	   fcntl()
	   fdatasync()
	   fork()
	   fpathconf()
	   fstat()
	   fsync()
	   ftruncate()
	   getegid()
	   geteuid()
	   getgid()
	   getgroups()
	   getpeername()
	   getpgrp()
	   getpid()
	   getppid()
	   getsockname()
	   getsockopt()
	   getuid()
	   kill()
	   link()
	   listen()
	   lseek()
	   lstat()
	   mkdir()
	   mkfifo()
	   open()
	   pathconf()
	   pause()
	   pipe()
	   poll()
	   posix_trace_event()
	   pselect()
	   raise()
	   read()
	   readlink()
	   recv()
	   recvfrom()
	   recvmsg()
	   rename()
	   rmdir()
	   select()
	   sem_post()
	   send()
	   sendmsg()
	   sendto()
	   setgid()
	   setpgid()
	   setsid()
	   setsockopt()
	   setuid()
	   shutdown()
	   sigaction()
	   sigaddset()
	   sigdelset()
	   sigemptyset()
	   sigfillset()
	   sigismember()
	   signal()
	   sigpause()
	   sigpending()
	   sigprocmask()
	   sigqueue()
	   sigset()
	   sigsuspend()
	   sleep()
	   sockatmark()
	   socket()
	   socketpair()
	   stat()
	   symlink()
	   sysconf()
	   tcdrain()
	   tcflow()
	   tcflush()
	   tcgetattr()
	   tcgetpgrp()
	   tcsendbreak()
	   tcsetattr()
	   tcsetpgrp()
	   time()
	   timer_getoverrun()
	   timer_gettime()
	   timer_settime()
	   times()
	   umask()
	   uname()
	   unlink()
	   utime()
	   wait()
	   waitpid()
	   write()

       POSIX.1-2008 removes fpathconf(), pathconf(), and  sysconf()  from  the
       above list, and adds the	following functions:

	   execl()
	   execv()
	   faccessat()
	   fchmodat()
	   fchownat()
	   fexecve()
	   fstatat()
	   futimens()
	   linkat()
	   mkdirat()
	   mkfifoat()
	   mknod()
	   mknodat()
	   openat()
	   readlinkat()
	   renameat()
	   symlinkat()
	   unlinkat()
	   utimensat()
	   utimes()

   Interruption	of System Calls	and Library Functions by Signal	Handlers
       If  a signal handler is invoked while a system call or library function
       call is blocked,	then either:

       * the call is automatically restarted after the signal handler returns;
	 or

       * the call fails	with the error EINTR.

       Which  of  these	 two  behaviors	 occurs	 depends  on the interface and
       whether or not the signal handler was established using the  SA_RESTART
       flag  (see sigaction(2)).  The details vary across Unix systems;	below,
       the details for Linux.

       If a blocked call to one	of the following interfaces is interrupted  by
       a  signal  handler, then	the call will be automatically restarted after
       the signal handler returns if the SA_RESTART flag was  used;  otherwise
       the call	will fail with the error EINTR:

	   * read(2),  readv(2),  write(2),  writev(2),	 and ioctl(2) calls on
	     "slow" devices.  A	"slow" device is one where the	I/O  call  may
	     block  for	 an indefinite time, for example, a terminal, pipe, or
	     socket.  (A disk is not a slow device according to	 this  defini-
	     tion.)   If  an I/O call on a slow	device has already transferred
	     some data by the time it is interrupted by	a signal handler, then
	     the  call	will  return a success status (normally, the number of
	     bytes transferred).

	   * open(2), if  it  can  block  (e.g.,  when	opening	 a  FIFO;  see
	     fifo(7)).

	   * wait(2), wait3(2),	wait4(2), waitid(2), and waitpid(2).

	   * Socket  interfaces:  accept(2), connect(2), recv(2), recvfrom(2),
	     recvmsg(2), send(2), sendto(2), and sendmsg(2), unless a  timeout
	     has been set on the socket	(see below).

	   * File locking interfaces: flock(2) and fcntl(2) F_SETLKW.

	   * POSIX   message   queue   interfaces:   mq_receive(3),   mq_time-
	     dreceive(3), mq_send(3), and mq_timedsend(3).

	   * futex(2)  FUTEX_WAIT  (since  Linux  2.6.22;  beforehand,	always
	     failed with EINTR).

	   * POSIX  semaphore  interfaces:  sem_wait(3)	 and  sem_timedwait(3)
	     (since Linux 2.6.22; beforehand, always failed with EINTR).

       The following interfaces	are never restarted after being	interrupted by
       a signal	handler, regardless of the use of SA_RESTART; they always fail
       with the	error EINTR when interrupted by	a signal handler:

	   * Socket interfaces,	when a timeout has  been  set  on  the	socket
	     using   setsockopt(2):   accept(2),   recv(2),  recvfrom(2),  and
	     recvmsg(2), if a receive timeout (SO_RCVTIMEO) has	been set; con-
	     nect(2),  send(2),	 sendto(2),  and sendmsg(2), if	a send timeout
	     (SO_SNDTIMEO) has been set.

	   * Interfaces	used to	wait  for  signals:  pause(2),	sigsuspend(2),
	     sigtimedwait(2), and sigwaitinfo(2).

	   * File    descriptor	   multiplexing	  interfaces:	epoll_wait(2),
	     epoll_pwait(2), poll(2), ppoll(2),	select(2), and pselect(2).

	   * System V IPC interfaces: msgrcv(2), msgsnd(2), semop(2), and sem-
	     timedop(2).

	   * Sleep    interfaces:    clock_nanosleep(2),   nanosleep(2),   and
	     usleep(3).

	   * read(2) from an inotify(7)	file descriptor.

	   * io_getevents(2).

       The sleep(3) function is	also never restarted if	interrupted by a  han-
       dler,  but  gives  a success return: the	number of seconds remaining to
       sleep.

   Interruption	of System Calls	and Library Functions by Stop Signals
       On Linux, even in the absence  of  signal  handlers,  certain  blocking
       interfaces  can	fail with the error EINTR after	the process is stopped
       by one of the stop signals and then resumed via SIGCONT.	 This behavior
       is not sanctioned by POSIX.1, and doesn't occur on other	systems.

       The Linux interfaces that display this behavior are:

	   * Socket  interfaces,  when	a  timeout  has	been set on the	socket
	     using  setsockopt(2):  accept(2),	 recv(2),   recvfrom(2),   and
	     recvmsg(2), if a receive timeout (SO_RCVTIMEO) has	been set; con-
	     nect(2), send(2), sendto(2), and sendmsg(2), if  a	 send  timeout
	     (SO_SNDTIMEO) has been set.

	   * epoll_wait(2), epoll_pwait(2).

	   * semop(2), semtimedop(2).

	   * sigtimedwait(2), sigwaitinfo(2).

	   * read(2) from an inotify(7)	file descriptor.

	   * Linux  2.6.21 and earlier:	futex(2) FUTEX_WAIT, sem_timedwait(3),
	     sem_wait(3).

	   * Linux 2.6.8 and earlier: msgrcv(2), msgsnd(2).

	   * Linux 2.4 and earlier: nanosleep(2).

CONFORMING TO
       POSIX.1,	except as noted.

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),	getrlimit(2), kill(2), killpg(2), setitimer(2),	 setrlimit(2),
       sgetmask(2), sigaction(2), sigaltstack(2), signal(2), signalfd(2), sig-
       pending(2), sigprocmask(2), sigqueue(2),	sigsuspend(2), sigwaitinfo(2),
       abort(3), bsd_signal(3),	longjmp(3), raise(3), sigset(3), sigsetops(3),
       sigvec(3), sigwait(3), strsignal(3), sysv_signal(3), core(5),  proc(5),
       pthreads(7)

COLOPHON
       This  page  is  part of release 3.25 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found	at http://www.kernel.org/doc/man-pages/.

Linux				  2010-06-12			     SIGNAL(7)

NAME | DESCRIPTION | CONFORMING TO | BUGS | SEE ALSO | COLOPHON

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