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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(3)     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 x86, arm, and most	 other	architectures,
       and  the	 last one for mips.  (Values for parisc are not shown; see the
       Linux kernel source for signal numbering on that	 architecture.)	  A  -
       denotes that a signal is absent on the corresponding 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 terminal
       SIGTTIN	 21,21,26    Stop    Terminal input for background process
       SIGTTOU	 22,22,27    Stop    Terminal 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 (unused)
       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
       Starting	 with  version 2.2, Linux supports real-time signals as origi-
       nally 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 imple-
       mentation support at least _POSIX_RTSIG_MAX (8) real-time signals.

       The  Linux  kernel  supports a range of 33 different real-time signals,
       numbered 32 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.

       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(3),  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.

       The  addition  or real-time signals required the widening of the signal
       set structure (sigset_t) from 32 to  64	bits.	Consequently,  various
       system  calls  were  superseded	by new system calls that supported the
       larger signal sets.  The old and new system calls are as follows:

       Linux 2.0 and earlier   Linux 2.2 and later
       sigaction(2)	       rt_sigaction(2)
       sigpending(2)	       rt_sigpending(2)
       sigprocmask(2)	       rt_sigprocmask(2)
       sigreturn(2)	       rt_sigreturn(2)
       sigsuspend(2)	       rt_sigsuspend(2)
       sigtimedwait(2)	       rt_sigtimedwait(2)

   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()

       POSIX.1-2008  Technical	Corrigendum  1 (2013) adds the following func-
       tions:

	   fchdir()
	   pthread_kill()
	   pthread_self()
	   pthread_sigmask()

   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.   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).  Note that a  (local)	disk  is  not  a  slow
	     device  according	to  this  definition;  I/O  operations on disk
	     devices are not interrupted by signals.

	   * 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),
	     recvmmsg(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  the	 F_SETLKW  and
	     F_OFD_SETLKW operations of fcntl(2)

	   * 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).

	   * getrandom(2).

	   * pthread_mutex_lock(3), pthread_cond_wait(3), and related APIs.

	   * futex(2) FUTEX_WAIT_BITSET.

	   * 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:

	   * "Input"  socket interfaces, when a timeout (SO_RCVTIMEO) has been
	     set  on  the  socket  using  setsockopt(2):  accept(2),  recv(2),
	     recvfrom(2), recvmmsg(2) (also with a non-NULL timeout argument),
	     and recvmsg(2).

	   * "Output" socket interfaces, when a timeout (SO_RCVTIMEO) has been
	     set  on  the  socket  using  setsockopt(2):  connect(2), send(2),
	     sendto(2), and sendmsg(2).

	   * 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:

	   * "Input"  socket interfaces, when a timeout (SO_RCVTIMEO) has been
	     set  on  the  socket  using  setsockopt(2):  accept(2),  recv(2),
	     recvfrom(2), recvmmsg(2) (also with a non-NULL timeout argument),
	     and recvmsg(2).

	   * "Output" socket interfaces, when a timeout (SO_RCVTIMEO) has been
	     set  on  the  socket  using  setsockopt(2):  connect(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.

SEE ALSO
       kill(1),	  getrlimit(2),	  kill(2),   killpg(2),	   restart_syscall(2),
       rt_sigqueueinfo(2),  setitimer(2),  setrlimit(2),  sgetmask(2),	sigac-
       tion(2), sigaltstack(2), signal(2),  signalfd(2),  sigpending(2),  sig-
       procmask(2),  sigsuspend(2),  sigwaitinfo(2),  abort(3), bsd_signal(3),
       longjmp(3),  raise(3),  pthread_sigqueue(3),  sigqueue(3),   sigset(3),
       sigsetops(3),   sigvec(3),  sigwait(3),	strsignal(3),  sysv_signal(3),
       core(5), proc(5), nptl(7), pthreads(7), sigevent(7)

COLOPHON
       This page is part of release 4.04 of the Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at
       http://www.kernel.org/doc/man-pages/.



Linux				  2015-12-05			     SIGNAL(7)