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NAME

       signal - overview of signals

DESCRIPTION

       Linux  supports  both  POSIX  reliable  signals  (hereinafter "standard
       signals") 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
       (less  portably)  signal(2).   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
       sigaltstack(2) for a discussion of how to do this and when it might  be
       useful.)

       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
       dispositions.  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
       execution 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
         information  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
         signal.

   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
       indicates 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
       executing  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
       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 ix86, ia64, ppc, s390, arm and sh, and
       the last one for mips.  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 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,-,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,
       SIGXCPU, 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,-     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.1-2001, but nevertheless appears on
       most other Unix systems, where  its  default  action  is  typically  to
       terminate 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.

   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,
       programs  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
           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_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
       interfaces 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 handling routine established by sigaction(2) or signal(2) must
       be  very careful, since processing elsewhere 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
             definition.)  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_timedreceive(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;
             connect(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
             semtimedop(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
       handler, 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;
             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.

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),
       sigpending(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 3.23 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/.