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



NAME
       signal - list of available 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
       (less  portably)  signal(2).   Using  these system calls, a process can
       elect one of the following behaviors to occur on delivery of  the  sig-
       nal: perform the default action; ignore the signal; or catch the signal
       with a signal handler, a programmer-defined function that is  automati-
       cally invoked when the signal is delivered.

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

   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  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.

   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.

       l   c  c  l  ____  lB  c  c  l.   Signal    Value     Action    Comment
       SIGHUP     1    Term Hangup   detected    on    controlling    terminal
                      or     death     of     controlling     process     SIG-
       INT     2   Term Interrupt from keyboard SIGQUIT    3   Core Quit  from
       keyboard       SIGILL     4   Core Illegal       Instruction      SIGA-
       BRT    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 Ter-
       mination    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    SIG-
       CONT   19,18,25  Cont Continue if stopped SIGSTOP   17,19,23  Stop Stop
       process    SIGTSTP   18,20,24  Stop Stop    typed    at    tty    SIGT-
       TIN   21,21,26  Stop tty   input   for   background    process    SIGT-
       TOU   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.

       l c c l ____ lB  c  c  l.   Signal    Value     Action    Comment  SIG-
       BUS    10,7,10   Core Bus    error    (bad    memory    access)    SIG-
       POLL        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)    SIG-
       TRAP   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.

       l  c  c  l  ____  lB  c  c  l.    Signal    Value     Action    Comment
       SIGIOT    6    Core IOT     trap.     A     synonym     for     SIGABRT
       SIGEMT    7,-,7     Term SIGSTKFLT -,16,-    Term Stack fault on copro-
       cessor (unused) SIGIO     23,29,22  Term I/O now possible (4.2BSD) SIG-
       CLD    -,-,18    Ign  A      synonym       for       SIGCHLD       SIG-
       PWR    29,30,19  Term Power      failure      (System      V)      SIG-
       INFO   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  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.

   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 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()

   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).

           * 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:

           * 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:

           * 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 3.05 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                             2008-07-07                         SIGNAL(7)