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NAME

       signal - list of available signals

DESCRIPTION

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

   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 i386, ppc and sh, and the last one for
       mips.  A - denotes  that  a  signal  is  absent  on  the  corresponding
       architecture.)

       The  entries  in  the  "Action" column of the table specify the default
       action for the signal, as follows:

       Term   Default action is to terminate the process.

       Ign    Default action is to ignore the signal.

       Core   Default action is to terminate the process and dump core.

       Stop   Default action is to stop the process.

       First the signals described in the original POSIX.1 standard.

       Signal     Value     Action   Comment
       -------------------------------------------------------------------------
       SIGHUP        1       Term    Hangup detected on controlling terminal
                                     or death of controlling process
       SIGINT        2       Term    Interrupt from keyboard
       SIGQUIT       3       Core    Quit from keyboard
       SIGILL        4       Core    Illegal Instruction
       SIGABRT       6       Core    Abort signal from abort(3)
       SIGFPE        8       Core    Floating point exception
       SIGKILL       9       Term    Kill signal
       SIGSEGV      11       Core    Invalid memory reference
       SIGPIPE      13       Term    Broken pipe: write to pipe with no readers
       SIGALRM      14       Term    Timer signal from alarm(2)
       SIGTERM      15       Term    Termination signal
       SIGUSR1   30,10,16    Term    User-defined signal 1
       SIGUSR2   31,12,17    Term    User-defined signal 2
       SIGCHLD   20,17,18    Ign     Child stopped or terminated
       SIGCONT   19,18,25            Continue if stopped
       SIGSTOP   17,19,23    Stop    Stop process
       SIGTSTP   18,20,24    Stop    Stop typed at tty
       SIGTTIN   21,21,26    Stop    tty input for background process
       SIGTTOU   22,22,27    Stop    tty output for background process

       The signals SIGKILL and SIGSTOP cannot be caught, blocked, or  ignored.

       Next the signals not in the POSIX.1 standard but described in SUSv2 and
       SUSv3 / POSIX 1003.1-2001.

       Signal       Value     Action   Comment

       -------------------------------------------------------------------------
       SIGBUS      10,7,10     Core    Bus error (bad memory access)
       SIGPOLL                 Term    Pollable event (Sys V). Synonym of SIGIO
       SIGPROF     27,27,29    Term    Profiling timer expired
       SIGSYS      12,-,12     Core    Bad argument to routine (SVID)
       SIGTRAP        5        Core    Trace/breakpoint trap
       SIGURG      16,23,21    Ign     Urgent condition on socket (4.2 BSD)
       SIGVTALRM   26,26,28    Term    Virtual alarm clock (4.2 BSD)
       SIGXCPU     24,24,30    Core    CPU time limit exceeded (4.2 BSD)
       SIGXFSZ     25,25,31    Core    File size limit exceeded (4.2 BSD)

       Up to and including  Linux  2.2,  the  default  behaviour  for  SIGSYS,
       SIGXCPU,  SIGXFSZ,  and  (on  architectures  other than SPARC and MIPS)
       SIGBUS was to terminate the process (without a core  dump).   (On  some
       other Unices the default action for SIGXCPU and SIGXFSZ is to terminate
       the process without a core dump.)  Linux  2.4  conforms  to  the  POSIX
       1003.1-2001  requirements  for  these  signals, terminating the process
       with a core dump.

       Next various other signals.

       Signal       Value     Action   Comment
       --------------------------------------------------------------------
       SIGIOT         6        Core    IOT trap. A synonym for SIGABRT
       SIGEMT       7,-,7      Term
       SIGSTKFLT    -,16,-     Term    Stack fault on coprocessor (unused)
       SIGIO       23,29,22    Term    I/O now possible (4.2 BSD)
       SIGCLD       -,-,18     Ign     A synonym for SIGCHLD
       SIGPWR      29,30,19    Term    Power failure (System V)
       SIGINFO      29,-,-             A synonym for SIGPWR
       SIGLOST      -,-,-      Term    File lock lost
       SIGWINCH    28,28,20    Ign     Window resize signal (4.3 BSD, Sun)
       SIGUNUSED    -,31,-     Term    Unused signal (will be SIGSYS)

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

       SIGEMT is not specified in POSIX 1003.1-2001, but nevertheless  appears
       on  most  other  Unices,  where  its  default  action  is  typically to
       terminate the process with a core dump.

       SIGPWR (which is not  specified  in  POSIX  1003.1-2001)  is  typically
       ignored by default on those other Unices where it appears.

       SIGIO  (which  is  not  specified  in  POSIX 1003.1-2001) is ignored by
       default on several other Unices.

   Real-time Signals
       Linux supports real-time signals as originally defined in  the  POSIX.4
       real-time  extensions  (and  now included in POSIX 1003.1-2001).  Linux
       supports 32 real-time  signals,  numbered  from  32  (SIGRTMIN)  to  63
       (SIGRTMAX).   (Programs  should always refer to real-time signals using
       notation SIGRTMIN+n, since the range of real-time signal numbers varies
       across Unices.)

       Unlike standard signals, real-time signals have no predefined meanings:
       the entire set of real-time signals can be used for application-defined
       purposes.   (Note,  however,  that the LinuxThreads implementation uses
       the first three real-time signals.)

       The default action for an unhandled real-time signal  is  to  terminate
       the receiving process.

       Real-time signals are distinguished by the following:

       1.  Multiple   instances  of  real-time  signals  can  be  queued.   By
           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.)

       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.

CONFORMING TO

       POSIX.1

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),  kill(2), setitimer(2), setrlimit(2), sigaction(2), signal(2),
       sigpending(2), sigprocmask(2), sigqueue(2),  sigsuspend(2),  sigvec(3),
       sigset(3), proc(5), pthreads(7)