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signal



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  stan­
       dard.

       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

       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 (with­
       out 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 neverth­
       less 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
           By  contrast, if multiple instances of a standard sig­
           nal are  delivered  while  that  signal  is  currently
           blocked, then only one instance is queued.

       2.  If the signal is sent using sigqueue(2), an accompany­
           ing 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_SIGACTION  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 deliv­
           ered 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 pri­
       ority 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, rather than placing a  per-
       process  limit,  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.


CONFORMING TO

       POSIX.1


BUGS

       SIGIO and SIGLOST have the same value.  The latter is com­
       mented  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),  sigaction(2),  signal(2),
       sigprocmask(2), sigqueue(2)

Linux 2.4.18                2002-06-13                  SIGNAL(7)
  




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