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



NAME
       signal(2,7) - list of available signals

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

   Standard Signals
       Linux supports the standard signals listed below. Several  signal(2,7)  num-
       bers  are  architecture  dependent, as indicated in(1,8) 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(2,7) is absent on the corresponding  archi-
       tecture.)

       The  entries  in(1,8)  the  "Action" column of the table specify the default
       action for the signal(2,7), 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(1,8) 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(2,7) from abort(3,7)(3)
       SIGFPE        8       Core    Floating point exception
       SIGKILL       9       Term    Kill signal(2,7)
       SIGSEGV      11       Core    Invalid memory reference
       SIGPIPE      13       Term    Broken pipe: write(1,2) to pipe(2,8) with no readers
       SIGALRM      14       Term    Timer signal(2,7) from alarm(1,2)(2)
       SIGTERM      15       Term    Termination signal(2,7)
       SIGUSR1   30,10,16    Term    User-defined signal(2,7) 1
       SIGUSR2   31,12,17    Term    User-defined signal(2,7) 2
       SIGCHLD   20,17,18    Ign     Child stopped or terminated
       SIGCONT   19,18,25            Continue if(3,n) stopped
       SIGSTOP   17,19,23    Stop    Stop process
       SIGTSTP   18,20,24    Stop    Stop typed at tty(1,4)
       SIGTTIN   21,21,26    Stop    tty(1,4) input for background process
       SIGTTOU   22,22,27    Stop    tty(1,4) output for background process

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

       Next the signals not in(1,8) the POSIX.1 standard but described in(1,8) SUSv2 and
       SUSv3 / POSIX 1003.1-2001.



       Signal       Value     Action   Comment

       -------------------------------------------------------------------------
       SIGBUS      10,7,10     Core    Bus error(8,n) (bad memory access(2,5))
       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(2,7,n) (4.2 BSD)
       SIGVTALRM   26,26,28    Term    Virtual alarm(1,2) clock(3,n) (4.2 BSD)
       SIGXCPU     24,24,30    Core    CPU time(1,2,n) 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,  SIGX-
       CPU,  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(2,7) (4.3 BSD, Sun)
       SIGUNUSED    -,31,-     Term    Unused signal(2,7) (will be SIGSYS)

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

       SIGEMT is not specified in(1,8) POSIX 1003.1-2001, but  neverthless  appears
       on  most  other Unices, where its default action is typically to termi-
       nate the process with a core dump.

       SIGPWR (which is not  specified  in(1,8)  POSIX  1003.1-2001)  is  typically
       ignored by default on those other Unices where it appears.

       SIGIO  (which  is  not  specified  in(1,8)  POSIX 1003.1-2001) is ignored by
       default on several other Unices.

   Real-time Signals
       Linux supports real-time signals as originally defined in(1,8)  the  POSIX.4
       real-time  extensions  (and  now included in(1,8) 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(2,7) numbers varies
       across Unices.)

       Unlike standard signals, real-time signals have no predefined meanings:
       the entire set(7,n,1 builtins) 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(2,7)  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(3,n) multiple instances of a  standard  signal(2,7)  are  delivered
           while  that  signal(2,7) is currently blocked, then only one instance is
           queued.

       2.  If the signal(2,7) 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(2,7)  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(1,8) a guaranteed order.  Multiple
           real-time signals of the same type are delivered in(1,8) 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(1,8) 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 sys-
       tem-wide limit on the number of queued real-time signals for  all  pro-
       cesses.   This limit can be viewed and (with privilege) changed via the
       /proc(5,n)/sys/kernel/rtsig-max  file.   A  related   file(1,n),   /proc(5,n)/sys/ker-
       nel/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 commented out  in(1,8)
       the  kernel source, but the build process of some software still thinks
       that signal(2,7) 29 is SIGLOST.

SEE ALSO
       kill(1,2,1 builtins)(1),  kill(1,2,1 builtins)(2),  setitimer(2),  sigaction(2),  signal(2,7)(2),   sigproc-
       mask(2), sigqueue(2)



Linux 2.4.18                      2002-06-13                         SIGNAL(7)

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