signal(7)



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 behaviours 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(2)).  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 i386, ppc and sh, and the last one for
       mips.  A - denotes that a signal is absent on the corresponding  archi-
       tecture.)

       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 of 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 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.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 Unices, 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 Unices where it appears.

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

   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).   Linux  sup-
       ports  32  real-time signals, numbered from 32 (SIGRTMIN) to 63 (SIGRT-
       MAX).  (Programs should always refer to real-time signals  using  nota-
       tion  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  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.)

       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.

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), killpg(2), setitimer(2), setrlimit(2),  sigaction(2),
       signal(2),  sigpending(2),  sigprocmask(2), sigqueue(2), sigsuspend(2),
       sigwaitinfo(2), raise(3), sigvec(3), sigset(3), strsignal(3),  core(5),
       proc(5), pthreads(7)

Linux 2.4.18                      2002-06-13                         SIGNAL(7)

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