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

       signal - list of available signals

       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

       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 neverthless 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

       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_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 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, 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-max, can be used to find out how many real-time
       signals are currently queued.


       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.

       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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