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NAME

       signal - overview of signals

DESCRIPTION

       Linux  supports  both  POSIX  reliable  signals (hereinafter "standard signals") 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  signal(2).   (The
       latter  is  less  portable when establishing a signal handler; see signal(2) for details.)
       Using these system calls, a process can elect one of the following behaviors to  occur  on
       delivery of the signal: perform the default action; ignore the signal; or catch the signal
       with a signal handler, a programmer-defined function that is  automatically  invoked  when
       the signal is delivered.  (By default, the signal handler is invoked on the normal process
       stack.  It is possible to arrange that the signal handler uses  an  alternate  stack;  see
       sigaltstack(2) for a discussion of how to do this and when it might be useful.)

       The  signal  disposition  is  a per-process attribute: in a multithreaded application, the
       disposition of a particular signal is the same for all threads.

       A child created via fork(2) inherits a copy of its parent's signal  dispositions.   During
       an  execve(2),  the  dispositions  of  handled  signals  are  reset  to  the  default; the
       dispositions of ignored signals are left unchanged.

   Sending a signal
       The following system calls and library functions allow the caller to send a signal:

       raise(3)        Sends a signal to the calling thread.

       kill(2)         Sends a signal to a specified process,  to  all  members  of  a  specified
                       process group, or to all processes on the system.

       killpg(3)       Sends a signal to all of the members of a specified process group.

       pthread_kill(3) Sends  a  signal  to  a  specified POSIX thread in the same process as the
                       caller.

       tgkill(2)       Sends a signal to a specified thread within a specific process.  (This  is
                       the system call used to implement pthread_kill(3).)

       sigqueue(3)     Sends a real-time signal with accompanying data to a specified process.

   Waiting for a signal to be caught
       The  following  system  calls  suspend  execution of the calling process or thread until a
       signal is caught (or an unhandled signal terminates the process):

       pause(2)        Suspends execution until any signal is caught.

       sigsuspend(2)   Temporarily changes the signal mask (see  below)  and  suspends  execution
                       until one of the unmasked signals is caught.

   Synchronously accepting a signal
       Rather  than  asynchronously  catching  a  signal  via a signal handler, it is possible to
       synchronously accept the  signal,  that  is,  to  block  execution  until  the  signal  is
       delivered,  at  which point the kernel returns information about the signal to the caller.
       There are two general ways to do this:

       * sigwaitinfo(2), sigtimedwait(2), and sigwait(3)  suspend  execution  until  one  of  the
         signals  in a specified set is delivered.  Each of these calls returns information about
         the delivered signal.

       * signalfd(2) returns a file descriptor that can be used to read information about signals
         that  are  delivered to the caller.  Each read(2) from this file descriptor blocks until
         one of the signals in the set specified in the signalfd(2)  call  is  delivered  to  the
         caller.  The buffer returned by read(2) contains a structure describing the signal.

   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 indicates 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 child created via fork(2) inherits a copy of its parent's signal mask; the  signal  mask
       is preserved across execve(2).

       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 executing a specific machine-language instruction
       are thread directed, as are signals targeted at a specific thread using  pthread_kill(3)).
       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.

       A  child created via fork(2) initially has an empty pending signal set; the pending signal
       set is preserved across an execve(2).

   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  x86,  arm,
       and  most  other  architectures,  and  the  last one for mips.  (Values for parisc are not
       shown; see the Linux kernel source for signal numbering on that architecture.)  A dash (-)
       denotes that a signal is absent on the corresponding architecture.

       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; see pipe(7)
       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 terminal
       SIGTTIN   21,21,26    Stop    Terminal input for background process
       SIGTTOU   22,22,27    Stop    Terminal 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 for SIGIO
       SIGPROF     27,27,29    Term    Profiling timer expired
       SIGSYS      12,31,12    Core    Bad system call (SVr4);
                                       see also seccomp(2)
       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);
                                       see setrlimit(2)
       SIGXFSZ     25,25,31    Core    File size limit exceeded (4.2BSD);
                                       see setrlimit(2)

       Up  to and including Linux 2.2, the default behavior 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 UNIX systems 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    Emulator trap
       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 (unused)
       SIGWINCH    28,28,20    Ign     Window resize signal (4.3BSD, Sun)
       SIGUNUSED    -,31,-     Core    Synonymous with 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 UNIX
       systems, 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 UNIX systems where it appears.

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

       Where defined, SIGUNUSED is synonymous with SIGSYS on  most  architectures.   Since  glibc
       2.26, SIGUNUSED is no longer defined on any architecture.

   Real-time signals
       Starting  with  version 2.2, Linux supports real-time signals as originally defined in the
       POSIX.1b real-time extensions (and now included in POSIX.1-2001).  The range of  supported
       real-time  signals  is defined by the macros SIGRTMIN and SIGRTMAX.  POSIX.1-2001 requires
       that an implementation support at least _POSIX_RTSIG_MAX (8) real-time signals.

       The Linux kernel supports a range of 33 different real-time signals, numbered  32  to  64.
       However,  the  glibc  POSIX threads implementation internally uses two (for NPTL) or three
       (for LinuxThreads) real-time signals (see pthreads(7)), and adjusts the value of  SIGRTMIN
       suitably (to 34 or 35).  Because the range of available real-time signals varies according
       to the glibc threading implementation (and this variation can occur at run time  according
       to  the  available  kernel  and  glibc),  and indeed the range of real-time signals varies
       across UNIX systems, programs should never refer to  real-time  signals  using  hard-coded
       numbers,  but  instead  should  always  refer  to  real-time  signals  using  the notation
       SIGRTMIN+n, and include  suitable  (run-time)  checks  that  SIGRTMIN+n  does  not  exceed
       SIGRTMAX.

       Unlike  standard signals, real-time signals have no predefined meanings: the entire set of
       real-time signals can be used for application-defined purposes.

       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(3), 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.)  By contrast, if multiple
           standard signals are pending for a process, the order in which they are  delivered  is
           unspecified.

       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.

       The addition of real-time signals required  the  widening  of  the  signal  set  structure
       (sigset_t)  from 32 to 64 bits.  Consequently, various system calls were superseded by new
       system calls that supported the larger signal sets.  The old and new system calls  are  as
       follows:

       Linux 2.0 and earlier   Linux 2.2 and later
       sigaction(2)            rt_sigaction(2)
       sigpending(2)           rt_sigpending(2)
       sigprocmask(2)          rt_sigprocmask(2)
       sigreturn(2)            rt_sigreturn(2)
       sigsuspend(2)           rt_sigsuspend(2)
       sigtimedwait(2)         rt_sigtimedwait(2)

   Interruption of system calls and library functions by signal handlers
       If  a  signal  handler is invoked while a system call or library function call is blocked,
       then either:

       * the call is automatically restarted after the signal handler returns; or

       * the call fails with the error EINTR.

       Which of these two behaviors occurs depends on the interface and whether or not the signal
       handler  was  established  using the SA_RESTART flag (see sigaction(2)).  The details vary
       across UNIX systems; below, the details for Linux.

       If a blocked call to one of the following interfaces is interrupted by a  signal  handler,
       then  the  call  is  automatically  restarted  after  the  signal  handler  returns if the
       SA_RESTART flag was used; otherwise the call fails with the error EINTR:

       * read(2), readv(2), write(2), writev(2), and ioctl(2) calls on "slow" devices.  A  "slow"
         device  is  one  where  the  I/O  call  may block for an indefinite time, for example, a
         terminal, pipe, or socket.  If an I/O call on a slow device has already transferred some
         data  by  the  time  it  is interrupted by a signal handler, then the call will return a
         success status (normally, the number of bytes transferred).  Note that a (local) disk is
         not  a  slow device according to this definition; I/O operations on disk devices are not
         interrupted by signals.

       * open(2), if it can block (e.g., when opening a FIFO; see fifo(7)).

       * wait(2), wait3(2), wait4(2), waitid(2), and waitpid(2).

       * Socket interfaces: accept(2), connect(2), recv(2), recvfrom(2), recvmmsg(2), recvmsg(2),
         send(2),  sendto(2),  and  sendmsg(2),  unless a timeout has been set on the socket (see
         below).

       * File locking interfaces: flock(2)  and  the  F_SETLKW  and  F_OFD_SETLKW  operations  of
         fcntl(2)

       * POSIX  message  queue  interfaces:  mq_receive(3),  mq_timedreceive(3),  mq_send(3), and
         mq_timedsend(3).

       * futex(2) FUTEX_WAIT (since Linux 2.6.22; beforehand, always failed with EINTR).

       * getrandom(2).

       * pthread_mutex_lock(3), pthread_cond_wait(3), and related APIs.

       * futex(2) FUTEX_WAIT_BITSET.

       * POSIX semaphore  interfaces:  sem_wait(3)  and  sem_timedwait(3)  (since  Linux  2.6.22;
         beforehand, always failed with EINTR).

       * read(2)  from  an inotify(7) file descriptor (since Linux 3.8; beforehand, always failed
         with EINTR).

       The following interfaces are never restarted after being interrupted by a signal  handler,
       regardless  of  the  use  of  SA_RESTART;  they  always  fail  with  the  error EINTR when
       interrupted by a signal handler:

       * "Input" socket interfaces, when a timeout (SO_RCVTIMEO) has been set on the socket using
         setsockopt(2):  accept(2),  recv(2),  recvfrom(2),  recvmmsg(2)  (also  with  a non-NULL
         timeout argument), and recvmsg(2).

       * "Output" socket interfaces, when a timeout (SO_RCVTIMEO) has  been  set  on  the  socket
         using setsockopt(2): connect(2), send(2), sendto(2), and sendmsg(2).

       * Interfaces  used  to  wait  for  signals:  pause(2), sigsuspend(2), sigtimedwait(2), and
         sigwaitinfo(2).

       * File  descriptor  multiplexing  interfaces:  epoll_wait(2),   epoll_pwait(2),   poll(2),
         ppoll(2), select(2), and pselect(2).

       * System V IPC interfaces: msgrcv(2), msgsnd(2), semop(2), and semtimedop(2).

       * Sleep interfaces: clock_nanosleep(2), nanosleep(2), and usleep(3).

       * io_getevents(2).

       The  sleep(3)  function  is  also never restarted if interrupted by a handler, but gives a
       success return: the number of seconds remaining to sleep.

   Interruption of system calls and library functions by stop signals
       On Linux, even in the absence of signal handlers, certain  blocking  interfaces  can  fail
       with  the  error  EINTR  after  the process is stopped by one of the stop signals and then
       resumed via SIGCONT.  This behavior is not sanctioned by POSIX.1,  and  doesn't  occur  on
       other systems.

       The Linux interfaces that display this behavior are:

       * "Input" socket interfaces, when a timeout (SO_RCVTIMEO) has been set on the socket using
         setsockopt(2): accept(2),  recv(2),  recvfrom(2),  recvmmsg(2)  (also  with  a  non-NULL
         timeout argument), and recvmsg(2).

       * "Output"  socket  interfaces,  when  a  timeout (SO_RCVTIMEO) has been set on the socket
         using setsockopt(2): connect(2), send(2), sendto(2), and sendmsg(2), if a  send  timeout
         (SO_SNDTIMEO) has been set.

       * epoll_wait(2), epoll_pwait(2).

       * semop(2), semtimedop(2).

       * sigtimedwait(2), sigwaitinfo(2).

       * Linux 3.7 and earlier: read(2) from an inotify(7) file descriptor

       * Linux 2.6.21 and earlier: futex(2) FUTEX_WAIT, sem_timedwait(3), sem_wait(3).

       * Linux 2.6.8 and earlier: msgrcv(2), msgsnd(2).

       * Linux 2.4 and earlier: nanosleep(2).

CONFORMING TO

       POSIX.1, except as noted.

NOTES

       For a discussion of async-signal-safe functions, see signal-safety(7).

SEE ALSO

       kill(1),  getrlimit(2),  kill(2),  restart_syscall(2),  rt_sigqueueinfo(2),  setitimer(2),
       setrlimit(2),   sgetmask(2),   sigaction(2),   sigaltstack(2),   signal(2),   signalfd(2),
       sigpending(2),  sigprocmask(2),  sigreturn(2),  sigsuspend(2),  sigwaitinfo(2),  abort(3),
       bsd_signal(3),  killpg(3),   longjmp(3),   pthread_sigqueue(3),   raise(3),   sigqueue(3),
       sigset(3),  sigsetops(3),  sigvec(3),  sigwait(3),  strsignal(3), sysv_signal(3), core(5),
       proc(5), nptl(7), pthreads(7), sigevent(7)

COLOPHON

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