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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 table 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,  a  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 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.   The  second  column  of  the  table
       indicates  which standard (if any) specified the signal: "P1990" indicates that the signal
       is described in the original POSIX.1-1990 standard; "P2001" indicates that the signal  was
       added in SUSv2 and POSIX.1-2001.

       Signal      Standard   Action   Comment
       ────────────────────────────────────────────────────────────────────────
       SIGABRT      P1990      Core    Abort signal from abort(3)
       SIGALRM      P1990      Term    Timer signal from alarm(2)
       SIGBUS       P2001      Core    Bus error (bad memory access)
       SIGCHLD      P1990      Ign     Child stopped or terminated
       SIGCLD         -        Ign     A synonym for SIGCHLD
       SIGCONT      P1990      Cont    Continue if stopped
       SIGEMT         -        Term    Emulator trap
       SIGFPE       P1990      Core    Floating-point exception
       SIGHUP       P1990      Term    Hangup detected on controlling terminal

                                       or death of controlling process
       SIGILL       P1990      Core    Illegal Instruction
       SIGINFO        -                A synonym for SIGPWR
       SIGINT       P1990      Term    Interrupt from keyboard
       SIGIO          -        Term    I/O now possible (4.2BSD)
       SIGIOT         -        Core    IOT trap. A synonym for SIGABRT
       SIGKILL      P1990      Term    Kill signal
       SIGLOST        -        Term    File lock lost (unused)
       SIGPIPE      P1990      Term    Broken pipe: write to pipe with no
                                       readers; see pipe(7)
       SIGPOLL      P2001      Term    Pollable event (Sys V).
                                       Synonym for SIGIO
       SIGPROF      P2001      Term    Profiling timer expired
       SIGPWR         -        Term    Power failure (System V)
       SIGQUIT      P1990      Core    Quit from keyboard
       SIGSEGV      P1990      Core    Invalid memory reference
       SIGSTKFLT      -        Term    Stack fault on coprocessor (unused)
       SIGSTOP      P1990      Stop    Stop process
       SIGTSTP      P1990      Stop    Stop typed at terminal
       SIGSYS       P2001      Core    Bad system call (SVr4);
                                       see also seccomp(2)
       SIGTERM      P1990      Term    Termination signal
       SIGTRAP      P2001      Core    Trace/breakpoint trap
       SIGTTIN      P1990      Stop    Terminal input for background process
       SIGTTOU      P1990      Stop    Terminal output for background process
       SIGUNUSED      -        Core    Synonymous with SIGSYS
       SIGURG       P2001      Ign     Urgent condition on socket (4.2BSD)
       SIGUSR1      P1990      Term    User-defined signal 1
       SIGUSR2      P1990      Term    User-defined signal 2
       SIGVTALRM    P2001      Term    Virtual alarm clock (4.2BSD)
       SIGXCPU      P2001      Core    CPU time limit exceeded (4.2BSD);
                                       see setrlimit(2)
       SIGXFSZ      P2001      Core    File size limit exceeded (4.2BSD);
                                       see setrlimit(2)
       SIGWINCH       -        Ign     Window resize signal (4.3BSD, Sun)

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

       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.

       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.

   Queueing and delivery semantics for standard signals
       If multiple standard signals are pending for a process, the order in which the signals are
       delivered is unspecified.

       Standard  signals  do not queue.  If multiple instances of a standard signal are generated
       while that signal is blocked, then only one instance of the signal is  marked  as  pending
       (and  the  signal  will be delivered just once when it is unblocked).  In the case where a
       standard signal is already pending, the siginfo_t structure (see sigaction(2))  associated
       with that signal is not overwritten on arrival of subsequent instances of the same signal.
       Thus, the process will receive the information associated with the first instance  of  the
       signal.

   Signal numbering for standard signals
       The  numeric  value  for  each signal is given in the table below.  As shown in the table,
       many signals have different numeric values on different architectures.  The first  numeric
       value in each table row shows the signal number on x86, ARM, and most other architectures;
       the second value is for Alpha and SPARC; the third is  for  MIPS;  and  the  last  is  for
       PARISC.  A dash (-) denotes that a signal is absent on the corresponding architecture.

       Signal        x86/ARM     Alpha/   MIPS   PARISC   Notes
                   most others   SPARC
       ─────────────────────────────────────────────────────────────────
       SIGHUP           1           1       1       1
       SIGINT           2           2       2       2
       SIGQUIT          3           3       3       3
       SIGILL           4           4       4       4
       SIGTRAP          5           5       5       5
       SIGABRT          6           6       6       6
       SIGIOT           6           6       6       6
       SIGBUS           7          10      10      10
       SIGEMT           -           7       7      -
       SIGFPE           8           8       8       8
       SIGKILL          9           9       9       9
       SIGUSR1         10          30      16      16
       SIGSEGV         11          11      11      11
       SIGUSR2         12          31      17      17
       SIGPIPE         13          13      13      13
       SIGALRM         14          14      14      14
       SIGTERM         15          15      15      15
       SIGSTKFLT       16          -       -        7
       SIGCHLD         17          20      18      18
       SIGCLD           -          -       18      -
       SIGCONT         18          19      25      26
       SIGSTOP         19          17      23      24
       SIGTSTP         20          18      24      25
       SIGTTIN         21          21      26      27
       SIGTTOU         22          22      27      28
       SIGURG          23          16      21      29
       SIGXCPU         24          24      30      12
       SIGXFSZ         25          25      31      30
       SIGVTALRM       26          26      28      20
       SIGPROF         27          27      29      21
       SIGWINCH        28          28      20      23
       SIGIO           29          23      22      22
       SIGPOLL                                            Same as SIGIO
       SIGPWR          30         29/-     19      19
       SIGINFO          -         29/-     -       -
       SIGLOST          -         -/29     -       -
       SIGSYS          31          12      12      31
       SIGUNUSED       31          -       -       31

       Note the following:

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

       *  Signal 29 is SIGINFO/SIGPWR (synonyms for the same  value)  on  Alpha  but  SIGLOST  on
          SPARC.

   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).

       The  /proc/[pid]/task/[tid]/status file contains various fields that show the signals that
       a thread is blocking (SigBlk), catching (SigCgt),  or  ignoring  (SigIgn).   (The  set  of
       signals  that  are  caught  or  ignored will be the same across all threads in a process.)
       Other fields show the set of pending signals that are directed to the thread  (SigPnd)  as
       well  as  the set of pending signals that are directed to the process as a whole (ShdPnd).
       The corresponding fields in /proc/[pid]/status show the information for the  main  thread.
       See proc(5) for further details.

SEE ALSO

       kill(1),   clone(2),   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

       This page is part of release 5.02 of the Linux man-pages project.  A  description  of  the
       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at https://www.kernel.org/doc/man-pages/.