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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.

       pidfd_send_signal(2)
              Sends a signal to a process identified by a PID file descriptor.

       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 process-directed or thread-directed.  A  process-directed  signal  is  one
       that  is  targeted  at  (and  thus  pending  for) the process as a whole.  A signal may be
       process-directed because it was generated by the kernel for reasons other than a  hardware
       exception,  or because it was sent using kill(2) or sigqueue(3).  A thread-directed signal
       is one that is targeted at a specific thread.  A signal may be thread-directed because  it
       was  generated  as a consequence of executing a specific machine-language instruction that
       triggered a hardware exception (e.g., SIGSEGV for an invalid memory access, or SIGFPE  for
       a  math  error),  or because it was targeted at a specific thread using interfaces such as
       tgkill(2) or 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).

   Execution of signal handlers
       Whenever  there  is  a transition from kernel-mode to user-mode execution (e.g., on return
       from a system call or scheduling of a thread onto the  CPU),  the  kernel  checks  whether
       there  is  a  pending  unblocked  signal  for  which  the process has established a signal
       handler.  If there is such a pending signal, the following steps occur:

       1. The kernel performs the  necessary  preparatory  steps  for  execution  of  the  signal
          handler:

          a) The signal is removed from the set of pending signals.

          b) If  the  signal  handler  was installed by a call to sigaction(2) that specified the
             SA_ONSTACK flag and  the  thread  has  defined  an  alternate  signal  stack  (using
             sigaltstack(2)), then that stack is installed.

          c) Various  pieces  of  signal-related  context  are saved into a special frame that is
             created on the stack.  The saved information includes:

             + the program counter register (i.e., the address of the  next  instruction  in  the
               main program that should be executed when the signal handler returns);

             + architecture-specific   register  state  required  for  resuming  the  interrupted
               program;

             + the thread's current signal mask;

             + the thread's alternate signal stack settings.

             (If the signal handler was installed using the sigaction(2)  SA_SIGINFO  flag,  then
             the  above information is accessible via the ucontext_t object that is pointed to by
             the third argument of the signal handler.)

          d) Any  signals  specified  in  act->sa_mask  when   registering   the   handler   with
             sigprocmask(2) are added to the thread's signal mask.  The signal being delivered is
             also added to the signal mask, unless SA_NODEFER was specified when registering  the
             handler.  These signals are thus blocked while the handler executes.

       2. The kernel constructs a frame for the signal handler on the stack.  The kernel sets the
          program counter for the thread to point to the first instruction of the signal  handler
          function,  and  configures  the return address for that function to point to a piece of
          user-space code known as the signal trampoline (described in sigreturn(2)).

       3. The kernel passes control back to user-space, where execution commences at the start of
          the signal handler function.

       4. When the signal handler returns, control passes to the signal trampoline code.

       5. The  signal  trampoline  calls sigreturn(2), a system call that uses the information in
          the stack frame created in step 1 to restore the thread to its state before the  signal
          handler  was  called.  The thread's signal mask and alternate signal stack settings are
          restored as part of this procedure.  Upon completion of the call to  sigreturn(2),  the
          kernel  transfers  control  back to user space, and the thread recommences execution at
          the point where it was interrupted by the signal handler.

       Note that if the signal handler does not return (e.g., control is transferred out  of  the
       handler  using  siglongjmp(3), or the handler executes a new program with execve(2)), then
       the final step is not performed.  In particular, in such scenarios it is the  programmer's
       responsibility  to  restore  the state of the signal mask (using sigprocmask(2)), if it is
       desired to unblock the signals that were blocked on entry to the  signal  handler.   (Note
       that siglongjmp(3) may or may not restore the signal mask, depending on the savesigs value
       that was specified in the corresponding call to sigsetjmp(3).)

       From the kernel's point of view, execution of the signal handler code is exactly the  same
       as the execution of any other user-space code.  That is to say, the kernel does not record
       any special state information indicating that the thread is currently  excuting  inside  a
       signal handler.  All necessary state information is maintained in user-space registers and
       the user-space stack.  The depth to which nested signal handlers may be  invoked  is  thus
       limited only by the user-space stack (and sensible software design!).

   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.

BUGS

       There are six signals that can be delivered as a  consequence  of  a  hardware  exception:
       SIGBUS,  SIGEMT,  SIGFPE,  SIGILL,  SIGSEGV,  and  SIGTRAP.   Which  of  these  signals is
       delivered, for any given hardware exception, is not documented and does  not  always  make
       sense.

       For  example,  an  invalid  memory  access  that  causes  delivery  of  SIGSEGV on one CPU
       architecture may cause delivery of SIGBUS on another architecture, or vice versa.

       For another example, using the x86 int instruction with a forbidden argument  (any  number
       other than 3 or 128) causes delivery of SIGSEGV, even though SIGILL would make more sense,
       because of how the CPU reports the forbidden operation to the kernel.

SEE ALSO

       kill(1),  clone(2),  getrlimit(2),  kill(2),   pidfd_send_signal(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),
       swapcontext(3), sysv_signal(3), core(5), proc(5), nptl(7), pthreads(7), sigevent(7)

COLOPHON

       This  page  is  part of release 5.10 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/.