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

       signal - overview of signals

       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 table below specify the de-
       fault 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

       Stop   Default action is to stop the process.

       Cont   Default  action  is  to  continue the process if it is currently

       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

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

   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

       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  un-
       til 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

   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  exe-
       cution 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 un-
         til 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 infor-
         mation 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 sig-

   Signal mask and pending signals
       A signal may be blocked, which means that it will not be delivered  un-
       til  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 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-di-
       rected 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  ex-
       ception  (e.g.,  SIGSEGV  for an invalid memory access, or SIGFPE for a
       math error), or because it was targeted at a specific thread using  in-
       terfaces 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 arbi-
       trary 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,  SIGX-
       CPU,  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 ter-
       minate 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  in-
       stance  of  the signal is marked as pending (and the signal will be de-
       livered 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 in-
       formation 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 dif-
       ferent 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 corre-
       sponding 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  origi-
       nally 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 imple-
       mentation 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 in-
       ternally uses two (for NPTL) or three (for LinuxThreads) real-time sig-
       nals  (see pthreads(7)), and adjusts the value of SIGRTMIN suitably (to
       34 or 35).  Because the range of available real-time signals varies ac-
       cording  to  the glibc threading implementation (and this variation can
       occur at run time according to the available kernel and glibc), and in-
       deed  the  range  of real-time signals varies across UNIX systems, pro-
       grams 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

       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

       2.  If the signal is sent using sigqueue(3), an accompanying value (ei-
           ther 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 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.

       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;

       * 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 defini-
         tion; 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;  before-
         hand, 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), sig-
         timedwait(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 semtime-

       * 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  han-
       dler,  but  gives  a success return: the number of seconds remaining to

   Interruption of system calls and library functions by stop signals
       On Linux, even in the absence of signal handlers, certain blocking  in-
       terfaces  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),

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

       * Linux 2.4 and earlier: nanosleep(2).

       POSIX.1, except as noted.

       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.

       There are six signals that can be delivered as a consequence of a hard-
       ware 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 archi-
       tecture, or vice versa.

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

       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), sig-
       pending(2),  sigprocmask(2),  sigreturn(2),   sigsuspend(2),   sigwait-
       info(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)

       This page is part of release 5.07 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

Linux                             2020-04-11                         SIGNAL(7)

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