signal(7)



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

NAME
       signal - overview of signals

DESCRIPTION
       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 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 sigalt-
       stack(2)  for  a discussion of how to do this and when it might be use-
       ful.)

       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 dis-
       positions.   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  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
         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 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-
         nal.

   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  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 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 exe-
       cuting 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 num-
       bers 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,  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.

       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 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.

       Where defined, SIGUNUSED is synonymous with SIGSYS  on  most  architec-
       tures.   Since glibc 2.26, SIGUNUSED is no longer defined on any archi-
       tecture.

   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
       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, 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
       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  con-
           trast,  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 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;
         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 will be automatically restarted after
       the signal handler returns if the SA_RESTART flag was  used;  otherwise
       the call will fail 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-
         dop(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  han-
       dler,  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
       This page is part of release 4.13 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/.

Linux                             2017-09-15                         SIGNAL(7)

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