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

NAME
       signal - overview of signals

DESCRIPTION
       Linux  supports  both  POSIX  reliable  signals (hereinafter "standard signals") and POSIX
       real-time signals.

   Signal dispositions
       Each signal has a current disposition, which determines how the process behaves when it is
       delivered the signal.

       The entries in the "Action" column of the tables below specify the default disposition for
       each signal, as follows:

       Term   Default action is to terminate the process.

       Ign    Default action is to ignore the signal.

       Core   Default action is to terminate the process and dump core (see core(5)).

       Stop   Default action is to stop the process.

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

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

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

       A child created via fork(2) inherits a copy of its parent's signal  dispositions.   During
       an  execve(2),  the dispositions of handled signals are reset to the default; the disposi‐
       tions 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(2)       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 sig‐
       nal 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 syn‐
       chronously 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  sig‐
         nals 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 sig‐
       nals 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 cur‐
       rently have the signal blocked.  If more than one of the threads has the signal unblocked,
       then the kernel chooses an arbitrary thread to which to deliver the signal.

       A  thread can obtain the set of signals that it currently has pending using sigpending(2).
       This set will consist of the union of the set of pending process-directed signals and  the
       set of signals pending for the calling thread.

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

   Standard signals
       Linux supports the standard signals listed below.  Several signal  numbers  are  architec‐
       ture-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 - 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
       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 argument to routine (SVr4)
       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)
       SIGXFSZ     25,25,31    Core    File size limit exceeded (4.2BSD)

       Up  to and including Linux 2.2, the default behavior for SIGSYS, SIGXCPU, SIGXFSZ, and (on
       architectures other than SPARC and MIPS) SIGBUS was to terminate the  process  (without  a
       core  dump).  (On some other UNIX systems the default action for SIGXCPU and SIGXFSZ is to
       terminate the process without a core  dump.)   Linux  2.4  conforms  to  the  POSIX.1-2001
       requirements for these signals, terminating the process with a core dump.

       Next various other signals.

       Signal       Value     Action   Comment
       ────────────────────────────────────────────────────────────────────
       SIGIOT         6        Core    IOT trap. A synonym for SIGABRT
       SIGEMT       7,-,7      Term
       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 sys‐
       tems, where its default action is typically to terminate the process with a core dump.

       SIGPWR (which is not specified in POSIX.1-2001) is typically ignored by default  on  those
       other UNIX systems where it appears.

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

       Where defined, SIGUNUSED is synonymous with SIGSYS on most architectures.

   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  SIGRT‐
       MIN+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 sig‐
           nals are sent to a process, they are delivered starting with the lowest-numbered  sig‐
           nal.   (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 unspeci‐
       fied 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 or 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)

   Async-signal-safe functions
       A  signal  handler function must be very careful, since processing elsewhere may be inter‐
       rupted at some arbitrary point in the execution of the program.  POSIX has the concept  of
       "safe  function".  If a signal interrupts the execution of an unsafe function, and handler
       calls an unsafe function, then the behavior of the program is undefined.

       POSIX.1-2004 (also known as POSIX.1-2001 Technical Corrigendum 2) requires an  implementa‐
       tion  to  guarantee that the following functions can be safely called inside a signal han‐
       dler:

           _Exit()
           _exit()
           abort()
           accept()
           access()
           aio_error()
           aio_return()
           aio_suspend()
           alarm()
           bind()
           cfgetispeed()
           cfgetospeed()
           cfsetispeed()
           cfsetospeed()
           chdir()
           chmod()
           chown()
           clock_gettime()
           close()
           connect()
           creat()
           dup()
           dup2()
           execle()
           execve()
           fchmod()
           fchown()
           fcntl()
           fdatasync()
           fork()
           fpathconf()
           fstat()
           fsync()
           ftruncate()
           getegid()
           geteuid()
           getgid()
           getgroups()
           getpeername()
           getpgrp()
           getpid()
           getppid()
           getsockname()
           getsockopt()
           getuid()
           kill()
           link()
           listen()
           lseek()
           lstat()
           mkdir()
           mkfifo()
           open()
           pathconf()
           pause()
           pipe()
           poll()
           posix_trace_event()
           pselect()
           raise()
           read()
           readlink()
           recv()
           recvfrom()
           recvmsg()
           rename()
           rmdir()
           select()
           sem_post()
           send()
           sendmsg()
           sendto()
           setgid()
           setpgid()
           setsid()
           setsockopt()
           setuid()
           shutdown()
           sigaction()
           sigaddset()
           sigdelset()
           sigemptyset()
           sigfillset()
           sigismember()
           signal()
           sigpause()
           sigpending()
           sigprocmask()
           sigqueue()
           sigset()
           sigsuspend()
           sleep()
           sockatmark()
           socket()
           socketpair()
           stat()
           symlink()
           sysconf()
           tcdrain()
           tcflow()
           tcflush()
           tcgetattr()
           tcgetpgrp()
           tcsendbreak()
           tcsetattr()
           tcsetpgrp()
           time()
           timer_getoverrun()
           timer_gettime()
           timer_settime()
           times()
           umask()
           uname()
           unlink()
           utime()
           wait()
           waitpid()
           write()

       POSIX.1-2008 removes fpathconf(), pathconf(), and sysconf() from the above list, and  adds
       the following functions:

           execl()
           execv()
           faccessat()
           fchmodat()
           fchownat()
           fexecve()
           fstatat()
           futimens()
           linkat()
           mkdirat()
           mkfifoat()
           mknod()
           mknodat()
           openat()
           readlinkat()
           renameat()
           symlinkat()
           unlinkat()
           utimensat()
           utimes()

       POSIX.1-2008 Technical Corrigendum 1 (2013) adds the following functions:

           fchdir()
           pthread_kill()
           pthread_self()
           pthread_sigmask()

   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 exam‐
             ple, 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 opera‐
             tions 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).

       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  inter‐
       rupted 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).

           * read(2) from an inotify(7) file descriptor.

           * 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 time‐
             out (SO_SNDTIMEO) has been set.

           * epoll_wait(2), epoll_pwait(2).

           * semop(2), semtimedop(2).

           * sigtimedwait(2), sigwaitinfo(2).

           * 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.

SEE ALSO
       kill(1),  getrlimit(2),  kill(2),   killpg(2),   restart_syscall(2),   rt_sigqueueinfo(2),
       setitimer(2),  setrlimit(2),  sgetmask(2),  sigaction(2),  sigaltstack(2), signal(2), sig‐
       nalfd(2), sigpending(2), sigprocmask(2), sigsuspend(2), sigwaitinfo(2), abort(3), bsd_sig‐
       nal(3),  longjmp(3),  raise(3), pthread_sigqueue(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 4.04 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 http://www.kernel.org/doc/man-pages/.

Linux                                       2015-12-05                                  SIGNAL(7)

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