signal


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.
                       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.
       (Where three values are given, the first one is usually valid for alpha
       and sparc, the middle one for ix86, ia64, ppc, s390, arm  and  sh,  and
       the last one for mips.  A - denotes that a signal is absent on the cor-
       responding 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 tty
       SIGTTIN   21,21,26    Stop    tty input for background process
       SIGTTOU   22,22,27    Stop    tty 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, 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
       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
       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.

   Real-time Signals
       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 32  different  real-time  signals,
       numbered  33  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
           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.

   Async-signal-safe functions
       A signal handler function must be very careful, since processing  else-
       where  may  be  interrupted 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  implementation  to guarantee that the following functions
       can be safely called inside a signal handler:

           _Exit()
           _exit()
           abort()
           accept()
           access()
           aio_error()
           aio_return()
           aio_suspend()
           alarm()
           bind()
           cfgetispeed()
           cfgetospeed()
           cfsetispeed()
           cfsetospeed()
           chdir()
           chmod()
           chown()
           clock_gettime()
           close()
           connect()
           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()
           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()

   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.

             block  for  an indefinite time, for example, a terminal, pipe, or
             socket.  (A disk is not a slow device according to  this  defini-
             tion.)   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).

           * 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),
             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 fcntl(2) F_SETLKW.

           * POSIX   message   queue   interfaces:   mq_receive(3),   mq_time-
             dreceive(3), mq_send(3), and mq_timedsend(3).

           * futex(2)  FUTEX_WAIT  (since  Linux  2.6.22;  beforehand,  always
             failed with EINTR).

           * 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 interrupted by a signal handler:

           * Socket interfaces, when a timeout has  been  set  on  the  socket
             using   setsockopt(2):   accept(2),   recv(2),  recvfrom(2),  and
             recvmsg(2), if a receive timeout (SO_RCVTIMEO) has been set; con-
             nect(2),  send(2),  sendto(2),  and sendmsg(2), if a send timeout
             (SO_SNDTIMEO) has been set.

           * 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 sem-
             timedop(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  han-
             using  setsockopt(2):  accept(2),   recv(2),   recvfrom(2),   and
             recvmsg(2), if a receive timeout (SO_RCVTIMEO) has been set; con-
             nect(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).

           * 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.

BUGS
       SIGIO and SIGLOST have the same value.  The latter is commented out  in
       the  kernel source, but the build process of some software still thinks
       that signal 29 is SIGLOST.

SEE ALSO
       kill(1),   getrlimit(2),   kill(2),   killpg(2),    rt_sigqueueinfo(2),
       setitimer(2),  setrlimit(2), sgetmask(2), sigaction(2), sigaltstack(2),
       signal(2), signalfd(2), sigpending(2),  sigprocmask(2),  sigsuspend(2),
       sigwaitinfo(2),    abort(3),   bsd_signal(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),
       pthreads(7), sigevent(7)

COLOPHON
       This page is part of release 3.35 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://man7.org/linux/man-pages/.



Linux                             2011-09-18                         SIGNAL(7)
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