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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 disposi-
       tion 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  (less
       portably) signal(2).  Using these system calls, a process can elect one of the fol-
       lowing 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 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 speci-
                       fied 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(2)     Sends a real-time signal with accompanying data to a specified pro-
                       cess.

   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 execu-
                       tion until one of the unmasked signals is caught.

   Synchronously Accepting a Signal
       Rather than asynchronously catching a signal via a signal handler, it  is  possible
       to synchronously accept the signal, that is, to block execution until the signal is
       delivered, at which point the kernel returns information about the  signal  to  the
       caller.  There are two general ways to do this:

       * sigwaitinfo(2),  sigtimedwait(2),  and  sigwait(3) suspend execution until one of
         the signals in a specified set is delivered.  Each of these calls returns  infor-
         mation about the delivered signal.

       * signalfd(2)  returns a file descriptor that can be used to read information about
         signals that are delivered to the caller.  Each read(2) from this file descriptor
         blocks  until  one of the signals in the set specified in the signalfd(2) call is
         delivered to the caller.  The buffer returned by  read(2)  contains  a  structure
         describing the signal.

   Signal Mask and Pending Signals
       A  signal  may  be  blocked,  which means that it will not be delivered until it is
       later unblocked.  Between the time when it is generated and when it is delivered  a
       signal is said to be pending.

       Each thread in a process has an independent signal mask, which indicates the set of
       signals that the thread is currently blocking.  A thread can manipulate its  signal
       mask  using pthread_sigmask(3).  In a traditional single-threaded application, sig-
       procmask(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-lan-
       guage 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 sigpend-
       ing(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
       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
       ix86, ia64, ppc, s390, arm and sh, and the last one for mips.  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 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,-,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 pro-
       cess (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  pro-
       cess 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
       SIGWINCH    28,28,20    Ign     Window resize signal (4.3BSD, Sun)
       SIGUNUSED    -,31,-     Term    Unused signal (will be 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 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.

   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, programs should never  refer
       to  real-time  signals using hard-coded numbers, but instead should always refer to
       real-time signals using the notation SIGRTMIN+n, and  include  suitable  (run-time)
       checks that SIGRTMIN+n does not exceed SIGRTMAX.

       Unlike  standard signals, real-time signals have no predefined meanings: the entire
       set of real-time signals can be used for application-defined purposes.  (Note, how-
       ever, that the LinuxThreads implementation uses the first three real-time signals.)

       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 multi-
           ple 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(2), an accompanying value (either an inte-
           ger or a pointer) can be sent with the signal.  If the receiving process estab-
           lishes 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 sig-
           nals 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 dif-
       ferently.  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.

   Async-signal-safe functions
       A signal handling routine established by sigaction(2) or  signal(2)  must  be  very
       careful,  since  processing elsewhere may be interrupted at some arbitrary point in
       the execution of the program.  POSIX has the concept of "safe function".  If a sig-
       nal  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()
           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()

   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.  (A disk is not a slow device
             according to this definition.)  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  trans-
             ferred).

           * 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_timedreceive(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  setsock-
             opt(2): accept(2), recv(2), recvfrom(2), and recvmsg(2), if a receive timeout
             (SO_RCVTIMEO) has been set; connect(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), sigtimed-
             wait(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:

           * Socket  interfaces,  when a timeout has been set on the socket using setsock-
             opt(2): accept(2), recv(2), recvfrom(2), and recvmsg(2), if a receive timeout
             (SO_RCVTIMEO)  has  been set; 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).

           * 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), setitimer(2), setrlimit(2), sgetmask(2),
       sigaction(2),  sigaltstack(2),  signal(2),  signalfd(2),  sigpending(2),   sigproc-
       mask(2),   sigqueue(2),  sigsuspend(2),  sigwaitinfo(2),  abort(3),  bsd_signal(3),
       longjmp(3), raise(3), sigset(3), sigsetops(3), sigvec(3), sigwait(3), strsignal(3),
       sysv_signal(3), core(5), proc(5), pthreads(7)

COLOPHON
       This page is part of release 3.22 of the Linux man-pages project.  A description of
       the project, and information about reporting bugs, can be found at  http://www.ker-
       nel.org/doc/man-pages/.



Linux                             2008-10-15                         SIGNAL(7)

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