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man:signal

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 is automatically restarted after the
     signal handler returns if the SA_RESTART flag was used;  otherwise  the
     call fails with the error EINTR:
  • read(2), readv(2), write(2), writev(2), and ioctl(2) calls on "slow"

devices. A "slow" device is one where the I/O call may block for an

       indefinite time, for example, a terminal, pipe, or socket.  If an I/O
       call on a slow device has already transferred some data by  the  time
       it  is  interrupted  by a signal handler, then the call will return a
       success status (normally, the number  of  bytes  transferred).   Note
       that  a  (local)  disk is not a slow device according to this defini-
       tion; I/O operations on disk devices are not interrupted by  signals.
  • open(2), if it can block (e.g., when opening a FIFO; see fifo(7)).
  • wait(2), wait3(2), wait4(2), waitid(2), and waitpid(2).
  • Socket interfaces: accept(2), connect(2), recv(2), recvfrom(2),

recvmmsg(2), recvmsg(2), send(2), sendto(2), and sendmsg(2), unless a

       timeout has been set on the socket (see below).
  • File locking interfaces: flock(2) and the F_SETLKW and F_OFD_SETLKW

operations of fcntl(2)

  • POSIX message queue interfaces: mq_receive(3), mq_timedreceive(3),

mq_send(3), and mq_timedsend(3).

  • futex(2) FUTEX_WAIT (since Linux 2.6.22; beforehand, always failed

with EINTR).

  • getrandom(2).
  • pthread_mutex_lock(3), pthread_cond_wait(3), and related APIs.
  • futex(2) FUTEX_WAIT_BITSET.
  • POSIX semaphore interfaces: sem_wait(3) and sem_timedwait(3) (since

Linux 2.6.22; beforehand, always failed with EINTR).

  • read(2) from an inotify(7) file descriptor (since Linux 3.8; before-

hand, always failed with EINTR).

     The following interfaces are never restarted after being interrupted by
     a signal handler, regardless of the use of SA_RESTART; they always fail
     with the error EINTR when interrupted by a signal handler:
  • "Input" socket interfaces, when a timeout (SO_RCVTIMEO) has been set

on the socket using setsockopt(2): accept(2), recv(2), recvfrom(2),

       recvmmsg(2) (also with a non-NULL timeout argument), and  recvmsg(2).
  • "Output" socket interfaces, when a timeout (SO_RCVTIMEO) has been set

on the socket using setsockopt(2): connect(2), send(2), sendto(2),

       and sendmsg(2).
  • Interfaces used to wait for signals: pause(2), sigsuspend(2), sig-

timedwait(2), and sigwaitinfo(2).

  • File descriptor multiplexing interfaces: epoll_wait(2),

epoll_pwait(2), poll(2), ppoll(2), select(2), and pselect(2).

  • System V IPC interfaces: msgrcv(2), msgsnd(2), semop(2), and semtime-

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

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     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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