signal

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 table 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 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 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
       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 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 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
       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 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.  The second column of
       the table indicates which standard (if any) specified the signal:
       "P1990" indicates that the signal is described in the original
       POSIX.1-1990 standard; "P2001" indicates that the signal was added in
       SUSv2 and POSIX.1-2001.

       Signal      Standard   Action   Comment
       ────────────────────────────────────────────────────────────────────────
       SIGABRT      P1990      Core    Abort signal from abort(3)
       SIGALRM      P1990      Term    Timer signal from alarm(2)
       SIGBUS       P2001      Core    Bus error (bad memory access)
       SIGCHLD      P1990      Ign     Child stopped or terminated
       SIGCLD         -        Ign     A synonym for SIGCHLD
       SIGCONT      P1990      Cont    Continue if stopped
       SIGEMT         -        Term    Emulator trap
       SIGFPE       P1990      Core    Floating-point exception
       SIGHUP       P1990      Term    Hangup detected on controlling terminal
                                       or death of controlling process
       SIGILL       P1990      Core    Illegal Instruction
       SIGINFO        -                A synonym for SIGPWR
       SIGINT       P1990      Term    Interrupt from keyboard
       SIGIO          -        Term    I/O now possible (4.2BSD)
       SIGIOT         -        Core    IOT trap. A synonym for SIGABRT
       SIGKILL      P1990      Term    Kill signal
       SIGLOST        -        Term    File lock lost (unused)
       SIGPIPE      P1990      Term    Broken pipe: write to pipe with no
                                       readers; see pipe(7)
       SIGPOLL      P2001      Term    Pollable event (Sys V).
                                       Synonym for SIGIO
       SIGPROF      P2001      Term    Profiling timer expired
       SIGPWR         -        Term    Power failure (System V)
       SIGQUIT      P1990      Core    Quit from keyboard
       SIGSEGV      P1990      Core    Invalid memory reference
       SIGSTKFLT      -        Term    Stack fault on coprocessor (unused)
       SIGSTOP      P1990      Stop    Stop process
       SIGTSTP      P1990      Stop    Stop typed at terminal
       SIGSYS       P2001      Core    Bad system call (SVr4);
                                       see also seccomp(2)
       SIGTERM      P1990      Term    Termination signal
       SIGTRAP      P2001      Core    Trace/breakpoint trap
       SIGTTIN      P1990      Stop    Terminal input for background process
       SIGTTOU      P1990      Stop    Terminal output for background process
       SIGUNUSED      -        Core    Synonymous with SIGSYS
       SIGURG       P2001      Ign     Urgent condition on socket (4.2BSD)
       SIGUSR1      P1990      Term    User-defined signal 1
       SIGUSR2      P1990      Term    User-defined signal 2
       SIGVTALRM    P2001      Term    Virtual alarm clock (4.2BSD)
       SIGXCPU      P2001      Core    CPU time limit exceeded (4.2BSD);
                                       see setrlimit(2)
       SIGXFSZ      P2001      Core    File size limit exceeded (4.2BSD);
                                       see setrlimit(2)
       SIGWINCH       -        Ign     Window resize signal (4.3BSD, Sun)

       The signals SIGKILL and SIGSTOP cannot be caught, blocked, or ignored.

       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.

       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.

   Signal numbering for standard signals
       The numeric value for each signal is given in the table below.  As
       shown in the table, many signals have different numeric values on
       different architectures.  The first numeric value in each table row
       shows the signal number on x86, ARM, and most other architectures; the
       second value is for Alpha and SPARC; the third is for MIPS; and the
       last is for PARISC.  A dash (-) denotes that a signal is absent on the
       corresponding architecture.

       Signal        x86/ARM     Alpha/   MIPS   PARISC   Notes
                   most others   SPARC
       ─────────────────────────────────────────────────────────────────
       SIGHUP           1           1       1       1
       SIGINT           2           2       2       2
       SIGQUIT          3           3       3       3
       SIGILL           4           4       4       4
       SIGTRAP          5           5       5       5
       SIGABRT          6           6       6       6
       SIGIOT           6           6       6       6
       SIGBUS           7          10      10      10
       SIGEMT           -           7       7      -
       SIGFPE           8           8       8       8
       SIGKILL          9           9       9       9
       SIGUSR1         10          30      16      16
       SIGSEGV         11          11      11      11
       SIGUSR2         12          31      17      17
       SIGPIPE         13          13      13      13
       SIGALRM         14          14      14      14
       SIGTERM         15          15      15      15
       SIGSTKFLT       16          -       -        7
       SIGCHLD         17          20      18      18
       SIGCLD           -          -       18      -
       SIGCONT         18          19      25      26
       SIGSTOP         19          17      23      24
       SIGTSTP         20          18      24      25
       SIGTTIN         21          21      26      27
       SIGTTOU         22          22      27      28
       SIGURG          23          16      21      29
       SIGXCPU         24          24      30      12
       SIGXFSZ         25          25      31      30
       SIGVTALRM       26          26      28      20
       SIGPROF         27          27      29      21
       SIGWINCH        28          28      20      23
       SIGIO           29          23      22      22
       SIGPOLL                                            Same as SIGIO
       SIGPWR          30         29/-     19      19
       SIGINFO          -         29/-     -       -
       SIGLOST          -         -/29     -       -
       SIGSYS          31          12      12      31
       SIGUNUSED       31          -       -       31

       Note the following:

       *  Where defined, SIGUNUSED is synonymous with SIGSYS.  Since glibc
          2.26, SIGUNUSED is no longer defined on any architecture.

       *  Signal 29 is SIGINFO/SIGPWR (synonyms for the same value) on Alpha
          but SIGLOST on SPARC.

   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 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
           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 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
       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 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
         definition; 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;
         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:

       * "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).

       * 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 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), clone(2), getrlimit(2), kill(2), restart_syscall(2),
       rt_sigqueueinfo(2), setitimer(2), setrlimit(2), sgetmask(2),
       sigaction(2), sigaltstack(2), signal(2), signalfd(2), sigpending(2),
       sigprocmask(2), sigreturn(2), sigsuspend(2), sigwaitinfo(2), abort(3),
       bsd_signal(3), killpg(3), longjmp(3), pthread_sigqueue(3), raise(3),
       sigqueue(3), sigset(3), sigsetops(3), sigvec(3), sigwait(3),
       strsignal(3), sysv_signal(3), core(5), proc(5), nptl(7), pthreads(7),
       sigevent(7)

COLOPHON
       This page is part of release 5.01 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.



Linux                             2019-03-06                         SIGNAL(7)