signal(7) — Linux manual page


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

NAME         top

       signal - overview of signals

DESCRIPTION         top

       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

       Stop   Default action is to stop the process.

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

       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

       By default, a 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

       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 process-directed or thread-directed.  A process-
       directed signal is one that is targeted at (and thus pending for) the
       process as a whole.  A signal may be process-directed because it was
       generated by the kernel for reasons other than a hardware exception,
       or because it was sent using kill(2) or sigqueue(3).  A thread-
       directed signal is one that is targeted at a specific thread.  A
       signal may be thread-directed because it was generated as a
       consequence of executing a specific machine-language instruction that
       triggered a hardware exception (e.g., SIGSEGV for an invalid memory
       access, or SIGFPE for a math error), or because it was targeted at a
       specific thread using interfaces such as tgkill(2) or

       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

       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.

   Queueing and delivery semantics for standard signals
       If multiple standard signals are pending for a process, the order in
       which the signals are delivered is unspecified.

       Standard signals do not queue.  If multiple instances of a standard
       signal are generated while that signal is blocked, then only one
       instance of the signal is marked as pending (and the signal will be
       delivered just once when it is unblocked).  In the case where a
       standard signal is already pending, the siginfo_t structure (see
       sigaction(2)) associated with that signal is not overwritten on
       arrival of subsequent instances of the same signal.  Thus, the
       process will receive the information associated with the first
       instance of the signal.

   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

       * 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

       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

       * 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),

       * Linux 2.6.8 and earlier: msgrcv(2), msgsnd(2).

       * Linux 2.4 and earlier: nanosleep(2).

CONFORMING TO         top

       POSIX.1, except as noted.

NOTES         top

       For a discussion of async-signal-safe functions, see

       The /proc/[pid]/task/[tid]/status file contains various fields that
       show the signals that a thread is blocking (SigBlk), catching
       (SigCgt), or ignoring (SigIgn).  (The set of signals that are caught
       or ignored will be the same across all threads in a process.)  Other
       fields show the set of pending signals that are directed to the
       thread (SigPnd) as well as the set of pending signals that are
       directed to the process as a whole (ShdPnd).  The corresponding
       fields in /proc/[pid]/status show the information for the main
       thread.  See proc(5) for further details.

BUGS         top

       There are six signals that can be delivered as a consequence of a
       hardware exception: SIGBUS, SIGEMT, SIGFPE, SIGILL, SIGSEGV, and
       SIGTRAP.  Which of these signals is delivered, for any given hardware
       exception, is not documented and does not always make sense.

       For example, an invalid memory access that causes delivery of SIGSEGV
       on one CPU architecture may cause delivery of SIGBUS on another
       architecture, or vice versa.

       For another example, using the x86 int instruction with a forbidden
       argument (any number other than 3 or 128) causes delivery of SIGSEGV,
       even though SIGILL would make more sense, because of how the CPU
       reports the forbidden operation to the kernel.

SEE ALSO         top

       kill(1), clone(2), getrlimit(2), kill(2), pidfd_send_signal(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         top

       This page is part of release 5.09 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
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Linux                            2020-08-13                        SIGNAL(7)

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