NAME | DESCRIPTION | CONFORMING TO | SEE ALSO | COLOPHON

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 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 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(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(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
       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.  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 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 - 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 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 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 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
       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
       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.

       Where defined, SIGUNUSED is synonymous with SIGSYS on most
       architectures.

   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.

       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.

   Async-signal-safe functions
       A signal handler function 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
       signal interrupts the execution of an unsafe function, and handler
       calls an unsafe function, then the behavior of the program is
       undefined.

       POSIX.1-2004 (also known as POSIX.1-2001 Technical Corrigendum 2)
       requires an implementation to guarantee that the following functions
       can be safely called inside a signal handler:

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

           * open(2), if it can block (e.g., when opening a FIFO; see
             fifo(7)).

           * wait(2), wait3(2), wait4(2), waitid(2), and waitpid(2).

           * Socket interfaces: accept(2), connect(2), recv(2), recvfrom(2),
             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 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).

           * pthread_mutex_lock(3), pthread_cond_wait(3), and related APIs.

           * 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:

           * "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).

           * 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:

           * "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).

           * 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         top

       POSIX.1, except as noted.

SEE ALSO         top

       kill(1), getrlimit(2), kill(2), killpg(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), sigsuspend(2), sigwaitinfo(2), abort(3),
       bsd_signal(3), longjmp(3), raise(3), pthread_sigqueue(3),
       sigqueue(3), sigset(3), sigsetops(3), sigvec(3), sigwait(3),
       strsignal(3), sysv_signal(3), core(5), proc(5), pthreads(7),
       sigevent(7)

COLOPHON         top

       This page is part of release 3.75 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
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Linux                            2014-10-15                        SIGNAL(7)