Man Pages

signal(7) - phpMan signal(7) - phpMan

Command: man perldoc info search(apropos)  


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 sig-
       nal.

       The entries in the "Action" column of the tables below specify the default disposition for each signal, as fol-
       lows:

       Term   Default action is to terminate the process.

       Ign    Default action is to ignore the signal.

       Core   Default action is to terminate the process and dump core (see core(5)).

       Stop   Default action is to stop the process.

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

       A  process can change the disposition of a signal using sigaction(2) or (less portably) signal(2).  Using these
       system calls, a process can elect one of the 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 useful.)

       The signal disposition is a per-process attribute: in a multithreaded application, the disposition of a partic-
       ular 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 dis-
       positions 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(2)     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  informa-
       tion 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 sin-
       gle-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  con-
       sist 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 indi-
       cated in the "Value" column.  (Where three values are given, the first one  is  usually  valid  for  alpha  and
       sparc,  the  middle  one  for ix86, ia64, ppc, s390, arm and sh, and the last one for mips.  A - denotes that a
       signal is absent on the corresponding architecture.)

       First the signals described in the original POSIX.1-1990 standard.

       Signal     Value     Action   Comment
       ----------------------------------------------------------------------
       SIGHUP        1       Term    Hangup detected on controlling terminal
                                     or death of controlling process
       SIGINT        2       Term    Interrupt from keyboard
       SIGQUIT       3       Core    Quit from keyboard
       SIGILL        4       Core    Illegal Instruction
       SIGABRT       6       Core    Abort signal from abort(3)
       SIGFPE        8       Core    Floating point exception
       SIGKILL       9       Term    Kill signal
       SIGSEGV      11       Core    Invalid memory reference
       SIGPIPE      13       Term    Broken pipe: write to pipe with no
                                     readers
       SIGALRM      14       Term    Timer signal from alarm(2)
       SIGTERM      15       Term    Termination signal
       SIGUSR1   30,10,16    Term    User-defined signal 1
       SIGUSR2   31,12,17    Term    User-defined signal 2
       SIGCHLD   20,17,18    Ign     Child stopped or terminated
       SIGCONT   19,18,25    Cont    Continue if stopped

       SIGSTOP   17,19,23    Stop    Stop process
       SIGTSTP   18,20,24    Stop    Stop typed at tty
       SIGTTIN   21,21,26    Stop    tty input for background process
       SIGTTOU   22,22,27    Stop    tty output for background process

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

       Next the signals not in the POSIX.1-1990 standard but described in SUSv2 and POSIX.1-2001.

       Signal       Value     Action   Comment
       --------------------------------------------------------------------
       SIGBUS      10,7,10     Core    Bus error (bad memory access)
       SIGPOLL                 Term    Pollable event (Sys V).
                                       Synonym for SIGIO
       SIGPROF     27,27,29    Term    Profiling timer expired
       SIGSYS      12,-,12     Core    Bad argument to routine (SVr4)
       SIGTRAP        5        Core    Trace/breakpoint trap
       SIGURG      16,23,21    Ign     Urgent condition on socket (4.2BSD)
       SIGVTALRM   26,26,28    Term    Virtual alarm clock (4.2BSD)
       SIGXCPU     24,24,30    Core    CPU time limit exceeded (4.2BSD)
       SIGXFSZ     25,25,31    Core    File size limit exceeded (4.2BSD)

       Up to and including Linux 2.2, the default behavior for SIGSYS, SIGXCPU, SIGXFSZ, and (on  architectures  other
       than  SPARC  and  MIPS) SIGBUS was to terminate the 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  con-
       forms 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
       SIGWINCH    28,28,20    Ign     Window resize signal (4.3BSD, Sun)
       SIGUNUSED    -,31,-     Term    Unused signal (will be SIGSYS)

       (Signal 29 is SIGINFO / SIGPWR on an alpha but SIGLOST on a sparc.)

       SIGEMT is not specified in POSIX.1-2001, but nevertheless appears on most other Unix systems, where its default
       action is typically to terminate the process with a core dump.

       SIGPWR (which is not specified in POSIX.1-2001) is typically ignored by default on  those  other  Unix  systems
       where it appears.

       SIGIO (which is not specified in POSIX.1-2001) is ignored by default on several other Unix systems.

   Real-time Signals
       Linux  supports  real-time signals as originally defined in the POSIX.1b real-time extensions (and now included
       in POSIX.1-2001).  The range of supported real-time signals is defined by the  macros  SIGRTMIN  and  SIGRTMAX.
       POSIX.1-2001 requires that an implementation support at least _POSIX_RTSIG_MAX (8) real-time signals.

       The  Linux  kernel  supports  a range of 32 different real-time signals, numbered 33 to 64.  However, the glibc
       POSIX threads implementation internally uses two (for NPTL) or three (for LinuxThreads) real-time signals  (see
       pthreads(7)),  and  adjusts the value of SIGRTMIN suitably (to 34 or 35).  Because the range of available real-
       time signals varies according to the glibc threading implementation (and this variation can occur at  run  time
       according to the available kernel and glibc), and indeed the range of real-time signals varies across Unix sys-
       tems, 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 SIGRT-
       MIN+n does not exceed SIGRTMAX.

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

       The default action for an unhandled real-time signal is to terminate the receiving process.

       Real-time signals are distinguished by the following:

       1.  Multiple instances of real-time signals can be queued.  By contrast, if 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(2), 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 deliv-
       ered 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/ker-
       nel/rtsig-nr, can be used to find out how many real-time signals are currently queued.  In Linux  2.6.8,  these
       /proc  interfaces  were  replaced by the RLIMIT_SIGPENDING resource limit, which specifies a per-user limit for
       queued signals; see setrlimit(2) for further details.

   Async-signal-safe functions
       A signal handling routine established by sigaction(2) or signal(2) must be very careful, since processing else-
       where  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 func-
       tion, 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  func-
       tions:

           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 estab-
       lished 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 trans-
             ferred 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),  recvmsg(2), send(2), sendto(2), and
             sendmsg(2), unless a timeout has been set on the socket (see below).

           * File locking interfaces: flock(2) and fcntl(2) F_SETLKW.

           * POSIX message queue interfaces: mq_receive(3), mq_timedreceive(3), mq_send(3), and mq_timedsend(3).

           * futex(2) FUTEX_WAIT (since Linux 2.6.22; beforehand, always failed with EINTR).

           * POSIX semaphore interfaces: sem_wait(3) and sem_timedwait(3)  (since  Linux  2.6.22;  beforehand,  always
             failed with EINTR).

       The following interfaces are never restarted after being interrupted by a signal handler, regardless of the use
       of SA_RESTART; they always fail with the error EINTR when interrupted by a signal handler:

           * Socket interfaces, when a timeout has been set on the socket  using  setsockopt(2):  accept(2),  recv(2),
             recvfrom(2),  and  recvmsg(2),  if  a  receive  timeout  (SO_RCVTIMEO) has been set; connect(2), send(2),
             sendto(2), and sendmsg(2), if a send timeout (SO_SNDTIMEO) has been set.

           * Interfaces used to wait for signals: pause(2), sigsuspend(2), 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 num-
       ber of seconds remaining to sleep.

   Interruption of System Calls and Library Functions by Stop Signals
       On Linux, even in the absence of signal handlers, certain blocking interfaces can fail  with  the  error  EINTR
       after  the  process  is  stopped by one of the stop signals and then resumed via SIGCONT.  This behavior is not
       sanctioned by POSIX.1, and doesn't occur on other systems.

       The Linux interfaces that display this behavior are:

           * Socket interfaces, when a timeout has been set on the socket  using  setsockopt(2):  accept(2),  recv(2),
             recvfrom(2),  and  recvmsg(2),  if  a  receive  timeout  (SO_RCVTIMEO) has been set; connect(2), send(2),
             sendto(2), and sendmsg(2), if a send timeout (SO_SNDTIMEO) has been set.

           * epoll_wait(2), epoll_pwait(2).

           * semop(2), semtimedop(2).

           * sigtimedwait(2), sigwaitinfo(2).

           * read(2) from an inotify(7) file descriptor.

           * Linux 2.6.21 and earlier: futex(2) FUTEX_WAIT, sem_timedwait(3), sem_wait(3).

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

           * Linux 2.4 and earlier: nanosleep(2).

CONFORMING TO
       POSIX.1, except as noted.

BUGS
       SIGIO and SIGLOST have the same value.  The latter is commented out in the kernel source, but the build process
       of some software still thinks that signal 29 is SIGLOST.

SEE ALSO
       kill(1),  getrlimit(2),  kill(2),  killpg(2),  setitimer(2),  setrlimit(2),  sgetmask(2), sigaction(2), sigalt-
       stack(2), signal(2), signalfd(2), sigpending(2), sigprocmask(2),  sigqueue(2),  sigsuspend(2),  sigwaitinfo(2),
       abort(3),  bsd_signal(3),  longjmp(3),  raise(3), sigset(3), sigsetops(3), sigvec(3), sigwait(3), strsignal(3),
       sysv_signal(3), core(5), proc(5), pthreads(7)

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



Linux                             2008-10-15                         SIGNAL(7)