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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 (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 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(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 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 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 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       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 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. (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 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 handling routine established by sigaction(2) or signal(2) 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),             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 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:           * 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 top

       POSIX.1, except as noted.

BUGS top

       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.

COLOPHON top

       This page is part of release 3.21 of the Linux man-pages project.  A       description of the project, and information about reporting bugs, can be found       at http://www.kernel.org/doc/man-pages/.Linux                                 2008-10-15                            SIGNAL(7)