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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.
SEE ALSO top
kill(1), getrlimit(2), kill(2), killpg(2), setitimer(2), setrlimit(2), sgetmask(2), sigaction(2), sigaltstack(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 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)
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