go/src/os/signal/doc.go

// Copyright 2015 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

/*
Package signal implements access to incoming signals.

Signals are primarily used on Unix-like systems. For the use of this
package on Windows and Plan 9, see below.

# Types of signals

The signals SIGKILL and SIGSTOP may not be caught by a program, and
therefore cannot be affected by this package.

Synchronous signals are signals triggered by errors in program
execution: SIGBUS, SIGFPE, and SIGSEGV. These are only considered
synchronous when caused by program execution, not when sent using
[os.Process.Kill] or the kill program or some similar mechanism. In
general, except as discussed below, Go programs will convert a
synchronous signal into a run-time panic.

The remaining signals are asynchronous signals. They are not
triggered by program errors, but are instead sent from the kernel or
from some other program.

Of the asynchronous signals, the SIGHUP signal is sent when a program
loses its controlling terminal. The SIGINT signal is sent when the
user at the controlling terminal presses the interrupt character,
which by default is ^C (Control-C). The SIGQUIT signal is sent when
the user at the controlling terminal presses the quit character, which
by default is ^\ (Control-Backslash). In general you can cause a
program to simply exit by pressing ^C, and you can cause it to exit
with a stack dump by pressing ^\.

# Default behavior of signals in Go programs

By default, a synchronous signal is converted into a run-time panic. A
SIGHUP, SIGINT, or SIGTERM signal causes the program to exit. A
SIGQUIT, SIGILL, SIGTRAP, SIGABRT, SIGSTKFLT, SIGEMT, or SIGSYS signal
causes the program to exit with a stack dump. A SIGTSTP, SIGTTIN, or
SIGTTOU signal gets the system default behavior (these signals are
used by the shell for job control). The SIGPROF signal is handled
directly by the Go runtime to implement runtime.CPUProfile. Other
signals will be caught but no action will be taken.

If the Go program is started with either SIGHUP or SIGINT ignored
(signal handler set to SIG_IGN), they will remain ignored.

If the Go program is started with a non-empty signal mask, that will
generally be honored. However, some signals are explicitly unblocked:
the synchronous signals, SIGILL, SIGTRAP, SIGSTKFLT, SIGCHLD, SIGPROF,
and, on Linux, signals 32 (SIGCANCEL) and 33 (SIGSETXID)
(SIGCANCEL and SIGSETXID are used internally by glibc). Subprocesses
started by [os.Exec], or by [os/exec], will inherit the
modified signal mask.

# Changing the behavior of signals in Go programs

The functions in this package allow a program to change the way Go
programs handle signals.

Notify disables the default behavior for a given set of asynchronous
signals and instead delivers them over one or more registered
channels. Specifically, it applies to the signals SIGHUP, SIGINT,
SIGQUIT, SIGABRT, and SIGTERM. It also applies to the job control
signals SIGTSTP, SIGTTIN, and SIGTTOU, in which case the system
default behavior does not occur. It also applies to some signals that
otherwise cause no action: SIGUSR1, SIGUSR2, SIGPIPE, SIGALRM,
SIGCHLD, SIGCONT, SIGURG, SIGXCPU, SIGXFSZ, SIGVTALRM, SIGWINCH,
SIGIO, SIGPWR, SIGINFO, SIGTHR, SIGWAITING, SIGLWP, SIGFREEZE,
SIGTHAW, SIGLOST, SIGXRES, SIGJVM1, SIGJVM2, and any real time signals
used on the system. Note that not all of these signals are available
on all systems.

If the program was started with SIGHUP or SIGINT ignored, and [Notify]
is called for either signal, a signal handler will be installed for
that signal and it will no longer be ignored. If, later, [Reset] or
[Ignore] is called for that signal, or [Stop] is called on all channels
passed to Notify for that signal, the signal will once again be
ignored. Reset will restore the system default behavior for the
signal, while Ignore will cause the system to ignore the signal
entirely.

If the program is started with a non-empty signal mask, some signals
will be explicitly unblocked as described above. If Notify is called
for a blocked signal, it will be unblocked. If, later, Reset is
called for that signal, or Stop is called on all channels passed to
Notify for that signal, the signal will once again be blocked.

# SIGPIPE

When a Go program writes to a broken pipe, the kernel will raise a
SIGPIPE signal.

If the program has not called Notify to receive SIGPIPE signals, then
the behavior depends on the file descriptor number. A write to a
broken pipe on file descriptors 1 or 2 (standard output or standard
error) will cause the program to exit with a SIGPIPE signal. A write
to a broken pipe on some other file descriptor will take no action on
the SIGPIPE signal, and the write will fail with a [syscall.EPIPE]
error.

If the program has called Notify to receive SIGPIPE signals, the file
descriptor number does not matter. The SIGPIPE signal will be
delivered to the Notify channel, and the write will fail with a
[syscall.EPIPE] error.

This means that, by default, command line programs will behave like
typical Unix command line programs, while other programs will not
crash with SIGPIPE when writing to a closed network connection.

# Go programs that use cgo or SWIG

In a Go program that includes non-Go code, typically C/C++ code
accessed using cgo or SWIG, Go's startup code normally runs first. It
configures the signal handlers as expected by the Go runtime, before
the non-Go startup code runs. If the non-Go startup code wishes to
install its own signal handlers, it must take certain steps to keep Go
working well. This section documents those steps and the overall
effect changes to signal handler settings by the non-Go code can have
on Go programs. In rare cases, the non-Go code may run before the Go
code, in which case the next section also applies.

If the non-Go code called by the Go program does not change any signal
handlers or masks, then the behavior is the same as for a pure Go
program.

If the non-Go code installs any signal handlers, it must use the
SA_ONSTACK flag with sigaction. Failing to do so is likely to cause
the program to crash if the signal is received. Go programs routinely
run with a limited stack, and therefore set up an alternate signal
stack.

If the non-Go code installs a signal handler for any of the
synchronous signals (SIGBUS, SIGFPE, SIGSEGV), then it should record
the existing Go signal handler. If those signals occur while
executing Go code, it should invoke the Go signal handler (whether the
signal occurs while executing Go code can be determined by looking at
the PC passed to the signal handler). Otherwise some Go run-time
panics will not occur as expected.

If the non-Go code installs a signal handler for any of the
asynchronous signals, it may invoke the Go signal handler or not as it
chooses. Naturally, if it does not invoke the Go signal handler, the
Go behavior described above will not occur. This can be an issue with
the SIGPROF signal in particular.

The non-Go code should not change the signal mask on any threads
created by the Go runtime. If the non-Go code starts new threads
itself, those threads may set the signal mask as they please.

If the non-Go code starts a new thread, changes the signal mask, and
then invokes a Go function in that thread, the Go runtime will
automatically unblock certain signals: the synchronous signals,
SIGILL, SIGTRAP, SIGSTKFLT, SIGCHLD, SIGPROF, SIGCANCEL, and
SIGSETXID. When the Go function returns, the non-Go signal mask will
be restored.

If the Go signal handler is invoked on a non-Go thread not running Go
code, the handler generally forwards the signal to the non-Go code, as
follows. If the signal is SIGPROF, the Go handler does
nothing. Otherwise, the Go handler removes itself, unblocks the
signal, and raises it again, to invoke any non-Go handler or default
system handler. If the program does not exit, the Go handler then
reinstalls itself and continues execution of the program.

If a SIGPIPE signal is received, the Go program will invoke the
special handling described above if the SIGPIPE is received on a Go
thread.  If the SIGPIPE is received on a non-Go thread the signal will
be forwarded to the non-Go handler, if any; if there is none the
default system handler will cause the program to terminate.

# Non-Go programs that call Go code

When Go code is built with options like -buildmode=c-shared, it will
be run as part of an existing non-Go program. The non-Go code may
have already installed signal handlers when the Go code starts (that
may also happen in unusual cases when using cgo or SWIG; in that case,
the discussion here applies).  For -buildmode=c-archive the Go runtime
will initialize signals at global constructor time.  For
-buildmode=c-shared the Go runtime will initialize signals when the
shared library is loaded.

If the Go runtime sees an existing signal handler for the SIGCANCEL or
SIGSETXID signals (which are used only on Linux), it will turn on
the SA_ONSTACK flag and otherwise keep the signal handler.

For the synchronous signals and SIGPIPE, the Go runtime will install a
signal handler. It will save any existing signal handler. If a
synchronous signal arrives while executing non-Go code, the Go runtime
will invoke the existing signal handler instead of the Go signal
handler.

Go code built with -buildmode=c-archive or -buildmode=c-shared will
not install any other signal handlers by default. If there is an
existing signal handler, the Go runtime will turn on the SA_ONSTACK
flag and otherwise keep the signal handler. If Notify is called for an
asynchronous signal, a Go signal handler will be installed for that
signal. If, later, Reset is called for that signal, the original
handling for that signal will be reinstalled, restoring the non-Go
signal handler if any.

Go code built without -buildmode=c-archive or -buildmode=c-shared will
install a signal handler for the asynchronous signals listed above,
and save any existing signal handler. If a signal is delivered to a
non-Go thread, it will act as described above, except that if there is
an existing non-Go signal handler, that handler will be installed
before raising the signal.

# Windows

On Windows a ^C (Control-C) or ^BREAK (Control-Break) normally cause
the program to exit. If Notify is called for [os.Interrupt], ^C or ^BREAK
will cause [os.Interrupt] to be sent on the channel, and the program will
not exit. If Reset is called, or Stop is called on all channels passed
to Notify, then the default behavior will be restored.

Additionally, if Notify is called, and Windows sends CTRL_CLOSE_EVENT,
CTRL_LOGOFF_EVENT or CTRL_SHUTDOWN_EVENT to the process, Notify will
return syscall.SIGTERM. Unlike Control-C and Control-Break, Notify does
not change process behavior when either CTRL_CLOSE_EVENT,
CTRL_LOGOFF_EVENT or CTRL_SHUTDOWN_EVENT is received - the process will
still get terminated unless it exits. But receiving syscall.SIGTERM will
give the process an opportunity to clean up before termination.

# Plan 9

On Plan 9, signals have type syscall.Note, which is a string. Calling
Notify with a syscall.Note will cause that value to be sent on the
channel when that string is posted as a note.
*/
package signal