| // 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 can not 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 GNU/Linux, signals 32 (SIGCANCEL) and 33 (SIGSETXID) |
| (SIGCANCEL and SIGSETXID are used internally by glibc). Subprocesses |
| started by os.Exec, or by the os/exec package, 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, SIGSYS, 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 an 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 an 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. Also, the Go standard library expects that any signal handlers |
| will use the SA_RESTART flag. Failing to do so may cause some library |
| calls to return "interrupted system call" errors. |
| |
| 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 of |
| its own, it may set the signal mask as it pleases. |
| |
| 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. |
| |
| 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 GNU/Linux), it will turn on |
| the SA_ONSTACK flag and otherwise keep the signal handler. |
| |
| For the synchronous signals, 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.SIGINT, ^C or ^BREAK |
| will cause os.SIGINT 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. |
| |
| 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 |