1 // Copyright 2015 The Go Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style 3 // license that can be found in the LICENSE file. 4 5 /* 6 Package signal implements access to incoming signals. 7 8 Signals are primarily used on Unix-like systems. For the use of this 9 package on Windows and Plan 9, see below. 10 11 Types of signals 12 13 The signals SIGKILL and SIGSTOP may not be caught by a program, and 14 therefore cannot be affected by this package. 15 16 Synchronous signals are signals triggered by errors in program 17 execution: SIGBUS, SIGFPE, and SIGSEGV. These are only considered 18 synchronous when caused by program execution, not when sent using 19 os.Process.Kill or the kill program or some similar mechanism. In 20 general, except as discussed below, Go programs will convert a 21 synchronous signal into a run-time panic. 22 23 The remaining signals are asynchronous signals. They are not 24 triggered by program errors, but are instead sent from the kernel or 25 from some other program. 26 27 Of the asynchronous signals, the SIGHUP signal is sent when a program 28 loses its controlling terminal. The SIGINT signal is sent when the 29 user at the controlling terminal presses the interrupt character, 30 which by default is ^C (Control-C). The SIGQUIT signal is sent when 31 the user at the controlling terminal presses the quit character, which 32 by default is ^\ (Control-Backslash). In general you can cause a 33 program to simply exit by pressing ^C, and you can cause it to exit 34 with a stack dump by pressing ^\. 35 36 Default behavior of signals in Go programs 37 38 By default, a synchronous signal is converted into a run-time panic. A 39 SIGHUP, SIGINT, or SIGTERM signal causes the program to exit. A 40 SIGQUIT, SIGILL, SIGTRAP, SIGABRT, SIGSTKFLT, SIGEMT, or SIGSYS signal 41 causes the program to exit with a stack dump. A SIGTSTP, SIGTTIN, or 42 SIGTTOU signal gets the system default behavior (these signals are 43 used by the shell for job control). The SIGPROF signal is handled 44 directly by the Go runtime to implement runtime.CPUProfile. Other 45 signals will be caught but no action will be taken. 46 47 If the Go program is started with either SIGHUP or SIGINT ignored 48 (signal handler set to SIG_IGN), they will remain ignored. 49 50 If the Go program is started with a non-empty signal mask, that will 51 generally be honored. However, some signals are explicitly unblocked: 52 the synchronous signals, SIGILL, SIGTRAP, SIGSTKFLT, SIGCHLD, SIGPROF, 53 and, on GNU/Linux, signals 32 (SIGCANCEL) and 33 (SIGSETXID) 54 (SIGCANCEL and SIGSETXID are used internally by glibc). Subprocesses 55 started by os.Exec, or by the os/exec package, will inherit the 56 modified signal mask. 57 58 Changing the behavior of signals in Go programs 59 60 The functions in this package allow a program to change the way Go 61 programs handle signals. 62 63 Notify disables the default behavior for a given set of asynchronous 64 signals and instead delivers them over one or more registered 65 channels. Specifically, it applies to the signals SIGHUP, SIGINT, 66 SIGQUIT, SIGABRT, and SIGTERM. It also applies to the job control 67 signals SIGTSTP, SIGTTIN, and SIGTTOU, in which case the system 68 default behavior does not occur. It also applies to some signals that 69 otherwise cause no action: SIGUSR1, SIGUSR2, SIGPIPE, SIGALRM, 70 SIGCHLD, SIGCONT, SIGURG, SIGXCPU, SIGXFSZ, SIGVTALRM, SIGWINCH, 71 SIGIO, SIGPWR, SIGSYS, SIGINFO, SIGTHR, SIGWAITING, SIGLWP, SIGFREEZE, 72 SIGTHAW, SIGLOST, SIGXRES, SIGJVM1, SIGJVM2, and any real time signals 73 used on the system. Note that not all of these signals are available 74 on all systems. 75 76 If the program was started with SIGHUP or SIGINT ignored, and Notify 77 is called for either signal, a signal handler will be installed for 78 that signal and it will no longer be ignored. If, later, Reset or 79 Ignore is called for that signal, or Stop is called on all channels 80 passed to Notify for that signal, the signal will once again be 81 ignored. Reset will restore the system default behavior for the 82 signal, while Ignore will cause the system to ignore the signal 83 entirely. 84 85 If the program is started with a non-empty signal mask, some signals 86 will be explicitly unblocked as described above. If Notify is called 87 for a blocked signal, it will be unblocked. If, later, Reset is 88 called for that signal, or Stop is called on all channels passed to 89 Notify for that signal, the signal will once again be blocked. 90 91 SIGPIPE 92 93 When a Go program writes to a broken pipe, the kernel will raise a 94 SIGPIPE signal. 95 96 If the program has not called Notify to receive SIGPIPE signals, then 97 the behavior depends on the file descriptor number. A write to a 98 broken pipe on file descriptors 1 or 2 (standard output or standard 99 error) will cause the program to exit with a SIGPIPE signal. A write 100 to a broken pipe on some other file descriptor will take no action on 101 the SIGPIPE signal, and the write will fail with an EPIPE error. 102 103 If the program has called Notify to receive SIGPIPE signals, the file 104 descriptor number does not matter. The SIGPIPE signal will be 105 delivered to the Notify channel, and the write will fail with an EPIPE 106 error. 107 108 This means that, by default, command line programs will behave like 109 typical Unix command line programs, while other programs will not 110 crash with SIGPIPE when writing to a closed network connection. 111 112 Go programs that use cgo or SWIG 113 114 In a Go program that includes non-Go code, typically C/C++ code 115 accessed using cgo or SWIG, Go's startup code normally runs first. It 116 configures the signal handlers as expected by the Go runtime, before 117 the non-Go startup code runs. If the non-Go startup code wishes to 118 install its own signal handlers, it must take certain steps to keep Go 119 working well. This section documents those steps and the overall 120 effect changes to signal handler settings by the non-Go code can have 121 on Go programs. In rare cases, the non-Go code may run before the Go 122 code, in which case the next section also applies. 123 124 If the non-Go code called by the Go program does not change any signal 125 handlers or masks, then the behavior is the same as for a pure Go 126 program. 127 128 If the non-Go code installs any signal handlers, it must use the 129 SA_ONSTACK flag with sigaction. Failing to do so is likely to cause 130 the program to crash if the signal is received. Go programs routinely 131 run with a limited stack, and therefore set up an alternate signal 132 stack. Also, the Go standard library expects that any signal handlers 133 will use the SA_RESTART flag. Failing to do so may cause some library 134 calls to return "interrupted system call" errors. 135 136 If the non-Go code installs a signal handler for any of the 137 synchronous signals (SIGBUS, SIGFPE, SIGSEGV), then it should record 138 the existing Go signal handler. If those signals occur while 139 executing Go code, it should invoke the Go signal handler (whether the 140 signal occurs while executing Go code can be determined by looking at 141 the PC passed to the signal handler). Otherwise some Go run-time 142 panics will not occur as expected. 143 144 If the non-Go code installs a signal handler for any of the 145 asynchronous signals, it may invoke the Go signal handler or not as it 146 chooses. Naturally, if it does not invoke the Go signal handler, the 147 Go behavior described above will not occur. This can be an issue with 148 the SIGPROF signal in particular. 149 150 The non-Go code should not change the signal mask on any threads 151 created by the Go runtime. If the non-Go code starts new threads of 152 its own, it may set the signal mask as it pleases. 153 154 If the non-Go code starts a new thread, changes the signal mask, and 155 then invokes a Go function in that thread, the Go runtime will 156 automatically unblock certain signals: the synchronous signals, 157 SIGILL, SIGTRAP, SIGSTKFLT, SIGCHLD, SIGPROF, SIGCANCEL, and 158 SIGSETXID. When the Go function returns, the non-Go signal mask will 159 be restored. 160 161 If the Go signal handler is invoked on a non-Go thread not running Go 162 code, the handler generally forwards the signal to the non-Go code, as 163 follows. If the signal is SIGPROF, the Go handler does 164 nothing. Otherwise, the Go handler removes itself, unblocks the 165 signal, and raises it again, to invoke any non-Go handler or default 166 system handler. If the program does not exit, the Go handler then 167 reinstalls itself and continues execution of the program. 168 169 Non-Go programs that call Go code 170 171 When Go code is built with options like -buildmode=c-shared, it will 172 be run as part of an existing non-Go program. The non-Go code may 173 have already installed signal handlers when the Go code starts (that 174 may also happen in unusual cases when using cgo or SWIG; in that case, 175 the discussion here applies). For -buildmode=c-archive the Go runtime 176 will initialize signals at global constructor time. For 177 -buildmode=c-shared the Go runtime will initialize signals when the 178 shared library is loaded. 179 180 If the Go runtime sees an existing signal handler for the SIGCANCEL or 181 SIGSETXID signals (which are used only on GNU/Linux), it will turn on 182 the SA_ONSTACK flag and otherwise keep the signal handler. 183 184 For the synchronous signals and SIGPIPE, the Go runtime will install a 185 signal handler. It will save any existing signal handler. If a 186 synchronous signal arrives while executing non-Go code, the Go runtime 187 will invoke the existing signal handler instead of the Go signal 188 handler. 189 190 Go code built with -buildmode=c-archive or -buildmode=c-shared will 191 not install any other signal handlers by default. If there is an 192 existing signal handler, the Go runtime will turn on the SA_ONSTACK 193 flag and otherwise keep the signal handler. If Notify is called for an 194 asynchronous signal, a Go signal handler will be installed for that 195 signal. If, later, Reset is called for that signal, the original 196 handling for that signal will be reinstalled, restoring the non-Go 197 signal handler if any. 198 199 Go code built without -buildmode=c-archive or -buildmode=c-shared will 200 install a signal handler for the asynchronous signals listed above, 201 and save any existing signal handler. If a signal is delivered to a 202 non-Go thread, it will act as described above, except that if there is 203 an existing non-Go signal handler, that handler will be installed 204 before raising the signal. 205 206 Windows 207 208 On Windows a ^C (Control-C) or ^BREAK (Control-Break) normally cause 209 the program to exit. If Notify is called for os.Interrupt, ^C or ^BREAK 210 will cause os.Interrupt to be sent on the channel, and the program will 211 not exit. If Reset is called, or Stop is called on all channels passed 212 to Notify, then the default behavior will be restored. 213 214 Plan 9 215 216 On Plan 9, signals have type syscall.Note, which is a string. Calling 217 Notify with a syscall.Note will cause that value to be sent on the 218 channel when that string is posted as a note. 219 220 */ 221 package signal 222
