doc.go

     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 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.
   133  
   134  If the non-Go code installs a signal handler for any of the
   135  synchronous signals (SIGBUS, SIGFPE, SIGSEGV), then it should record
   136  the existing Go signal handler. If those signals occur while
   137  executing Go code, it should invoke the Go signal handler (whether the
   138  signal occurs while executing Go code can be determined by looking at
   139  the PC passed to the signal handler). Otherwise some Go run-time
   140  panics will not occur as expected.
   141  
   142  If the non-Go code installs a signal handler for any of the
   143  asynchronous signals, it may invoke the Go signal handler or not as it
   144  chooses. Naturally, if it does not invoke the Go signal handler, the
   145  Go behavior described above will not occur. This can be an issue with
   146  the SIGPROF signal in particular.
   147  
   148  The non-Go code should not change the signal mask on any threads
   149  created by the Go runtime. If the non-Go code starts new threads of
   150  its own, it may set the signal mask as it pleases.
   151  
   152  If the non-Go code starts a new thread, changes the signal mask, and
   153  then invokes a Go function in that thread, the Go runtime will
   154  automatically unblock certain signals: the synchronous signals,
   155  SIGILL, SIGTRAP, SIGSTKFLT, SIGCHLD, SIGPROF, SIGCANCEL, and
   156  SIGSETXID. When the Go function returns, the non-Go signal mask will
   157  be restored.
   158  
   159  If the Go signal handler is invoked on a non-Go thread not running Go
   160  code, the handler generally forwards the signal to the non-Go code, as
   161  follows. If the signal is SIGPROF, the Go handler does
   162  nothing. Otherwise, the Go handler removes itself, unblocks the
   163  signal, and raises it again, to invoke any non-Go handler or default
   164  system handler. If the program does not exit, the Go handler then
   165  reinstalls itself and continues execution of the program.
   166  
   167  Non-Go programs that call Go code
   168  
   169  When Go code is built with options like -buildmode=c-shared, it will
   170  be run as part of an existing non-Go program. The non-Go code may
   171  have already installed signal handlers when the Go code starts (that
   172  may also happen in unusual cases when using cgo or SWIG; in that case,
   173  the discussion here applies).  For -buildmode=c-archive the Go runtime
   174  will initialize signals at global constructor time.  For
   175  -buildmode=c-shared the Go runtime will initialize signals when the
   176  shared library is loaded.
   177  
   178  If the Go runtime sees an existing signal handler for the SIGCANCEL or
   179  SIGSETXID signals (which are used only on Linux), it will turn on
   180  the SA_ONSTACK flag and otherwise keep the signal handler.
   181  
   182  For the synchronous signals and SIGPIPE, the Go runtime will install a
   183  signal handler. It will save any existing signal handler. If a
   184  synchronous signal arrives while executing non-Go code, the Go runtime
   185  will invoke the existing signal handler instead of the Go signal
   186  handler.
   187  
   188  Go code built with -buildmode=c-archive or -buildmode=c-shared will
   189  not install any other signal handlers by default. If there is an
   190  existing signal handler, the Go runtime will turn on the SA_ONSTACK
   191  flag and otherwise keep the signal handler. If Notify is called for an
   192  asynchronous signal, a Go signal handler will be installed for that
   193  signal. If, later, Reset is called for that signal, the original
   194  handling for that signal will be reinstalled, restoring the non-Go
   195  signal handler if any.
   196  
   197  Go code built without -buildmode=c-archive or -buildmode=c-shared will
   198  install a signal handler for the asynchronous signals listed above,
   199  and save any existing signal handler. If a signal is delivered to a
   200  non-Go thread, it will act as described above, except that if there is
   201  an existing non-Go signal handler, that handler will be installed
   202  before raising the signal.
   203  
   204  Windows
   205  
   206  On Windows a ^C (Control-C) or ^BREAK (Control-Break) normally cause
   207  the program to exit. If Notify is called for os.Interrupt, ^C or ^BREAK
   208  will cause os.Interrupt to be sent on the channel, and the program will
   209  not exit. If Reset is called, or Stop is called on all channels passed
   210  to Notify, then the default behavior will be restored.
   211  
   212  Additionally, if Notify is called, and Windows sends CTRL_CLOSE_EVENT,
   213  CTRL_LOGOFF_EVENT or CTRL_SHUTDOWN_EVENT to the process, Notify will
   214  return syscall.SIGTERM. Unlike Control-C and Control-Break, Notify does
   215  not change process behavior when either CTRL_CLOSE_EVENT,
   216  CTRL_LOGOFF_EVENT or CTRL_SHUTDOWN_EVENT is received - the process will
   217  still get terminated unless it exits. But receiving syscall.SIGTERM will
   218  give the process an opportunity to clean up before termination.
   219  
   220  Plan 9
   221  
   222  On Plan 9, signals have type syscall.Note, which is a string. Calling
   223  Notify with a syscall.Note will cause that value to be sent on the
   224  channel when that string is posted as a note.
   225  
   226  */
   227  package signal
   228
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