1 \section{\module{signal
} ---
2 Set handlers for asynchronous events
}
4 \declaremodule{builtin
}{signal
}
5 \modulesynopsis{Set handlers for asynchronous events.
}
8 This module provides mechanisms to use signal handlers in Python.
9 Some general rules for working with signals and their handlers:
14 A handler for a particular signal, once set, remains installed until
15 it is explicitly reset (Python emulates the BSD style interface
16 regardless of the underlying implementation), with the exception of
17 the handler for
\constant{SIGCHLD
}, which follows the underlying
21 There is no way to ``block'' signals temporarily from critical
22 sections (since this is not supported by all
\UNIX{} flavors).
25 Although Python signal handlers are called asynchronously as far as
26 the Python user is concerned, they can only occur between the
27 ``atomic'' instructions of the Python interpreter. This means that
28 signals arriving during long calculations implemented purely in C
29 (such as regular expression matches on large bodies of text) may be
30 delayed for an arbitrary amount of time.
33 When a signal arrives during an I/O operation, it is possible that the
34 I/O operation raises an exception after the signal handler returns.
35 This is dependent on the underlying
\UNIX{} system's semantics regarding
36 interrupted system calls.
39 Because the
\C{} signal handler always returns, it makes little sense to
40 catch synchronous errors like
\constant{SIGFPE
} or
\constant{SIGSEGV
}.
43 Python installs a small number of signal handlers by default:
44 \constant{SIGPIPE
} is ignored (so write errors on pipes and sockets can be
45 reported as ordinary Python exceptions) and
\constant{SIGINT
} is translated
46 into a
\exception{KeyboardInterrupt
} exception. All of these can be
50 Some care must be taken if both signals and threads are used in the
51 same program. The fundamental thing to remember in using signals and
52 threads simultaneously is:\ always perform
\function{signal()
} operations
53 in the main thread of execution. Any thread can perform an
54 \function{alarm()
},
\function{getsignal()
}, or
\function{pause()
};
55 only the main thread can set a new signal handler, and the main thread
56 will be the only one to receive signals (this is enforced by the
57 Python
\module{signal
} module, even if the underlying thread
58 implementation supports sending signals to individual threads). This
59 means that signals can't be used as a means of inter-thread
60 communication. Use locks instead.
64 The variables defined in the
\module{signal
} module are:
66 \begin{datadesc
}{SIG_DFL
}
67 This is one of two standard signal handling options; it will simply
68 perform the default function for the signal. For example, on most
69 systems the default action for
\constant{SIGQUIT
} is to dump core
70 and exit, while the default action for
\constant{SIGCLD
} is to
74 \begin{datadesc
}{SIG_IGN
}
75 This is another standard signal handler, which will simply ignore
79 \begin{datadesc
}{SIG*
}
80 All the signal numbers are defined symbolically. For example, the
81 hangup signal is defined as
\constant{signal.SIGHUP
}; the variable names
82 are identical to the names used in C programs, as found in
84 The
\UNIX{} man page for `
\cfunction{signal()
}' lists the existing
85 signals (on some systems this is
\manpage{signal
}{2}, on others the
86 list is in
\manpage{signal
}{7}).
87 Note that not all systems define the same set of signal names; only
88 those names defined by the system are defined by this module.
91 \begin{datadesc
}{NSIG
}
92 One more than the number of the highest signal number.
95 The
\module{signal
} module defines the following functions:
97 \begin{funcdesc
}{alarm
}{time
}
98 If
\var{time
} is non-zero, this function requests that a
99 \constant{SIGALRM
} signal be sent to the process in
\var{time
} seconds.
100 Any previously scheduled alarm is canceled (only one alarm can
101 be scheduled at any time). The returned value is then the number of
102 seconds before any previously set alarm was to have been delivered.
103 If
\var{time
} is zero, no alarm is scheduled, and any scheduled
104 alarm is canceled. The return value is the number of seconds
105 remaining before a previously scheduled alarm. If the return value
106 is zero, no alarm is currently scheduled. (See the
\UNIX{} man page
111 \begin{funcdesc
}{getsignal
}{signalnum
}
112 Return the current signal handler for the signal
\var{signalnum
}.
113 The returned value may be a callable Python object, or one of the
114 special values
\constant{signal.SIG_IGN
},
\constant{signal.SIG_DFL
} or
115 \constant{None
}. Here,
\constant{signal.SIG_IGN
} means that the
116 signal was previously ignored,
\constant{signal.SIG_DFL
} means that the
117 default way of handling the signal was previously in use, and
118 \code{None
} means that the previous signal handler was not installed
122 \begin{funcdesc
}{pause
}{}
123 Cause the process to sleep until a signal is received; the
124 appropriate handler will then be called. Returns nothing. Not on
125 Windows. (See the
\UNIX{} man page
\manpage{signal
}{2}.)
128 \begin{funcdesc
}{signal
}{signalnum, handler
}
129 Set the handler for signal
\var{signalnum
} to the function
130 \var{handler
}.
\var{handler
} can be a callable Python object
131 taking two arguments (see below), or
132 one of the special values
\constant{signal.SIG_IGN
} or
133 \constant{signal.SIG_DFL
}. The previous signal handler will be returned
134 (see the description of
\function{getsignal()
} above). (See the
135 \UNIX{} man page
\manpage{signal
}{2}.)
137 When threads are enabled, this function can only be called from the
138 main thread; attempting to call it from other threads will cause a
139 \exception{ValueError
} exception to be raised.
141 The
\var{handler
} is called with two arguments: the signal number
142 and the current stack frame (
\code{None
} or a frame object;
143 for a description of frame objects, see the reference manual section
144 on the standard type hierarchy or see the attribute descriptions in
145 the
\refmodule{inspect
} module).
149 \nodename{Signal Example
}
151 Here is a minimal example program. It uses the
\function{alarm()
}
152 function to limit the time spent waiting to open a file; this is
153 useful if the file is for a serial device that may not be turned on,
154 which would normally cause the
\function{os.open()
} to hang
155 indefinitely. The solution is to set a
5-second alarm before opening
156 the file; if the operation takes too long, the alarm signal will be
157 sent, and the handler raises an exception.
162 def handler(signum, frame):
163 print 'Signal handler called with signal', signum
164 raise IOError, "Couldn't open device!"
166 # Set the signal handler and a
5-second alarm
167 signal.signal(signal.SIGALRM, handler)
170 # This open() may hang indefinitely
171 fd = os.open('/dev/ttyS0', os.O_RDWR)
173 signal.alarm(
0) # Disable the alarm