# features/pep-420 / Doc / lib / libsignal.tex

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  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 \section{\module{signal} --- Set handlers for asynchronous events} \declaremodule{builtin}{signal} \modulesynopsis{Set handlers for asynchronous events.} This module provides mechanisms to use signal handlers in Python. Some general rules for working with signals and their handlers: \begin{itemize} \item A handler for a particular signal, once set, remains installed until it is explicitly reset (Python emulates the BSD style interface regardless of the underlying implementation), with the exception of the handler for \constant{SIGCHLD}, which follows the underlying implementation. \item There is no way to block'' signals temporarily from critical sections (since this is not supported by all \UNIX{} flavors). \item Although Python signal handlers are called asynchronously as far as the Python user is concerned, they can only occur between the atomic'' instructions of the Python interpreter. This means that signals arriving during long calculations implemented purely in C (such as regular expression matches on large bodies of text) may be delayed for an arbitrary amount of time. \item When a signal arrives during an I/O operation, it is possible that the I/O operation raises an exception after the signal handler returns. This is dependent on the underlying \UNIX{} system's semantics regarding interrupted system calls. \item Because the \C{} signal handler always returns, it makes little sense to catch synchronous errors like \constant{SIGFPE} or \constant{SIGSEGV}. \item Python installs a small number of signal handlers by default: \constant{SIGPIPE} is ignored (so write errors on pipes and sockets can be reported as ordinary Python exceptions) and \constant{SIGINT} is translated into a \exception{KeyboardInterrupt} exception. All of these can be overridden. \item Some care must be taken if both signals and threads are used in the same program. The fundamental thing to remember in using signals and threads simultaneously is:\ always perform \function{signal()} operations in the main thread of execution. Any thread can perform an \function{alarm()}, \function{getsignal()}, or \function{pause()}; only the main thread can set a new signal handler, and the main thread will be the only one to receive signals (this is enforced by the Python \module{signal} module, even if the underlying thread implementation supports sending signals to individual threads). This means that signals can't be used as a means of inter-thread communication. Use locks instead. \end{itemize} The variables defined in the \module{signal} module are: \begin{datadesc}{SIG_DFL} This is one of two standard signal handling options; it will simply perform the default function for the signal. For example, on most systems the default action for \constant{SIGQUIT} is to dump core and exit, while the default action for \constant{SIGCLD} is to simply ignore it. \end{datadesc} \begin{datadesc}{SIG_IGN} This is another standard signal handler, which will simply ignore the given signal. \end{datadesc} \begin{datadesc}{SIG*} All the signal numbers are defined symbolically. For example, the hangup signal is defined as \constant{signal.SIGHUP}; the variable names are identical to the names used in C programs, as found in \code{}. The \UNIX{} man page for \cfunction{signal()}' lists the existing signals (on some systems this is \manpage{signal}{2}, on others the list is in \manpage{signal}{7}). Note that not all systems define the same set of signal names; only those names defined by the system are defined by this module. \end{datadesc} \begin{datadesc}{NSIG} One more than the number of the highest signal number. \end{datadesc} The \module{signal} module defines the following functions: \begin{funcdesc}{alarm}{time} If \var{time} is non-zero, this function requests that a \constant{SIGALRM} signal be sent to the process in \var{time} seconds. Any previously scheduled alarm is canceled (only one alarm can be scheduled at any time). The returned value is then the number of seconds before any previously set alarm was to have been delivered. If \var{time} is zero, no alarm is scheduled, and any scheduled alarm is canceled. If the return value is zero, no alarm is currently scheduled. (See the \UNIX{} man page \manpage{alarm}{2}.) Availability: \UNIX. \end{funcdesc} \begin{funcdesc}{getsignal}{signalnum} Return the current signal handler for the signal \var{signalnum}. The returned value may be a callable Python object, or one of the special values \constant{signal.SIG_IGN}, \constant{signal.SIG_DFL} or \constant{None}. Here, \constant{signal.SIG_IGN} means that the signal was previously ignored, \constant{signal.SIG_DFL} means that the default way of handling the signal was previously in use, and \code{None} means that the previous signal handler was not installed from Python. \end{funcdesc} \begin{funcdesc}{pause}{} Cause the process to sleep until a signal is received; the appropriate handler will then be called. Returns nothing. Not on Windows. (See the \UNIX{} man page \manpage{signal}{2}.) \end{funcdesc} \begin{funcdesc}{signal}{signalnum, handler} Set the handler for signal \var{signalnum} to the function \var{handler}. \var{handler} can be a callable Python object taking two arguments (see below), or one of the special values \constant{signal.SIG_IGN} or \constant{signal.SIG_DFL}. The previous signal handler will be returned (see the description of \function{getsignal()} above). (See the \UNIX{} man page \manpage{signal}{2}.) When threads are enabled, this function can only be called from the main thread; attempting to call it from other threads will cause a \exception{ValueError} exception to be raised. The \var{handler} is called with two arguments: the signal number and the current stack frame (\code{None} or a frame object; for a description of frame objects, see the reference manual section on the standard type hierarchy or see the attribute descriptions in the \refmodule{inspect} module). \end{funcdesc} \subsection{Example} \nodename{Signal Example} Here is a minimal example program. It uses the \function{alarm()} function to limit the time spent waiting to open a file; this is useful if the file is for a serial device that may not be turned on, which would normally cause the \function{os.open()} to hang indefinitely. The solution is to set a 5-second alarm before opening the file; if the operation takes too long, the alarm signal will be sent, and the handler raises an exception. \begin{verbatim} import signal, os def handler(signum, frame): print 'Signal handler called with signal', signum raise IOError, "Couldn't open device!" # Set the signal handler and a 5-second alarm signal.signal(signal.SIGALRM, handler) signal.alarm(5) # This open() may hang indefinitely fd = os.open('/dev/ttyS0', os.O_RDWR) signal.alarm(0) # Disable the alarm \end{verbatim} `