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| 55 | <H1><A NAME="SECTION009100000000000000000"> |
| 56 | 7.1 <tt class="module">signal</tt> -- |
| 57 | Set handlers for asynchronous events</A> |
| 58 | </H1> |
| 59 | |
| 60 | <P> |
| 61 | <A NAME="module-signal"></A> |
| 62 | |
| 63 | <P> |
| 64 | This module provides mechanisms to use signal handlers in Python. |
| 65 | Some general rules for working with signals and their handlers: |
| 66 | |
| 67 | <P> |
| 68 | |
| 69 | <UL> |
| 70 | <LI>A handler for a particular signal, once set, remains installed until |
| 71 | it is explicitly reset (Python emulates the BSD style interface |
| 72 | regardless of the underlying implementation), with the exception of |
| 73 | the handler for <tt class="constant">SIGCHLD</tt>, which follows the underlying |
| 74 | implementation. |
| 75 | |
| 76 | <P> |
| 77 | </LI> |
| 78 | <LI>There is no way to ``block'' signals temporarily from critical |
| 79 | sections (since this is not supported by all <span class="Unix">Unix</span> flavors). |
| 80 | |
| 81 | <P> |
| 82 | </LI> |
| 83 | <LI>Although Python signal handlers are called asynchronously as far as |
| 84 | the Python user is concerned, they can only occur between the |
| 85 | ``atomic'' instructions of the Python interpreter. This means that |
| 86 | signals arriving during long calculations implemented purely in C |
| 87 | (such as regular expression matches on large bodies of text) may be |
| 88 | delayed for an arbitrary amount of time. |
| 89 | |
| 90 | <P> |
| 91 | </LI> |
| 92 | <LI>When a signal arrives during an I/O operation, it is possible that the |
| 93 | I/O operation raises an exception after the signal handler returns. |
| 94 | This is dependent on the underlying <span class="Unix">Unix</span> system's semantics regarding |
| 95 | interrupted system calls. |
| 96 | |
| 97 | <P> |
| 98 | </LI> |
| 99 | <LI>Because the C signal handler always returns, it makes little sense to |
| 100 | catch synchronous errors like <tt class="constant">SIGFPE</tt> or <tt class="constant">SIGSEGV</tt>. |
| 101 | |
| 102 | <P> |
| 103 | </LI> |
| 104 | <LI>Python installs a small number of signal handlers by default: |
| 105 | <tt class="constant">SIGPIPE</tt> is ignored (so write errors on pipes and sockets can be |
| 106 | reported as ordinary Python exceptions) and <tt class="constant">SIGINT</tt> is translated |
| 107 | into a <tt class="exception">KeyboardInterrupt</tt> exception. All of these can be |
| 108 | overridden. |
| 109 | |
| 110 | <P> |
| 111 | </LI> |
| 112 | <LI>Some care must be taken if both signals and threads are used in the |
| 113 | same program. The fundamental thing to remember in using signals and |
| 114 | threads simultaneously is: always perform <tt class="function">signal()</tt> operations |
| 115 | in the main thread of execution. Any thread can perform an |
| 116 | <tt class="function">alarm()</tt>, <tt class="function">getsignal()</tt>, or <tt class="function">pause()</tt>; |
| 117 | only the main thread can set a new signal handler, and the main thread |
| 118 | will be the only one to receive signals (this is enforced by the |
| 119 | Python <tt class="module">signal</tt> module, even if the underlying thread |
| 120 | implementation supports sending signals to individual threads). This |
| 121 | means that signals can't be used as a means of inter-thread |
| 122 | communication. Use locks instead. |
| 123 | |
| 124 | <P> |
| 125 | </LI> |
| 126 | </UL> |
| 127 | |
| 128 | <P> |
| 129 | The variables defined in the <tt class="module">signal</tt> module are: |
| 130 | |
| 131 | <P> |
| 132 | <dl><dt><b><tt id='l2h-2588' xml:id='l2h-2588'>SIG_DFL</tt></b></dt> |
| 133 | <dd> |
| 134 | This is one of two standard signal handling options; it will simply |
| 135 | perform the default function for the signal. For example, on most |
| 136 | systems the default action for <tt class="constant">SIGQUIT</tt> is to dump core |
| 137 | and exit, while the default action for <tt class="constant">SIGCLD</tt> is to |
| 138 | simply ignore it. |
| 139 | </dd></dl> |
| 140 | |
| 141 | <P> |
| 142 | <dl><dt><b><tt id='l2h-2589' xml:id='l2h-2589'>SIG_IGN</tt></b></dt> |
| 143 | <dd> |
| 144 | This is another standard signal handler, which will simply ignore |
| 145 | the given signal. |
| 146 | </dd></dl> |
| 147 | |
| 148 | <P> |
| 149 | <dl><dt><b><tt id='l2h-2590' xml:id='l2h-2590'>SIG*</tt></b></dt> |
| 150 | <dd> |
| 151 | All the signal numbers are defined symbolically. For example, the |
| 152 | hangup signal is defined as <tt class="constant">signal.SIGHUP</tt>; the variable names |
| 153 | are identical to the names used in C programs, as found in |
| 154 | <code><signal.h></code>. |
| 155 | The <span class="Unix">Unix</span> man page for `<tt class="cfunction">signal()</tt>' lists the existing |
| 156 | signals (on some systems this is <span class="manpage"><i>signal</i>(2)</span>, on others the |
| 157 | list is in <span class="manpage"><i>signal</i>(7)</span>). |
| 158 | Note that not all systems define the same set of signal names; only |
| 159 | those names defined by the system are defined by this module. |
| 160 | </dd></dl> |
| 161 | |
| 162 | <P> |
| 163 | <dl><dt><b><tt id='l2h-2591' xml:id='l2h-2591'>NSIG</tt></b></dt> |
| 164 | <dd> |
| 165 | One more than the number of the highest signal number. |
| 166 | </dd></dl> |
| 167 | |
| 168 | <P> |
| 169 | The <tt class="module">signal</tt> module defines the following functions: |
| 170 | |
| 171 | <P> |
| 172 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> |
| 173 | <td><nobr><b><tt id='l2h-2592' xml:id='l2h-2592' class="function">alarm</tt></b>(</nobr></td> |
| 174 | <td><var>time</var>)</td></tr></table></dt> |
| 175 | <dd> |
| 176 | If <var>time</var> is non-zero, this function requests that a |
| 177 | <tt class="constant">SIGALRM</tt> signal be sent to the process in <var>time</var> seconds. |
| 178 | Any previously scheduled alarm is canceled (only one alarm can |
| 179 | be scheduled at any time). The returned value is then the number of |
| 180 | seconds before any previously set alarm was to have been delivered. |
| 181 | If <var>time</var> is zero, no alarm id scheduled, and any scheduled |
| 182 | alarm is canceled. The return value is the number of seconds |
| 183 | remaining before a previously scheduled alarm. If the return value |
| 184 | is zero, no alarm is currently scheduled. (See the <span class="Unix">Unix</span> man page |
| 185 | <span class="manpage"><i>alarm</i>(2)</span>.) |
| 186 | Availability: <span class="Unix">Unix</span>. |
| 187 | </dl> |
| 188 | |
| 189 | <P> |
| 190 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> |
| 191 | <td><nobr><b><tt id='l2h-2593' xml:id='l2h-2593' class="function">getsignal</tt></b>(</nobr></td> |
| 192 | <td><var>signalnum</var>)</td></tr></table></dt> |
| 193 | <dd> |
| 194 | Return the current signal handler for the signal <var>signalnum</var>. |
| 195 | The returned value may be a callable Python object, or one of the |
| 196 | special values <tt class="constant">signal.SIG_IGN</tt>, <tt class="constant">signal.SIG_DFL</tt> or |
| 197 | <tt class="constant">None</tt>. Here, <tt class="constant">signal.SIG_IGN</tt> means that the |
| 198 | signal was previously ignored, <tt class="constant">signal.SIG_DFL</tt> means that the |
| 199 | default way of handling the signal was previously in use, and |
| 200 | <code>None</code> means that the previous signal handler was not installed |
| 201 | from Python. |
| 202 | </dl> |
| 203 | |
| 204 | <P> |
| 205 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> |
| 206 | <td><nobr><b><tt id='l2h-2594' xml:id='l2h-2594' class="function">pause</tt></b>(</nobr></td> |
| 207 | <td><var></var>)</td></tr></table></dt> |
| 208 | <dd> |
| 209 | Cause the process to sleep until a signal is received; the |
| 210 | appropriate handler will then be called. Returns nothing. Not on |
| 211 | Windows. (See the <span class="Unix">Unix</span> man page <span class="manpage"><i>signal</i>(2)</span>.) |
| 212 | </dl> |
| 213 | |
| 214 | <P> |
| 215 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> |
| 216 | <td><nobr><b><tt id='l2h-2595' xml:id='l2h-2595' class="function">signal</tt></b>(</nobr></td> |
| 217 | <td><var>signalnum, handler</var>)</td></tr></table></dt> |
| 218 | <dd> |
| 219 | Set the handler for signal <var>signalnum</var> to the function |
| 220 | <var>handler</var>. <var>handler</var> can be a callable Python object |
| 221 | taking two arguments (see below), or |
| 222 | one of the special values <tt class="constant">signal.SIG_IGN</tt> or |
| 223 | <tt class="constant">signal.SIG_DFL</tt>. The previous signal handler will be returned |
| 224 | (see the description of <tt class="function">getsignal()</tt> above). (See the |
| 225 | <span class="Unix">Unix</span> man page <span class="manpage"><i>signal</i>(2)</span>.) |
| 226 | |
| 227 | <P> |
| 228 | When threads are enabled, this function can only be called from the |
| 229 | main thread; attempting to call it from other threads will cause a |
| 230 | <tt class="exception">ValueError</tt> exception to be raised. |
| 231 | |
| 232 | <P> |
| 233 | The <var>handler</var> is called with two arguments: the signal number |
| 234 | and the current stack frame (<code>None</code> or a frame object; |
| 235 | for a description of frame objects, see the reference manual section |
| 236 | on the standard type hierarchy or see the attribute descriptions in |
| 237 | the <tt class="module"><a href="module-inspect.html">inspect</a></tt> module). |
| 238 | </dl> |
| 239 | |
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