1 2:mod:`signal` --- Set handlers for asynchronous events 3====================================================== 4 5.. module:: signal 6 :synopsis: Set handlers for asynchronous events. 7 8 9This module provides mechanisms to use signal handlers in Python. Some general 10rules for working with signals and their handlers: 11 12* A handler for a particular signal, once set, remains installed until it is 13 explicitly reset (Python emulates the BSD style interface regardless of the 14 underlying implementation), with the exception of the handler for 15 :const:`SIGCHLD`, which follows the underlying implementation. 16 17* There is no way to "block" signals temporarily from critical sections (since 18 this is not supported by all Unix flavors). 19 20* Although Python signal handlers are called asynchronously as far as the Python 21 user is concerned, they can only occur between the "atomic" instructions of the 22 Python interpreter. This means that signals arriving during long calculations 23 implemented purely in C (such as regular expression matches on large bodies of 24 text) may be delayed for an arbitrary amount of time. 25 26* When a signal arrives during an I/O operation, it is possible that the I/O 27 operation raises an exception after the signal handler returns. This is 28 dependent on the underlying Unix system's semantics regarding interrupted system 29 calls. 30 31* Because the C signal handler always returns, it makes little sense to catch 32 synchronous errors like :const:`SIGFPE` or :const:`SIGSEGV`. 33 34* Python installs a small number of signal handlers by default: :const:`SIGPIPE` 35 is ignored (so write errors on pipes and sockets can be reported as ordinary 36 Python exceptions) and :const:`SIGINT` is translated into a 37 :exc:`KeyboardInterrupt` exception. All of these can be overridden. 38 39* Some care must be taken if both signals and threads are used in the same 40 program. The fundamental thing to remember in using signals and threads 41 simultaneously is: always perform :func:`signal` operations in the main thread 42 of execution. Any thread can perform an :func:`alarm`, :func:`getsignal`, 43 :func:`pause`, :func:`setitimer` or :func:`getitimer`; only the main thread 44 can set a new signal handler, and the main thread will be the only one to 45 receive signals (this is enforced by the Python :mod:`signal` module, even 46 if the underlying thread implementation supports sending signals to 47 individual threads). This means that signals can't be used as a means of 48 inter-thread communication. Use locks instead. 49 50The variables defined in the :mod:`signal` module are: 51 52 53.. data:: SIG_DFL 54 55 This is one of two standard signal handling options; it will simply perform 56 the default function for the signal. For example, on most systems the 57 default action for :const:`SIGQUIT` is to dump core and exit, while the 58 default action for :const:`SIGCHLD` is to simply ignore it. 59 60 61.. data:: SIG_IGN 62 63 This is another standard signal handler, which will simply ignore the given 64 signal. 65 66 67.. data:: SIG* 68 69 All the signal numbers are defined symbolically. For example, the hangup signal 70 is defined as :const:`signal.SIGHUP`; the variable names are identical to the 71 names used in C programs, as found in ``<signal.h>``. The Unix man page for 72 ':c:func:`signal`' lists the existing signals (on some systems this is 73 :manpage:`signal(2)`, on others the list is in :manpage:`signal(7)`). Note that 74 not all systems define the same set of signal names; only those names defined by 75 the system are defined by this module. 76 77 78.. data:: CTRL_C_EVENT 79 80 The signal corresponding to the :kbd:`Ctrl+C` keystroke event. This signal can 81 only be used with :func:`os.kill`. 82 83 Availability: Windows. 84 85 .. versionadded:: 2.7 86 87 88.. data:: CTRL_BREAK_EVENT 89 90 The signal corresponding to the :kbd:`Ctrl+Break` keystroke event. This signal can 91 only be used with :func:`os.kill`. 92 93 Availability: Windows. 94 95 .. versionadded:: 2.7 96 97 98.. data:: NSIG 99 100 One more than the number of the highest signal number. 101 102 103.. data:: ITIMER_REAL 104 105 Decrements interval timer in real time, and delivers :const:`SIGALRM` upon expiration. 106 107 108.. data:: ITIMER_VIRTUAL 109 110 Decrements interval timer only when the process is executing, and delivers 111 SIGVTALRM upon expiration. 112 113 114.. data:: ITIMER_PROF 115 116 Decrements interval timer both when the process executes and when the 117 system is executing on behalf of the process. Coupled with ITIMER_VIRTUAL, 118 this timer is usually used to profile the time spent by the application 119 in user and kernel space. SIGPROF is delivered upon expiration. 120 121 122The :mod:`signal` module defines one exception: 123 124.. exception:: ItimerError 125 126 Raised to signal an error from the underlying :func:`setitimer` or 127 :func:`getitimer` implementation. Expect this error if an invalid 128 interval timer or a negative time is passed to :func:`setitimer`. 129 This error is a subtype of :exc:`IOError`. 130 131 132The :mod:`signal` module defines the following functions: 133 134 135.. function:: alarm(time) 136 137 If *time* is non-zero, this function requests that a :const:`SIGALRM` signal be 138 sent to the process in *time* seconds. Any previously scheduled alarm is 139 canceled (only one alarm can be scheduled at any time). The returned value is 140 then the number of seconds before any previously set alarm was to have been 141 delivered. If *time* is zero, no alarm is scheduled, and any scheduled alarm is 142 canceled. If the return value is zero, no alarm is currently scheduled. (See 143 the Unix man page :manpage:`alarm(2)`.) Availability: Unix. 144 145 146.. function:: getsignal(signalnum) 147 148 Return the current signal handler for the signal *signalnum*. The returned value 149 may be a callable Python object, or one of the special values 150 :const:`signal.SIG_IGN`, :const:`signal.SIG_DFL` or :const:`None`. Here, 151 :const:`signal.SIG_IGN` means that the signal was previously ignored, 152 :const:`signal.SIG_DFL` means that the default way of handling the signal was 153 previously in use, and ``None`` means that the previous signal handler was not 154 installed from Python. 155 156 157.. function:: pause() 158 159 Cause the process to sleep until a signal is received; the appropriate handler 160 will then be called. Returns nothing. Not on Windows. (See the Unix man page 161 :manpage:`signal(2)`.) 162 163 164.. function:: setitimer(which, seconds[, interval]) 165 166 Sets given interval timer (one of :const:`signal.ITIMER_REAL`, 167 :const:`signal.ITIMER_VIRTUAL` or :const:`signal.ITIMER_PROF`) specified 168 by *which* to fire after *seconds* (float is accepted, different from 169 :func:`alarm`) and after that every *interval* seconds. The interval 170 timer specified by *which* can be cleared by setting seconds to zero. 171 172 When an interval timer fires, a signal is sent to the process. 173 The signal sent is dependent on the timer being used; 174 :const:`signal.ITIMER_REAL` will deliver :const:`SIGALRM`, 175 :const:`signal.ITIMER_VIRTUAL` sends :const:`SIGVTALRM`, 176 and :const:`signal.ITIMER_PROF` will deliver :const:`SIGPROF`. 177 178 The old values are returned as a tuple: (delay, interval). 179 180 Attempting to pass an invalid interval timer will cause an 181 :exc:`ItimerError`. Availability: Unix. 182 183 .. versionadded:: 2.6 184 185 186.. function:: getitimer(which) 187 188 Returns current value of a given interval timer specified by *which*. 189 Availability: Unix. 190 191 .. versionadded:: 2.6 192 193 194.. function:: set_wakeup_fd(fd) 195 196 Set the wakeup fd to *fd*. When a signal is received, a ``'\0'`` byte is 197 written to the fd. This can be used by a library to wakeup a poll or select 198 call, allowing the signal to be fully processed. 199 200 The old wakeup fd is returned (or -1 if file descriptor wakeup was not 201 enabled). If *fd* is -1, file descriptor wakeup is disabled. 202 If not -1, *fd* must be non-blocking. It is up to the library to remove 203 any bytes from *fd* before calling poll or select again. 204 205 When threads are enabled, this function can only be called from the main thread; 206 attempting to call it from other threads will cause a :exc:`ValueError` 207 exception to be raised. 208 209 .. versionadded:: 2.6 210 211 212.. function:: siginterrupt(signalnum, flag) 213 214 Change system call restart behaviour: if *flag* is :const:`False`, system 215 calls will be restarted when interrupted by signal *signalnum*, otherwise 216 system calls will be interrupted. Returns nothing. Availability: Unix (see 217 the man page :manpage:`siginterrupt(3)` for further information). 218 219 Note that installing a signal handler with :func:`signal` will reset the 220 restart behaviour to interruptible by implicitly calling 221 :c:func:`siginterrupt` with a true *flag* value for the given signal. 222 223 .. versionadded:: 2.6 224 225 226.. function:: signal(signalnum, handler) 227 228 Set the handler for signal *signalnum* to the function *handler*. *handler* can 229 be a callable Python object taking two arguments (see below), or one of the 230 special values :const:`signal.SIG_IGN` or :const:`signal.SIG_DFL`. The previous 231 signal handler will be returned (see the description of :func:`getsignal` 232 above). (See the Unix man page :manpage:`signal(2)`.) 233 234 When threads are enabled, this function can only be called from the main thread; 235 attempting to call it from other threads will cause a :exc:`ValueError` 236 exception to be raised. 237 238 The *handler* is called with two arguments: the signal number and the current 239 stack frame (``None`` or a frame object; for a description of frame objects, 240 see the :ref:`description in the type hierarchy <frame-objects>` or see the 241 attribute descriptions in the :mod:`inspect` module). 242 243 On Windows, :func:`signal` can only be called with :const:`SIGABRT`, 244 :const:`SIGFPE`, :const:`SIGILL`, :const:`SIGINT`, :const:`SIGSEGV`, or 245 :const:`SIGTERM`. A :exc:`ValueError` will be raised in any other case. 246 247 248.. _signal-example: 249 250Example 251------- 252 253Here is a minimal example program. It uses the :func:`alarm` function to limit 254the time spent waiting to open a file; this is useful if the file is for a 255serial device that may not be turned on, which would normally cause the 256:func:`os.open` to hang indefinitely. The solution is to set a 5-second alarm 257before opening the file; if the operation takes too long, the alarm signal will 258be sent, and the handler raises an exception. :: 259 260 import signal, os 261 262 def handler(signum, frame): 263 print 'Signal handler called with signal', signum 264 raise IOError("Couldn't open device!") 265 266 # Set the signal handler and a 5-second alarm 267 signal.signal(signal.SIGALRM, handler) 268 signal.alarm(5) 269 270 # This open() may hang indefinitely 271 fd = os.open('/dev/ttyS0', os.O_RDWR) 272 273 signal.alarm(0) # Disable the alarm 274 275