1 :mod:`_thread` --- Low-level threading API 2 ========================================== 3 4 .. module:: _thread 5 :synopsis: Low-level threading API. 6 7 .. index:: 8 single: light-weight processes 9 single: processes, light-weight 10 single: binary semaphores 11 single: semaphores, binary 12 13 -------------- 14 15 This module provides low-level primitives for working with multiple threads 16 (also called :dfn:`light-weight processes` or :dfn:`tasks`) --- multiple threads of 17 control sharing their global data space. For synchronization, simple locks 18 (also called :dfn:`mutexes` or :dfn:`binary semaphores`) are provided. 19 The :mod:`threading` module provides an easier to use and higher-level 20 threading API built on top of this module. 21 22 .. index:: 23 single: pthreads 24 pair: threads; POSIX 25 26 The module is optional. It is supported on Windows, Linux, SGI IRIX, Solaris 27 2.x, as well as on systems that have a POSIX thread (a.k.a. "pthread") 28 implementation. For systems lacking the :mod:`_thread` module, the 29 :mod:`_dummy_thread` module is available. It duplicates this module's interface 30 and can be used as a drop-in replacement. 31 32 It defines the following constants and functions: 33 34 35 .. exception:: error 36 37 Raised on thread-specific errors. 38 39 .. versionchanged:: 3.3 40 This is now a synonym of the built-in :exc:`RuntimeError`. 41 42 43 .. data:: LockType 44 45 This is the type of lock objects. 46 47 48 .. function:: start_new_thread(function, args[, kwargs]) 49 50 Start a new thread and return its identifier. The thread executes the function 51 *function* with the argument list *args* (which must be a tuple). The optional 52 *kwargs* argument specifies a dictionary of keyword arguments. When the function 53 returns, the thread silently exits. When the function terminates with an 54 unhandled exception, a stack trace is printed and then the thread exits (but 55 other threads continue to run). 56 57 58 .. function:: interrupt_main() 59 60 Raise a :exc:`KeyboardInterrupt` exception in the main thread. A subthread can 61 use this function to interrupt the main thread. 62 63 64 .. function:: exit() 65 66 Raise the :exc:`SystemExit` exception. When not caught, this will cause the 67 thread to exit silently. 68 69 .. 70 function:: exit_prog(status) 71 72 Exit all threads and report the value of the integer argument 73 *status* as the exit status of the entire program. 74 **Caveat:** code in pending :keyword:`finally` clauses, in this thread 75 or in other threads, is not executed. 76 77 78 .. function:: allocate_lock() 79 80 Return a new lock object. Methods of locks are described below. The lock is 81 initially unlocked. 82 83 84 .. function:: get_ident() 85 86 Return the 'thread identifier' of the current thread. This is a nonzero 87 integer. Its value has no direct meaning; it is intended as a magic cookie to 88 be used e.g. to index a dictionary of thread-specific data. Thread identifiers 89 may be recycled when a thread exits and another thread is created. 90 91 92 .. function:: stack_size([size]) 93 94 Return the thread stack size used when creating new threads. The optional 95 *size* argument specifies the stack size to be used for subsequently created 96 threads, and must be 0 (use platform or configured default) or a positive 97 integer value of at least 32,768 (32 KiB). If *size* is not specified, 98 0 is used. If changing the thread stack size is 99 unsupported, a :exc:`RuntimeError` is raised. If the specified stack size is 100 invalid, a :exc:`ValueError` is raised and the stack size is unmodified. 32 KiB 101 is currently the minimum supported stack size value to guarantee sufficient 102 stack space for the interpreter itself. Note that some platforms may have 103 particular restrictions on values for the stack size, such as requiring a 104 minimum stack size > 32 KiB or requiring allocation in multiples of the system 105 memory page size - platform documentation should be referred to for more 106 information (4 KiB pages are common; using multiples of 4096 for the stack size is 107 the suggested approach in the absence of more specific information). 108 Availability: Windows, systems with POSIX threads. 109 110 111 .. data:: TIMEOUT_MAX 112 113 The maximum value allowed for the *timeout* parameter of 114 :meth:`Lock.acquire`. Specifying a timeout greater than this value will 115 raise an :exc:`OverflowError`. 116 117 .. versionadded:: 3.2 118 119 120 Lock objects have the following methods: 121 122 123 .. method:: lock.acquire(waitflag=1, timeout=-1) 124 125 Without any optional argument, this method acquires the lock unconditionally, if 126 necessary waiting until it is released by another thread (only one thread at a 127 time can acquire a lock --- that's their reason for existence). 128 129 If the integer *waitflag* argument is present, the action depends on its 130 value: if it is zero, the lock is only acquired if it can be acquired 131 immediately without waiting, while if it is nonzero, the lock is acquired 132 unconditionally as above. 133 134 If the floating-point *timeout* argument is present and positive, it 135 specifies the maximum wait time in seconds before returning. A negative 136 *timeout* argument specifies an unbounded wait. You cannot specify 137 a *timeout* if *waitflag* is zero. 138 139 The return value is ``True`` if the lock is acquired successfully, 140 ``False`` if not. 141 142 .. versionchanged:: 3.2 143 The *timeout* parameter is new. 144 145 .. versionchanged:: 3.2 146 Lock acquires can now be interrupted by signals on POSIX. 147 148 149 .. method:: lock.release() 150 151 Releases the lock. The lock must have been acquired earlier, but not 152 necessarily by the same thread. 153 154 155 .. method:: lock.locked() 156 157 Return the status of the lock: ``True`` if it has been acquired by some thread, 158 ``False`` if not. 159 160 In addition to these methods, lock objects can also be used via the 161 :keyword:`with` statement, e.g.:: 162 163 import _thread 164 165 a_lock = _thread.allocate_lock() 166 167 with a_lock: 168 print("a_lock is locked while this executes") 169 170 **Caveats:** 171 172 .. index:: module: signal 173 174 * Threads interact strangely with interrupts: the :exc:`KeyboardInterrupt` 175 exception will be received by an arbitrary thread. (When the :mod:`signal` 176 module is available, interrupts always go to the main thread.) 177 178 * Calling :func:`sys.exit` or raising the :exc:`SystemExit` exception is 179 equivalent to calling :func:`_thread.exit`. 180 181 * It is not possible to interrupt the :meth:`acquire` method on a lock --- the 182 :exc:`KeyboardInterrupt` exception will happen after the lock has been acquired. 183 184 * When the main thread exits, it is system defined whether the other threads 185 survive. On most systems, they are killed without executing 186 :keyword:`try` ... :keyword:`finally` clauses or executing object 187 destructors. 188 189 * When the main thread exits, it does not do any of its usual cleanup (except 190 that :keyword:`try` ... :keyword:`finally` clauses are honored), and the 191 standard I/O files are not flushed. 192 193
