195 lines
6.9 KiB
ReStructuredText
195 lines
6.9 KiB
ReStructuredText
:mod:`_thread` --- Low-level threading API
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==========================================
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.. module:: _thread
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:synopsis: Low-level threading API.
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.. index::
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single: light-weight processes
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single: processes, light-weight
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single: binary semaphores
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single: semaphores, binary
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This module provides low-level primitives for working with multiple threads
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(also called :dfn:`light-weight processes` or :dfn:`tasks`) --- multiple threads of
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control sharing their global data space. For synchronization, simple locks
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(also called :dfn:`mutexes` or :dfn:`binary semaphores`) are provided.
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The :mod:`threading` module provides an easier to use and higher-level
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threading API built on top of this module.
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.. index::
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single: pthreads
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pair: threads; POSIX
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The module is optional. It is supported on Windows, Linux, SGI IRIX, Solaris
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2.x, as well as on systems that have a POSIX thread (a.k.a. "pthread")
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implementation. For systems lacking the :mod:`_thread` module, the
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:mod:`_dummy_thread` module is available. It duplicates this module's interface
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and can be used as a drop-in replacement.
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It defines the following constants and functions:
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.. exception:: error
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Raised on thread-specific errors.
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.. versionchanged:: 3.3
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This is now a synonym of the built-in :exc:`RuntimeError`.
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.. data:: LockType
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This is the type of lock objects.
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.. function:: start_new_thread(function, args[, kwargs])
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Start a new thread and return its identifier. The thread executes the function
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*function* with the argument list *args* (which must be a tuple). The optional
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*kwargs* argument specifies a dictionary of keyword arguments. When the function
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returns, the thread silently exits. When the function terminates with an
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unhandled exception, a stack trace is printed and then the thread exits (but
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other threads continue to run).
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.. function:: interrupt_main()
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Raise a :exc:`KeyboardInterrupt` exception in the main thread. A subthread can
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use this function to interrupt the main thread.
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.. function:: exit()
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Raise the :exc:`SystemExit` exception. When not caught, this will cause the
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thread to exit silently.
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..
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function:: exit_prog(status)
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Exit all threads and report the value of the integer argument
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*status* as the exit status of the entire program.
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**Caveat:** code in pending :keyword:`finally` clauses, in this thread
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or in other threads, is not executed.
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.. function:: allocate_lock()
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Return a new lock object. Methods of locks are described below. The lock is
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initially unlocked.
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.. function:: get_ident()
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Return the 'thread identifier' of the current thread. This is a nonzero
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integer. Its value has no direct meaning; it is intended as a magic cookie to
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be used e.g. to index a dictionary of thread-specific data. Thread identifiers
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may be recycled when a thread exits and another thread is created.
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.. function:: stack_size([size])
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Return the thread stack size used when creating new threads. The optional
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*size* argument specifies the stack size to be used for subsequently created
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threads, and must be 0 (use platform or configured default) or a positive
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integer value of at least 32,768 (32kB). If changing the thread stack size is
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unsupported, a :exc:`ThreadError` is raised. If the specified stack size is
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invalid, a :exc:`ValueError` is raised and the stack size is unmodified. 32kB
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is currently the minimum supported stack size value to guarantee sufficient
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stack space for the interpreter itself. Note that some platforms may have
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particular restrictions on values for the stack size, such as requiring a
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minimum stack size > 32kB or requiring allocation in multiples of the system
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memory page size - platform documentation should be referred to for more
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information (4kB pages are common; using multiples of 4096 for the stack size is
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the suggested approach in the absence of more specific information).
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Availability: Windows, systems with POSIX threads.
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.. data:: TIMEOUT_MAX
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The maximum value allowed for the *timeout* parameter of
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:meth:`Lock.acquire`. Specifying a timeout greater than this value will
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raise an :exc:`OverflowError`.
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.. versionadded:: 3.2
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Lock objects have the following methods:
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.. method:: lock.acquire(waitflag=1, timeout=-1)
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Without any optional argument, this method acquires the lock unconditionally, if
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necessary waiting until it is released by another thread (only one thread at a
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time can acquire a lock --- that's their reason for existence).
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If the integer *waitflag* argument is present, the action depends on its
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value: if it is zero, the lock is only acquired if it can be acquired
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immediately without waiting, while if it is nonzero, the lock is acquired
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unconditionally as above.
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If the floating-point *timeout* argument is present and positive, it
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specifies the maximum wait time in seconds before returning. A negative
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*timeout* argument specifies an unbounded wait. You cannot specify
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a *timeout* if *waitflag* is zero.
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The return value is ``True`` if the lock is acquired successfully,
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``False`` if not.
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.. versionchanged:: 3.2
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The *timeout* parameter is new.
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.. versionchanged:: 3.2
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Lock acquires can now be interrupted by signals on POSIX.
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.. method:: lock.release()
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Releases the lock. The lock must have been acquired earlier, but not
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necessarily by the same thread.
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.. method:: lock.locked()
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Return the status of the lock: ``True`` if it has been acquired by some thread,
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``False`` if not.
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In addition to these methods, lock objects can also be used via the
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:keyword:`with` statement, e.g.::
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import _thread
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a_lock = _thread.allocate_lock()
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with a_lock:
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print("a_lock is locked while this executes")
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**Caveats:**
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.. index:: module: signal
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* Threads interact strangely with interrupts: the :exc:`KeyboardInterrupt`
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exception will be received by an arbitrary thread. (When the :mod:`signal`
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module is available, interrupts always go to the main thread.)
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* Calling :func:`sys.exit` or raising the :exc:`SystemExit` exception is
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equivalent to calling :func:`_thread.exit`.
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* Not all built-in functions that may block waiting for I/O allow other threads
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to run. (The most popular ones (:func:`time.sleep`, :meth:`file.read`,
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:func:`select.select`) work as expected.)
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* It is not possible to interrupt the :meth:`acquire` method on a lock --- the
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:exc:`KeyboardInterrupt` exception will happen after the lock has been acquired.
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* When the main thread exits, it is system defined whether the other threads
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survive. On most systems, they are killed without executing
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:keyword:`try` ... :keyword:`finally` clauses or executing object
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destructors.
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* When the main thread exits, it does not do any of its usual cleanup (except
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that :keyword:`try` ... :keyword:`finally` clauses are honored), and the
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standard I/O files are not flushed.
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