Both processes and threads are independent sequences of execution. The typical difference is that threads (of the same process) run in a shared memory space, while processes run in separate memory spaces.
I'm not sure what "hardware" vs "software" threads you might be referring to. Threads are an operating environment feature, rather than a CPU feature (though the CPU typically has operations that make threads efficient).
Erlang uses the term "process" because it does not expose a shared-memory multiprogramming model. Calling them "threads" would imply that they have shared memory.
It is generally a bad pattern to kill a thread abruptly, in Python, and in any language. Think of the following cases:
- the thread is holding a critical resource that must be closed properly
- the thread has created several other threads that must be killed as well.
The nice way of handling this, if you can afford it (if you are managing your own threads), is to have an exit_request flag that each thread checks on a regular interval to see if it is time for it to exit.
"""Thread class with a stop() method. The thread itself has to check
regularly for the stopped() condition."""
def __init__(self, *args, **kwargs):
super(StoppableThread, self).__init__(*args, **kwargs)
self._stop_event = threading.Event()
In this code, you should call
stop() on the thread when you want it to exit, and wait for the thread to exit properly using
join(). The thread should check the stop flag at regular intervals.
There are cases, however, when you really need to kill a thread. An example is when you are wrapping an external library that is busy for long calls, and you want to interrupt it.
The following code allows (with some restrictions) to raise an Exception in a Python thread:
def _async_raise(tid, exctype):
'''Raises an exception in the threads with id tid'''
if not inspect.isclass(exctype):
raise TypeError("Only types can be raised (not instances)")
res = ctypes.pythonapi.PyThreadState_SetAsyncExc(ctypes.c_long(tid),
if res == 0:
raise ValueError("invalid thread id")
elif res != 1:
# "if it returns a number greater than one, you're in trouble,
# and you should call it again with exc=NULL to revert the effect"
raise SystemError("PyThreadState_SetAsyncExc failed")
'''A thread class that supports raising an exception in the thread from
"""determines this (self's) thread id
CAREFUL: this function is executed in the context of the caller
thread, to get the identity of the thread represented by this
if not self.isAlive():
raise threading.ThreadError("the thread is not active")
# do we have it cached?
if hasattr(self, "_thread_id"):
# no, look for it in the _active dict
for tid, tobj in threading._active.items():
if tobj is self:
self._thread_id = tid
# TODO: in python 2.6, there's a simpler way to do: self.ident
raise AssertionError("could not determine the thread's id")
def raiseExc(self, exctype):
"""Raises the given exception type in the context of this thread.
If the thread is busy in a system call (time.sleep(),
socket.accept(), ...), the exception is simply ignored.
If you are sure that your exception should terminate the thread,
one way to ensure that it works is:
t = ThreadWithExc( ... )
t.raiseExc( SomeException )
time.sleep( 0.1 )
t.raiseExc( SomeException )
If the exception is to be caught by the thread, you need a way to
check that your thread has caught it.
CAREFUL: this function is executed in the context of the
caller thread, to raise an exception in the context of the
thread represented by this instance.
_async_raise( self._get_my_tid(), exctype )
(Based on Killable Threads by Tomer Filiba. The quote about the return value of
PyThreadState_SetAsyncExc appears to be from an old version of Python.)
As noted in the documentation, this is not a magic bullet because if the thread is busy outside the Python interpreter, it will not catch the interruption.
A good usage pattern of this code is to have the thread catch a specific exception and perform the cleanup. That way, you can interrupt a task and still have proper cleanup.
You don't need to join a thread, but it is a good idea. Without calling pthread_join(), there is a possibility that the main() function will return before the thread terminates. In this case, pthread_join() makes the application wait until the other thread finishes processing. Plus, when you join the thread, it gives you the opportunity to check for return values and make sure that everything went smoothly, and it gives you the opportunity to clean up any resources you may have shared with the thread.
EDIT: A function that may be of interest to you is pthread_detach(). pthread_detach() allows the thread's storage to be cleaned up after the thread terminates, so there is no need to join the thread afterwards.