Re: problem mutex-thread "Unlocking mutex owned by another thread ???"

"James Kanze" <>
14 Apr 2007 16:43:27 -0700
On Apr 14, 7:11 pm, "NaeiKinDus" <> wrote:

Hello, i'm trying to program a thread that would be locked (by a
mutex) and that would only be unlocked once that a function
(generating data) is done. The purpose is to generate data, and unlock
the mutex in order to activate the thread once the data is generated.
I have to do it this way, i can only call the thread if the data are

I'm not sure I understand that paragraph. However, there are
one or two things that look more than dubious...

if ((thId = pthread_create(&thread, NULL, OBody::thFileSender, (void
*)params)) != 0)

What are you passing as third pointer? It has to be a global
function, with `extern "C"' linkage. If OBody is a namespace,
this could be OK, but the rest of the code makes me think that
OBody is member function of a class, and there is no way that a
member function of a class can be passed as an argument to a
function, period, and there's also no way that it can have "C"

i lock my thread :

void *OBody::thFileSender(void *lpParam)
        cout << "locking MUTX*****" << endl;
        cout << pthread_mutex_lock(params->mx) << endl;
        cout << "unlocked MUTX *****" << endl;

        ... (sending data)


Where do you unlock it? (Your output says "unlocked MUTX", but
it looks to me like you just locked it.) After this operation,
the thread which has executed the pthread_mutex_lock owns the
mutex. No one else can unlock it, for rather obvious reasons.

In C++, the usual way of handling this is to use a RAII class;
that way, the mutex will be unlocked even if there is an


What i understand is that the mutex is not locking the thread because
in the step3,

I'm not sure what you mean by "the mutex locking the thread". A
thread acquires a mutex. Once a thread has acquired a mutex,
then any other thread which attempts to acquire it will block
until the thread holding the mutex releases
(pthread_mutex_unlock) it.


Moreover, when i try to unlock the thread, i get the error
"Unlocking mutex owned by another thread.".

So you know exactly what your error is.

all the mutex functions (init, lock, unlock) return 0 which mean that
the function are running correctly.

i have been working on this problem for hours and hours, i'm not a
specialist of the mutex/thread.

You might want to check out the Butenhof. It's the standard
reference for Posix threading. (I also recommend it for people
doing Windows programming, as it has enough general information
to be of use to them as well.)

James Kanze (Gabi Software) email:
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