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

[NaeiKinDus]

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
generated.

********************************************************
step 1: initialize the mutex

mx (the mutex is part of the class "cl")

class Client
{
...

public:
pthread_mutex_t mx;

...
}

pthread_mutex_init(&cl->mx, NULL);
********************************************************

********************************************************
step 2:

as i'm calling a thread, i have to pass in param a structure with the
mutex :

struct S_PARAMS
{
...

pthread_mutex_t *mx;

...
} typedef T_PARAMS;

i create the thread and i copy the mutex in the param structure :

pthread_t thread;
params->mx = &cl->mx;

if ((thId = pthread_create(&thread, NULL, OBody::thFileSender, (void
*)params)) != 0)
{
THREAD NOT CREATED ... throw ...
}
else
{
I SET SOME FLAG TO KNOW MY THREAD HAS BEEN CREATED
}
********************************************************

********************************************************
step 3:

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)
}

********************************************************

********************************************************
step 4:

once i know my functions generating data are done, i call the thread
unlocking function.


cout << "*** before unlock ***" << endl;
pthread_mutex_unlock(&cl.mx);
cout << "*** after unlock ***" << endl;

********************************************************


Finally i get :

********************************************************

....

locking MUTX*****
0
unlocked MUTX *****

.... (generated data)


*** before unlock ***
server: Error detected by libpthread: Unlocking mutex owned by another
thread.
Detected by file "/root/netbsd-3.0/src/lib/libpthread/
pthread_mutex.c", line 347, function "pthread_mutex_unlock".
See pthread(3) for information.

....

********************************************************


What i understand is that the mutex is not locking the thread because
in the step3,
the "cout << "locking MUTX*****" << endl;" is done when is start the
program.

I put a sleep(10) in order to be sure that the thread was still
running without calling thread_unlock.

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

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.

Thank you for your help.

 

[James Kanze]

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
generated.

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"
linkage.

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
exception.

[...]

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.)

 

[NaeiKinDus]

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

Rephrased: I had to block a thread until the calling process says it
can do what it's meant to do.

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"
linkage.

Nevermind. Just a noob thing a mate did (we're working in group ) -_-'
By the way, why does the function actually works ? It was successfully
called !
Anyway... Fixed.

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.

The "Unlocked" cout was supposed to appear when the thread would have
been released...

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.

[...]

Got it Thanks, we'll work on it !

So you know exactly what your error is.


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.)

Again, thanks for your help !

 

[James Kanze]

Rephrased: I had to block a thread until the calling process says it
can do what it's meant to do.

You mean you start a thread, just so that it can wait until you
decide it can work. In that case, you need for the calling
thread to lock the mutex before starting the thread, then unlock
it when its OK. The called thread tries acquire the mutex,
blocking until the calling thread releases it. Just don't
forget to have the called thread release the mutex too, when it
is through.

Still, I would definitly consider just putting off starting the
thread until it can do what it's meant to do.

Nevermind. Just a noob thing a mate did (we're working in group ) -_-'
By the way, why does the function actually works ? It was successfully
called !

I don't know. As I said, to begin with, supposing that
thFileSender is a member function, the code shouldn't even
compile; you can't pass a member function as a parameter,
period, and there's no implicit conversion of member function to
pointer. (I think VC++ did implicitly convert a member function
to a pointer without a &.)

The second thing is that the type of the function is wrong. If
the function is not static, of course, the type is way, way
wrong, and I can't imagine it ever working (but again, I've
heard about some wierd behavior in VC++). But even if the
member function is static, it still won't be `extern "C"'. Now,
Posix doesn't define a C++ binding, and C++ doesn't say anything
about pthread_create, so it's up to the implementation. But in
all the implementations I know (Solaris and Linux),
pthread_create is NOT overloaded, and requires a function with
"C" linkage. The compiler should complain. (Sun CC does,
although it is only a warning, and not an error. G++ doesn't;
this is a bug in g++.)

Anyway... Fixed.

The "Unlocked" cout was supposed to appear when the thread would have
been released...

Before going any further, you're going to have to read a general
book about threading, at least enough to know the basic
vocabulary. What do you mean by "released"? A thread is
started, it runs, and it exits (terminates). If it hasn't been
detached, someone, somewhere, must join with it. But it's never
"released".