Guile's threads and mutex

[Sascha Ziemann <szi@aibon.ping.de>]

Hi,

Guiles documentation says about mutex:

 - primitive: unlock-mutex MUTEX
     Unlocks MUTEX if the calling thread owns the lock on MUTEX.
     Calling unlock-mutex on a mutex not owned by the current thread
     results in undefined behaviour. Once a mutex has been unlocked,
     one thread blocked on MUTEX is awakened and grabs the mutex lock.

When I understand this right, it means, that I can not use a mutex for
flow control between two threads.

I have two threads A and B and a queue, which can be written to by
thread A (and also by some other threads) and read from by thread
B. Thread B should block, when the queue is empty. When I want to use
a emtpy-mutex, it must be locked by the init thread, which creates the
mutex, because in the beginning the queue is empty. By reading the
queue thread B blocks. But it is not possible to unlock the
empty-mutex, because it is likely that the thread writing to the queue
is not the thread, who created the mutex.

How can I get such a "select" functionality for Guile threads?

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