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Subject: [boost] [threads] two threads gain lock on the same mutex??
From: David M. Cotter (me_at_[hidden])
Date: 2010-01-02 16:43:56

i thought this was impossible. but i've caught it in the debugger *right now*!

on thread 28, i've got this code:

boost::mutex::scoped_lock lock(i_mutex);

i_waitingB = true;

if (i_semID) {
        if (!i_signaledB) {
                boost::xtime xt;

                if (wait_duration == kDurationForever) {
                        xt = boost::xdelay(kEventDurationDay);
                } else {
                        xt = boost::xdelay(0, 0, kNanoPerMicro * wait_duration); // wait_duration is in microseconds

PC --> if (!i_semID->timed_wait(lock, xt)) {
                        err = kMPTimeoutErr;
        if (!err) {
                i_signaledB = false;
} else {
        err = kMPDeletedErr;

the PC is waiting (for 24 hours) in the timed_wait() call (wait_duration = kDurationForever)

that thread it just hanging, which is correct.

however on thread 30 i've got this:

if (!err) {
        boost::mutex::scoped_lock lock(i_mutex);
PC --> i_signaledB = true; < breakpoint set here, now hit

that should be impossible, right? the PC should in fact block on that scoped_lock, right??
yes: the mutex IS the same mutex on both threads
the i_mutex is of type:
            boost::mutex i_mutex;

it is not a shared mutex

i realize that if the scoped_lock were actually functioning as expected, that the signal could never be sent to the semaphore to wake up. i'll fix the code design after i figure out WHY the lock is not actually locking.
but the thread WAS waking up, my program is actually functioning as it should, but given the above code it should NOT be working.

what's going on here?

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