mbed #974ecf7 mutex not working (tested on K64F)

[marcuschangarm]

I'm using a mutex to protect access to a variable in a different thread. In the example below id should only be readable after it has been assigned a thread id, however, on a K64F the printf outputs 0 instead of the actual ID.

#include "mbed.h"
#include "rtos.h"

osThreadId id = 0;
Mutex mutex;

void threadFunction(const void* args)
{
    id = Thread::gettid();
    mutex.unlock();
}

int main()
{
    mutex.lock();
    Thread thread(threadFunction);

    mutex.lock();
    printf("ID: %p\r\n", id);

    for(;;)
    {
        wait(1);
    }
}

The equivalent using semaphores does produce the correct result:

#include "mbed.h"
#include "rtos.h"

osThreadId id = 0;
Semaphore semaphore(1);

void threadFunction(const void* args)
{
    id = Thread::gettid();
    semaphore.release();
}

int main()
{
    semaphore.wait();
    Thread thread(threadFunction);

    semaphore.wait();
    printf("ID: %p\r\n", id);

    for(;;)
    {
        wait(1);
    }
}

[saedelman]

A bit late, but you are not using Mutex correctly. You acquire the lock with the first mutex.lock(), but the second mutex.lock() does not block because the lock is already held by the main thread. Mutex is meant to synchronize between threads, so calling lock() multiple times from the same thread never blocks. I would have a look at using Thread::signal_set and Thread::signal_wait() instead.