CPP Multi-threading Notes

• Semaphore video MIT

Synchronization

• Precedence Relationships
• Mutual Exclusive relationships.

Mutex (Mutual Exclusion)

Alpha precedes Beta OR Beta precedes Alpha. (they cannot overlap execution)

    semaphore lock = 1;
    foo(int account, int amount)
    {
        wait(lock);
        t = balance[account];
        valance[account] = t - amount;
        signal(lock);
    }

Semaphore

Beta precedes Alpha.

Resources managed by semaphore:Characters in FIFO, Spaces in FIFO. (works only with single producer)

Shared Memory

char buf[N];
int in = 0, out = 0;
semaphore chars = 0;
semaphore mutex= 1; // allow one piece pf code, creates a "critical section" between

Only need shared Mutext if manipulating the ins and outs of the buffer(shared resource).

Producer

send (char c)
{
    wait(space);
    wait(mutex);
    buf[in] = c;
    in = (in + 1) % N;
    signal(chars);
    signal(mutex);
}

Consumer

char rcv()
{
    char c;
    wait(chars);
    wait(mutex);
    c = buff[out];
    out = (out + 1) % N;
    signal(mutex);
    signal(space);
    return c;
}