two semaphors, two functions and one function only is allowed to work at the same time

Question

I have the following exercise:

The given program snippet contains two functions mywrite() and myread(), which are in the same Write file or read from file. These two functions are used by a lot of threads called again and again. It should be ensured that at the same time either only read or can be written. Only one writing thread is permitted at a time will. Multiple reading threads are allowed at the same time. Complete the given program using only the given ones for your solution binary semaphores S and T, and the integer variable c. Your solution does not have to be fair be. For the initialization of a semaphore you can e.g. e.g. use S:=0, this means that the semaphore is busy. Use the functions introduced in the lecture: S.acquire() or S.release().

and the following solution:

Semaphor S := 1;
Semaphor T := 1;

int c = 0;
void mywrite()
{
    T.acquire();
    write_data_to_file();
    T.release();
}

void myread()
{
    S.acquire();
    if (c == 0)
    {
        T.acquire()
    }
    c++;
    S.release();
    read_data_from_file();
    S.acquire();
    c--;
    if (c == 0)
    {
        T.release()
    }
    S.release();
}

I do not know, why T.acquire() comes after S.acquire() in the myread() function. the counter increments, but why? the mywrite() function never checks the value of c.

Answer 1

S provides mutually exclusive access to c.

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Each writer acquires and releases T.

T is also acquired by a reader when there are no other active readers. It is released when there are no more active readers.

This ensures:

• Would-be writers wait when there's an active writer.
• Would-be readers wait when there's an active writer.
• Would-be writers wait when there's one or more active readers.

c is the count of active readers. A reader uses c to determine if there are other active readers. In other words, a reader uses c to determine if it needs to acquire T, and if it should release T.

You can't have multiple threads accessing c simultaneously, so some form of mutual exclusion is needed around access of c. S is used to provide that functionality. S isn't acquired before T is acquired so much as S is acquired before c is accessed.