Readers and writer programming with semaphores.

The C program below is from our textbook.
Chapter-5 pg. 271 in my textbook.
Page 243 in the 9th Edition hard-bound textbook.

The semaphores are  similar to POSIX .

semWait() means to wait for sem to be greater than zero, then decrement.
semSignal() means increment the semaphore.

Readers have priority...


int readcount = 0;
semaphore x = 1;
semaphore wsem = 1;

void reader()
{
    while (true) {
        semWait(x);
        readcount++;
        if (readcount == 1)
            semWait(wsem);
        semSignal(x);
        READUNIT();
        semWait(x);
        readcount--;
        if (readcount == 0)
            semSignal(wsem);
        semSignal(x);
    }
}

void writer()
{
    while (true) {
        semWait(wsem);
        WRITEUNIT();
        semSignal(wsem);
    }
}

void main()
{
    readcount = 0;
    parbegin(reader, writer);
}