This algorithm uses semaphores to solve the first readers-writers problem.
1 var wrt, mutex: semaphore; 2 readcount: integer; 3 begin 4 readcount := 0; 5 wrt := 1; 6 mutex := 1; 7 parbegin 8 repeat (* reader process *) 9 (* do something *) 10 down(mutex); 11 readcount := readcount + 1; 12 if readcount = 1 then 13 down(wrt); 14 up(mutex); 15 (* read the file *) 16 down(mutex); 17 readcount := readcount - 1; 18 if readcount = 0 then 19 up(wrt); 20 up(mutex); 21 (* do something else *) 22 until false; 23 repeat (* writer process *) 24 (* do something *) 25 down(wrt); 26 (* write to the file *) 27 up(wrt); 28 (* do something else *) 29 until false; 30 parend; 31 end.
lines 1-2 Here, readcount contains the number of processes reading the file, and mutex is a semaphore used to provide mutual exclusion when readcount is incremented or decremented. The semaphore wrt is common to both readers and writers and ensures that when one writer is accessing the file, no other readers or writers may do so.
lines 4-6 This just initializes all the semaphores. It is the only time anything other than a down or an up operation may be done to them. As no readers are yet reading the file, readcount is initialized to 0.
line 9 Since the file is not accessed here, we don't need to put semaphores around this part.
lines 10-15 Since the value of the shared variable readcount is going to be changed, the process must wait until no-one else is accessing it (down(mutex)). Since this process will read from the file, readcount is incremented by 1; if this is the only reader that will access the file, it waits until any writers have finished (down(wrt)). It then indicates other processes may access readcount (down(mutex)) and proceeds to read from the file.
lines 16-20 Now the reader is done reading the file (for now.) It must update the value of readcount to indicate this, so it waits until no-one else is accessing that variable (down(mutex)) and then decrements readcount. If no other readers are waiting to read (readcount = 0), it signals that any reader or writer who wishes to access the file may do so (up(wrt)). Finally, it indicates it is done with readcount (up(mutex)).
line 24 Since the file is not accessed here, we don't need to put semaphores around this part.
lines 25-26 The writer process waits (down(wrt)) until no other process is accessing the file; it then proceeds to write to the file.
line 27 When the writer is done writing to the file, it signals that anyone who wishes to access the file may do so (up(wrt)).
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