1. Project 5: Minix Semaphores
Coordinator: J. Rhett Aultman
11/09/2005

2. Topics Semaphores System call semantics Providing atomicity
Design in MM and kernel

3. System Call Semantics Four system calls to implement:
semOpen( )
semP( )
semV( )
semClose( )
Each takes an integer key for the semaphore
Each returns an integer- 0 on success and a negative number (specified in proj. description) on failure

4. semOpen( ) Checks the key passed in:
If key denotes a semaphore in use:
Check semaphore's reference count
If less than 2, increment reference count
Else, return failure
Else, determine if all 16 semaphores are in use
If so, return failure
Else, initialize a semaphore and increment its reference count to 1 and return success

5. semClose( ) Checks the key passed in:
If no initialized semaphore associated with key, return failure
Else, locate semaphore and decrement reference count
If decremented count == 0, scrap the semaphore
return success

6. semP( ) Checks the key passed in:
If no semaphore associated with key, return failure
Else, perform the poll-wait-decrement-return process associated with Dijkstra's P( )
That is, if the semaphore's value is 0, wait and poll until the value is 1, then decrement the semaphore and return

7. semV( ) Checks the key passed in:
If no semaphore associated with key, return failure
Else, perform the increment associated with Dijkstra's V( )
That is, increment the semaphore's value
The semaphore's value should never be greater than 1

8. Hey! Where's the Atomicity?
Incrementing, decrementing, and testing a variable are not atomic
Hence special hardware-supported instructions
Are we using special instructions here? No!
Minix system calls provide atomicity
Rendezvous principle- one message at a time processed
Thus, each invocation of _syscall( ) to the MM is atomic
Ensured strict ordering of increments, decrements, and tests

9. But how do we wait?
Because each _syscall( ) is atomic, if you wait on the semaphore while in the MM, you will wait forever
Where is the waiting done? In the library!
Until the library returns, the application is not in control
Run the polling loop in the library; issue syscalls to the MM to check the state of the semaphore
Return to application only when the syscall reports that the semaphore's value was decremented and the critical section is clear

10. Semaphore bookkeeping
As in project 4, you must track the reference counts, keys, and values of 16 semaphores:
#define MAXSEM 16
struct _semelm {
int key;
int refcount;
int value;
} sem_table [MAXSEM];
Don't forget that this array will initially contain random bits and must be initialized!

11. Getting Started
First, implement the “plumbing”- writing the .s files, editing Makefiles, picking call numbers, etc.
Only once “plumbing” for all four system calls is complete should further implementation be done
Implement and test shmOpen( ) and shmClose( )
Implement shmV( ) and shmP( )