CS 433 Operating Systems

Lab4 : Interprocess Synchronization by Semaphores

Out date : 4/15/04
In date : 5/6/04

1. Procedure: In this assignment you will design and implement the following:

a) semaphore object : It has allocated and value fields (int) and a process queue

b) Semaphore table that has N_SEM elements

b) Operations on the semaphore object :

-int make_sem(int count) : Allocates space for a semaphore in the semaphore table and initialises a semaphore to the count value. Returns the id of the semaphore made or error.

-int wait_sem(int sid): Decrements semaphore count and blocks the caller if value < 0

-int signal_sem(int sid): Increments semaphore count and unblocks the caller if value >= 0

-int del_sem(int sid) : deletes a semaphore from the semaphore table

2. Test :

i. Initialization : Make a pcb_table of 10 entries. Make 2 system processes called Producer and Consumer. Make a semaphore table of 10 semaphores. Make three semaphores called empty, full, cs initialized to N_BUF, 0 and 1 respectively.

ii. This is the example in the text book for producer/consumer problem. Producer process reads from a file (sample.txt on the course homepage) and writes its data into a bounded buffer of size 10 (N_BUF) , one character at a time,until the end of the file.The Producer starts by checking the empty semaphore to see if there is any space in the circular buffer. It then has to perform writing to the buffer in a critical section guarded by the cs semaphore. Finally it signals the Consumer process.

iii. The Consumer process does the reverse things. It waits on the full semaphore to check if there is any available data. It then reads from the buffer in a critical section. Normally, reading would not require critical section but we are manipulating pointers (read and write). It prints whatever it has read to the screen and then signals the Producer by the empty semaphore meaning there is one more free buffer (the one just read).