1. Today’s agenda Lab 1 Module 4: Process Management
Read Chapter 2 (2.8, 2.9)
Semaphores: wait(), signal(),semcreate(),semdelete(), …
Module 4: Process Management
Chap 4: List and Queue Manipulation
CS354-Fall2018

2. Lab 1: Producer-Consumer
Read Chapter 2.8, 2.9
ex4.c, ex5.c and ex6.c
two concurrent processes that each increments a shared integer n
Race condition (how to handle it?)
CS354-Fall2018

3. What if producer-consumer?
write
integer n
read
consumer
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4. How to synchronize independent processes?
Semaphore abstraction
wait()
signal()
semcreate()
semdelete()
Two semaphores:
One on which the consumer waits
One on which the producer waits
CS354-Fall2018

5. Ex5.c /* ex5.c - main, prod2, cons2 */ # include <xinu.h>
void prod2(sid32,sid32), cons2(sid32,sid32);
int32 n = 0; /* variable n has initial value zero */ /*
* main - producer and consumer processes sychronizaed with semaphores * */
void main(void) {
sid32 produced, consumed;
consumed = semcreate(0);
produced = semcreate(1);
resume (create (cons2, 1024, 20, "cons", 2, consumed, produced));
resume (create (prod2, 1024, 20, "prod", 2, consumed, produced)); }
CS354-Fall2018

6. * prod2 - Increment n 2000 times, waiting for it to be consumed. /*
* prod2 - Increment n 2000 times, waiting for it to be consumed. * */
  void prod2(
sid32 consumed,
sid32 produced) {
int32 i;
for (i=1; i<=2000; i++){
wait(consumed);
n++;
signal(produced); } /*
* cons2 - print n 2000 times, waiting for it to be produced * */
void cons2(
sid32 consumed,
sid32 produced) {
int32 i;
for (i=1; i<=2000; i++){
wait(produced);
printf("n is %d \n", n);
signal(consumed); }
CS354-Fall2018

7. Semaphores: Mutual Exclusion
Only one of two executing processes obtains access to a shared resource
what if not n? but a shared list?
Accessed by many processes
shared[n++] = item; /* n is updated over time */
CS354-Fall2018

8. # include <xinu.h>
/* ex6.c - additem */
# include <xinu.h>
sid32 mutex; /* assume initialized with semcreate */
int32 shared[100]; /* An array shared by many processes */
int32 n = 0; /* Count of items in the array */
int32 n = 0; /* variable n has initial value zero */ /*
* addiotem - obtain exclusive use of array shared and add an item to it. * */
void additem
(int32 item) {
wait(mutex);
shared[n++] = item;
signal(mutex); }
CS354-Fall2018

9. Lab1 https://www.cs.purdue.edu/homes/c s354/lab1/ Mutual exclusion
Circled buffer
Producer: insert into the buffer
Consumer: extract from the buffer
Multiple producers and consumers
CS354-Fall2018

10. Switch to Module 4
CS354-Fall2018

11. Chapter 4: List and Queue Manipulation
A linked list
A single data structure
A single set of nodes used by all levels of OS to maintain list of processes
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12. Doubly-linked List previous process key next Head Tail x - 4 2 - x 2514 …
greater than max key
Smaller than max key
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13. In Xinu a fixed upper bound on # of processes: NPROC
Relative pointer: each pointer occupies 4 bytes (32-bit system), address will be list[n] (continuous memory address)
Implicit data structure: queue table
queue.h, getitem.c, queue.c (see the codes)
CS354-Fall2018

14. /* queue.h - firstid, firstkey, isempty, lastkey, nonempty */
/* Queue structure declarations, constants, and inline functions */
/* Default # of queue entries: 1 per process plus 2 for ready list plus */
/* for sleep list plus 2 per semaphore */
#ifndef NQENT
#define NQENT (NPROC NSEM + NSEM)
#endif
#define EMPTY (-1) /* Null value for qnext or qprev index */
#define MAXKEY 0x7FFFFFFF /* Max key that can be stored in queue */
#define MINKEY 0x /* Min key that can be stored in queue */
struct qentry { /* One per process plus two per list */
int32 qkey; /* Key on which the queue is ordered */
qid16 qnext; /* Index of next process or tail */
qid16 qprev; /* Index of previous process or head */ };
extern struct qentry queuetab[];
/* Inline queue manipulation functions */
#define queuehead(q) (q)
#define queuetail(q) ((q) + 1)
#define firstid(q) (queuetab[queuehead(q)].qnext)
#define lastid(q) (queuetab[queuetail(q)].qprev)
#define isempty(q) (firstid(q) >= NPROC)
#define nonempty(q) (firstid(q) < NPROC)
#define firstkey(q) (queuetab[firstid(q)].qkey)
#define lastkey(q) (queuetab[ lastid(q)].qkey)
/* Inline to check queue id assumes interrupts are disabled */
#define isbadqid(x) (((int32)(x) < 0) || (int32)(x) >= NQENT-1)
CS354-Fall2018

15. /* getitem.c - getfirst, getlast, getitem */ #include <xinu.h>
#include <xinu.h> /*
* getfirst - Remove a process from the front of a queue * */
pid32 getfirst(
qid16 q /* ID of queue from which to */
) /* Remove a process (assumed */
/* valid with no check) */ {
pid32 head;
if (isempty(q)) {
return EMPTY; }
head = queuehead(q);
return getitem(queuetab[head].qnext); /*
* getlast - Remove a process from end of queue * */
pid32 getlast(
qid16 q /* ID of queue from which to */
) /* Remove a process (assumed */
/* valid with no check) */ {
pid32 tail;
if (isempty(q)) {
return EMPTY; }
tail = queuetail(q);
return getitem(queuetab[tail].qprev);
 /*
* getitem - Remove a process from an arbitrary point in a queue * */
pid32 getitem(
pid32 pid /* ID of process to remove */ ) {
pid32 prev, next;
next = queuetab[pid].qnext; /* Following node in list */
prev = queuetab[pid].qprev; /* Previous node in list */
queuetab[prev].qnext = next;
queuetab[next].qprev = prev;
return pid; }
CS354-Fall2018

16. FIFO Queue Manipulation
First-In-First-Out (FIFO): queue.c
enqueue() and dequeue()
CS354-Fall2018

17. #include <xinu.h>
struct qentry queuetab[NQENT]; /* Table of process queues */ /*
* enqueue - Insert a process at the tail of a queue * */
pid32 enqueue(
pid32 pid, /* ID of process to insert */
qid16 q /* ID of queue to use */ ) {
int tail, prev; /* Tail & previous node indexes */
if (isbadqid(q) || isbadpid(pid)) {
return SYSERR; }
tail = queuetail(q);
prev = queuetab[tail].qprev;
queuetab[pid].qnext = tail; /* Insert just before tail node */
queuetab[pid].qprev = prev;
queuetab[prev].qnext = pid;
queuetab[tail].qprev = pid;
return pid; /*
* dequeue - Remove and return the first process on a list * */
pid32 dequeue(
qid16 q /* ID queue to use */ ) {
pid32 pid; /* ID of process removed */
if (isbadqid(q)) {
return SYSERR;
} else if (isempty(q)) {
return EMPTY; }
pid = getfirst(q);
queuetab[pid].qprev = EMPTY;
queuetab[pid].qnext = EMPTY;
return pid;
CS354-Fall2018

18. How about Priority Queue?
FIFO is not the sole way
Process manager may select from a set of processes the one with highest priority
Priority: as a qkey in the list
Queue sorted by qkey
Delete(): easy (just delete it)
Insert(): searches for the one that is just below qkey
See insert.c
CS354-Fall2018

19. insert.c /*
* insert - Insert a process into a queue in descending key order * */
status insert(
pid32 pid, /* ID of process to insert */
qid16 q, /* ID of queue to use */
int32 key /* Key for the inserted process */ ) {
int16 curr; /* Runs through items in a queue*/
int16 prev; /* Holds previous node index */
…. /* ignored */
curr = firstid(q);
while (queuetab[curr].qkey >= key) {
curr = queuetab[curr].qnext; }
/* Insert process between curr node and previous node */
prev = queuetab[curr].qprev; /* Get index of previous node */
queuetab[pid].qnext = curr;
queuetab[pid].qprev = prev;
queuetab[pid].qkey = key;
queuetab[prev].qnext = pid;
queuetab[curr].qprev = pid;
return OK;
/* insert.c - insert */
#include <xinu.h> /*
* insert - Insert a process into a queue in descending key order * */
status insert(
pid32 pid, /* ID of process to insert */
qid16 q, /* ID of queue to use */
int32 key /* Key for the inserted process */ ) {
int16 curr; /* Runs through items in a queue*/
int16 prev; /* Holds previous node index */
if (isbadqid(q) || isbadpid(pid)) {
return SYSERR; }
curr = firstid(q);
while (queuetab[curr].qkey >= key) {
curr = queuetab[curr].qnext;
/* Insert process between curr node and previous node */
prev = queuetab[curr].qprev; /* Get index of previous node */
queuetab[pid].qnext = curr;
queuetab[pid].qprev = prev;
queuetab[pid].qkey = key;
queuetab[prev].qnext = pid;
queuetab[curr].qprev = pid;
return OK;
CS354-Fall2018

20. List Initialization? newqueue.c/*
* newqueue - Allocate and initialize a queue in the global queue table * */
qid16 newqueue(void) {
static qid16 nextqid=NPROC; /* Next list in queuetab to use */
qid16 q; /* ID of allocated queue */
q = nextqid;
if (q > NQENT) { /* Check for table overflow */
return SYSERR; }
nextqid += 2; /* Increment index for next call*/
/* Initialize head and tail nodes to form an empty queue */
queuetab[queuehead(q)].qnext = queuetail(q);
queuetab[queuehead(q)].qprev = EMPTY;
queuetab[queuehead(q)].qkey = MAXKEY;
queuetab[queuetail(q)].qnext = EMPTY;
queuetab[queuetail(q)].qprev = queuehead(q);
queuetab[queuetail(q)].qkey = MINKEY;
return q;
List Initialization?
newqueue.c
CS354-Fall2018