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1. “Critical Section Problem Using Peterson’s
Solution”
By,
SHREYAS V E - 4VV21CS147
PRAMUKH B V - 4VV21CS190
SHREYA CHAKRAVARTHY S - 4VV21CS145
SHANE MATHEW - 4VV21CS139
Subject: Operating Systems
Subject Code: 21CS52
Faculty in-charge: Dr. Madhusudhan H S

2. Algorithm:
ANALYSIS:
• Peterson's solution is restricted to two.
• The processes are numbered P0 and P1 or Pi
and Pj where j = 1-i
• Peterson's solution requires the two processes
to share two data items:
int turn;
boolean flag[2];
• The variable turn indicates whose turn it is to
enter its critical section.
• The flag array is used to indicate if a process is
ready to enter its critical section.
• To enter the critical section,
• process Pi first sets flag [i] = true
• turn = j
• If both processes try to enter at the same time,
turn will be set to both i and j at roughly the
same time.

3. To prove that solution is correct, then we need to show that
1. Mutual exclusion is preserved
• Only one process can enter the critical section.
• If Pi and Pj are the two processes that enter the critical section, the turn will be given to any one among i or j
but not both.
2. Progress requirement is satisfied
• Assume that the process is waiting continuously to enter the critical section i.e, Pi is in while loop and
flag[j]=true and turn==j
• However the process Pj in the critical section, flag[j] will become false and now Pi can enter into the critical
section to progress.
3. Bounded-waiting requirement is met
• Since each process gives the turn to another process bounded waiting is assured and each process will wait
at most one entry by another process.

4. #include <stdio.h>
#include <pthread.h>
#include <unistd.h> // for sleep()
#define TRUE 1
#define FALSE 0
int flag[2] = { FALSE, FALSE }; // Flags to indicate if process is ready to enter
critical section
int turn = 0; // Variable to indicate whose turn it is


5. void* process(void* arg) {
int process_id = *((int*)arg);
int other = 1 - process_id;
for (int i = 0; i < 5; i++) {
flag[process_id] = TRUE;
turn = other;
while (flag[other] == TRUE && turn == other) {
// Process waits because it's not its turn or the other process is ready
}
// Process enters critical section
printf("Process %d is in the critical section.n", process_id);
sleep(1); // Simulating critical section work
// Process exits critical section
flag[process_id] = FALSE;
}
pthread_exit(NULL);
}

6. int main() {
pthread_t threads[2];
int ids[2] = { 0, 1 };
int i;
// Creating threads
for (i = 0; i < 2; i++) {
if (pthread_create(&threads[i], NULL, process, &ids[i]) != 0) {
perror("pthread_create");
return -1;
}
}
// Joining threads
for (i = 0; i < 2; i++) {
if (pthread_join(threads[i], NULL) != 0) {
perror("pthread_join");
return -1;
}
}
return 0;
}

8. THANK YOU