Semaphore Implementation
typedef struct {
int value;
struct process *list;
} semaphore;
Each semaphore has an integer value and a list of processes list. When
a process must wait on a semaphore, it is added to the list of processes. A
signal() operation removes one process from the list of waiting processes
and awakens that process.
Now, the wait() semaphore operation can be defined as
wait(semaphore *S) {
S->value--;
if (S->value < 0) {
add this process to S->list;
block();
}
}
and the signal() semaphore operation can be defined as
signal(semaphore *S) {
S->value++;
if (S->value <= 0) {
remove a process P from S->list;
wakeup(P);
}
}
suppose S=3, we need to be able to have 3 processes in Critical section at max. Also when counting semaphore S has negative value we need to have Absolute value of S as no of processes waiting for critical section. Suppose $P_0,P_1,P_3$ want to enter in critical section in order then they will call wait(); one after one and wait();is being called 3 times and S value will be 0 at this time all 3 will be there in critical section Now, if two more process $P_4,P_5$ will come and call the wait(); one afer one then s value will be -2 and they will be block and added to list of waiting process now, suppose if $P_0$ comes out of critical section then it will call signal(); So S will be increment to -1 and it will see that S<=0 and it will call wakeup so, the process will be unblocked in same order($P_3,then P4$)
$\mathbf{MYDOUBT}$->now, if $P_3$ wants to enter in critical section then it will call wait(); and S value will be -2 but S-value<0 then how will P_3 will enter in critical section it will be again added to list of processs ,and if again $P_4$ comes out of critical section then again such thing will happen,
please tell me where i am making mistake ?
