A text file named \"common.txt\" allows N concurrent processes to read and write

Question

A text file named "common.txt" allows N concurrent processes to read and write to it. However, only one process can access a file at a time. At the beginning of the file is a counter. When a process accesses the file, it first reads the counter, increments its value by one, updates it and append a message like the following to the file:
I am process process_id who have visited you!

The process then closes the file and sleeps for a random amount of time between 0 to 3 seconds. If the file does not exist, the first process which tries to access it will create the file and set the counter to 0.
Make use of a POSIX semaphore to implement these capabilities and test your program with at least 2 processes. The following is sample content of the file "common.txt":

8
I am process 1 who have visited you!
I am process 2 who have visited you!
I am process 1 who have visited you!
I am process 1 who have visited you!
I am process 3 who have visited you!
I am process 2 who have visited you!
I am process 3 who have visited you!
...........

Explanation / Answer

#include <stdio.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>

#define P(s) semop(s, &pop, 1) /* pop is the structure we pass for doing
the P(s) operation */
#define V(s) semop(s, &vop, 1) /* vop is the structure we pass for doing
the V(s) operation */

int main()
{
semid1 = semget(ftok("/usr/lib",1), 1, 0777|IPC_CREAT|IPC_EXCL);

int i;

char j;

if(semid1 < 0)
{  

semid1 = semget(ftok("/usr/lib",1), 1, 0777|IPC_CREAT);     

}

else
{
semctl(semid1, 0, SETVAL, 0);
}
pop.sem_num = vop.sem_num = 0;
pop.sem_flg = vop.sem_flg = 0;
pop.sem_op = -1 ; vop.sem_op = 1 ;

P(semid1);

FILE * fp = fopen("common.txt","ar+");

fscanf(fp,"%d",&i);

i++;

}

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