Question

3. (50pts) Write a simple sequence-number system through which three concurrent processes, P, P2, and P3, each obtain unique integers in the range [, 500]. Use the fork) call to create P, P2, and P3. Given a file, F, containing single number, each process must perform the following steps: a a. Open F b. Read the sequence number N from the file c. Close F d. Output N and the process' PID (either on screen or test file) e. Increment N by 1 f. Open F g. Write N to F h. Flush F i. Close F Describe the behavior of your program and explore the reason for this behavior. Provide evidence for your conclusion in form of test-output. You must clearly document your code

**write program in C

what is not clear?

Answer 1

Solution:

Code:

#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
const char* file_name = "read.txt";
void process()
{
   pid_t pid; //Process ID
   FILE* file = fopen (file_name, "r"); //Open file
   int N = 0;
   fscanf (file, "%d", &N); //Read Number
   fclose (file); //Close file
   if ((pid = getpid()) < 0) { //Get Process ID
       printf("unable to get pid");
   } else {
       printf("N: %d Process ID: %d",N,pid);
   }
   file = fopen(file_name, "w");
   N++;
   putw(N,file);
   fflush(file);
   fclose(file);
}
int main()
{
   if (fork() == 0) {
       printf("Starting Process A\n");
       process();
   }
   if (fork() == 0) {
       printf("Starting Process B\n");
       process();
   }
   if (fork() == 0){
       printf("Starting Process C\n");
       process();
   }
   return 0;
}

Behavior:

There are three process A, B and C running same piece of code and there is no synchronization between them and they are accessing same variable N. This leads to race condition between Processes and because of which there is unexpected value of N.

Proof:

Initial value of N is 2 in the file.

Starting Process C

Starting Process B

N: 2 Process ID: 32114

Starting Process A

N: 1 Process ID: 32456

This behavior varies from machine to machine depending on the order in which process A,B and C are executed.