FCFS Scheduling Program in C
FCFS Scheduling Program in C
Problem Description:
Write an FCFS
Scheduling Program in C to determine the average waiting time and average
turnaround time given n processes and their burst times.
FCFS Scheduling Algorithm:
The
CPU scheduling algorithm First Come, First Served (FCFS), also known as First In, First Out (FIFO), allocates the CPU to the processes in the
order they are queued in the ready queue.
FCFS uses non-preemptive scheduling, which
means that once a CPU has been assigned to a process, it stays assigned to that
process until it is either terminated or may be interrupted by an I/O
interrupt.
Problem Solution
1.
Enter all the processes and their burst time.
2. Find waiting time, WT of all the processes.
3. For the 1st process, WT = 0.
4. For all the next processes i, WT[i] = BT[i-1] +
WT[i-1].
5. Calculate Turnaround time = WT + BT for all the processes.
6. Calculate average waiting time = total waiting time/no. of processes.
7. Calculate average turnaround time = total turnaround time/no. of
processes.
Example:
Process |
Arrival Time |
Burst Time |
P1 |
0 |
5 |
P2 |
0 |
11 |
P3 |
0 |
11 |
Gantt Chart:
Waiting
Time: Time Difference between turnaround time and burst time.
Waiting
Time = Turnaround Time – Burst Time
P1 waiting time: 0
P2 waiting time: 5
P3 waiting time: 16
Average
Waiting Time = (0 + 5 + 16)/3 = 21/3 = 7
Turnaround
Time: Difference between completion time and arrival time.
Turnaround
Time = Completion Time – Arrival Time
P1 turnaround
time: 5-0 = 5
P2 turnaround time: 16-0 = 16
P3 turnaround time: 27-0 = 27
Average Turnaround Time = (5+16+27)/3 = 16
Program/Source Code
Here is the source
code of the C program for the FCFS Scheduling. The C program is successfully
compiled and run on a Linux system. The program output is also shown below.
1.
/*
2.
*
FCFS Scheduling Program in C
3.
*/
4.
5.
#include
<stdio.h>
6.
int main()
7.
{
8.
int pid[15];
9.
int bt[15];
10. int n;
11. printf("Enter
the number of processes: ");
12. scanf("%d",&n);
13.
14. printf("Enter
process id of all the processes: ");
15. for(int i=0;i<n;i++)
16. {
17. scanf("%d",&pid[i]);
18. }
19.
20. printf("Enter
burst time of all the processes: ");
21. for(int i=0;i<n;i++)
22. {
23. scanf("%d",&bt[i]);
24. }
25.
26. int i, wt[n];
27. wt[0]=0;
28.
29. //for calculating waiting time of each process
30. for(i=1; i<n; i++)
31. {
32.
wt[i]= bt[i-1]+ wt[i-1];
33. }
34.
35. printf("Process
ID Burst Time Waiting Time TurnAround Time\n");
36. float twt=0.0;
37. float tat= 0.0;
38. for(i=0; i<n; i++)
39. {
40. printf("%d\t\t", pid[i]);
41. printf("%d\t\t", bt[i]);
42. printf("%d\t\t", wt[i]);
43.
44. //calculating and printing turnaround time of
each process
45. printf("%d\t\t", bt[i]+wt[i]);
46. printf("\n");
47.
48. //for calculating total waiting time
49.
twt += wt[i];
50.
51. //for calculating total turnaround time
52.
tat += (wt[i]+bt[i]);
53. }
54. float att,awt;
55.
56. //for calculating average waiting time
57. awt = twt/n;
58.
59. //for calculating average turnaround time
60. att = tat/n;
61. printf("Avg.
waiting time= %f\n",awt);
62. printf("Avg.
turnaround time= %f",att);
63.}
Program Explanation
1.
Initialize two array pid[] and bt[] of size 15.
2. Ask the user for number of processes n.
3. Ask the user for process id and burst time for all n processes and store them
into pid[] and bt[] respectively.
4. Calculate waiting time of each process by the formula wt[i] = wt[i-1] +
bt[i-1].
5. Print Process Id, Burst Time, waiting time and Turnaround time of each
process in tabular manner.
6. Calculate and print turnaround time of each process = bt[i] + wt[i].
7. Add waiting time of all the processes and store it in the variable twt.
8. Add turnaround time of all the processes and store it in the variable tat.
9. Calculate average waiting time as awt = twt/n.
10. Calculate average turnaround time as att = tat/n;
11. Print average waiting time and average turnaround
time.
12. Exit.
OUTPUT:
Enter the number of processes: 3
Enter process id of all the processes: 1 2 3
Enter burst time of all the processes: 5 11 11
Process ID Burst Time Waiting Time TurnAround Time
1 5 0 5
2 11 5 16
3 11 16 27
Avg. waiting time= 7.000000
Avg. turnaround time= 16.000000
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