Here is source code of the C Program to Update Details of Employee using Files. The C program is successfully compiled and run on a Linux system. The program output is also shown below.

/*
* C Program to Update Details of Employee using Files
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct emp
{
int empid;
char *name;
};

int count = 0;
void add_rec(char *a);
void display(char *a);
void update_rec(char *a);

void main(int argc, char *argv[])
{
int choice;
while (1)
{
printf("MENU:\n");
printf("1.Add a record\n");
printf("2.Display the file\n");
printf("3.Update the record\n");
printf("Enter your choice:");
scanf("%d", &choice);

switch(choice)
{
case 1:
add_rec(argv[1]);
break;
case 2:
display(argv[1]);
break;
case 3:
update_rec(argv[1]);
break;
case 4:
exit(0);
default:
printf("Wrong choice!!!\nEnter the correct choice\n");
}
}
}

void add_rec(char *a)
{
FILE *fp;
fp = fopen(a, "a+");
struct emp *temp = (struct emp *)malloc(sizeof(struct emp));
temp->name = (char *)malloc(50*sizeof(char));
if (fp == NULL)
printf("Error!!!");
else
{
printf("Enter the employee id\n");
scanf("%d", &temp->empid);
fwrite(&temp->empid, sizeof(int), 1, fp);
printf("enter the employee name\n");
scanf(" %[^\n]s", temp->name);
fwrite(temp->name, 50, 1, fp);
count++;
}
fclose(fp);
free(temp);
free(temp->name);
}

void display(char *a)
{
FILE *fp;
char ch;
int rec = count;
fp = fopen(a, "r");
struct emp *temp = (struct emp *)malloc(sizeof(struct emp));
temp->name = (char *)malloc(50*sizeof(char));
if (fp == NULL)
printf("Error!!");
else
{
while (rec)
{
fread(&temp->empid, sizeof(int), 1, fp);
printf("%d", temp->empid);
fread(temp->name, 50, 1, fp);
printf(" %s\n", temp->name);
rec--;
}
}
fclose(fp);
free(temp);
free(temp->name);
}

void update_rec(char *a)
{
FILE *fp;
char ch, name[5];
int rec, id, c;
fp = fopen(a, "r+");
struct emp *temp = (struct emp *)malloc(sizeof(struct emp));
temp->name = (char *)malloc(50*sizeof(char));
printf("Enter the employee id to update:\n");
scanf("%d", &id);
fseek(fp, 0, 0);
rec = count;
while (rec)
{
fread(&temp->empid, sizeof(int), 1, fp);
printf("%d", temp->empid);
if (id == temp->empid)
{
printf("Enter the employee name to be updated");
scanf(" %[^\n]s", name);
c = fwrite(name, 50, 1, fp);
break;
}
fread(temp->name, 50, 1, fp);
rec--;
}
if (c == 1)
printf("Record updated\n");
else
printf("Update not successful\n");
fclose(fp);
free(temp);
free(temp->name);
}


$ cc file5.c
$ a.out empl
MENU:
1.Add a record
2.Display the file
3.Update the record
Enter your choice:1
Enter the employee id
1
enter the employee name
aaa
MENU:
1.Add a record
2.Display the file
3.Update the record
Enter your choice:1
Enter the employee id
2
enter the employee name
bbb
MENU:
1.Add a record
2.Display the file
3.Update the record
Enter your choice:3
Enter the employee id to update:
1
1Enter the employee name to be updated1bc
Record updated
MENU:
1.Add a record
2.Display the file
3.Update the record
Enter your choice:2
1 1bc
2 bbb
MENU:
1.Add a record
2.Display the file
3.Update the record
Enter your choice:4