Uses of FILE pointer and Text File manipulation through C code

#include"stdio.h"

#include"string.h"

#include"stdlib.h"

char *Update(char *s, char *prev, char *new)

{

int pLength = strlen(prev);

int nLength = strlen(new);

int count=0;

int i;

for(i=0; s[i]!='\0';++i)

{

if(strstr(&s[i],prev)== &s[i]){

++count;

i=i+pLength-1;

}

}

char *replace = (char *)malloc(i+count*(nLength-pLength)+1);

i=0;

while (*s)

{

if(strstr(s,prev)==s){

strcpy(&replace[i],new);

i+=nLength;

s+=pLength;

}

else{

replace[i++] = *s++;

}

}

replace[i] = '\0';

return replace;

}

int main()

{

FILE *ptr1=NULL;

FILE *ptr2=NULL;

char s[202];

char temp[30];

char *pointer;

ptr1 = fopen("input.txt","r");//read

ptr2 = fopen("output.txt","a");//append

if(ptr1==NULL) printf("File doesn't exist.\n");

fgets(s, 200, ptr1);

printf("Enter your name: ");

scanf(" %[^\n]s",temp);

pointer = Update(s,"{{name}}",temp);

// printf("%s\n",pointer);

printf("Enter your ID: ");

scanf(" %[^\n]s",temp);

pointer = Update(pointer,"{{id}}",temp);

// printf("%s\n",pointer);

printf("Enter your Department: ");

scanf(" %[^\n]s",temp);

pointer = Update(pointer,"{{Department}}",temp);

// printf("%s\n",pointer);

printf("%s\n",pointer);

fprintf(ptr2,"%s",pointer);

fclose(ptr1);

fclose(ptr2);

free(pointer);

return 0;

}