reading last n lines from file in c/c++

Question

I have seen many posts but didn't find something like i want.
I am getting wrong output :

????????????????????????......  // may be this is EOF character

Going into infinite loop.

My algorithm:

1. Go to end of file.
2. decrease position of pointer by 1 and read character by character.
3. exit if we found our 10 lines or we reach beginning of file.
4. now i will scan the full file till EOF and print them //not implemented in code.

code:

#include<iostream>
#include<stdio.h>
#include<conio.h>
#include<stdlib.h>
#include<string.h>

using namespace std;
int main()
{
    FILE *f1=fopen("input.txt","r");
    FILE *f2=fopen("output.txt","w");
    int i,j,pos;
        int count=0;
        char ch;
        int begin=ftell(f1);
        // GO TO END OF FILE
        fseek(f1,0,SEEK_END);
        int end = ftell(f1);
        pos=ftell(f1);

        while(count<10)
        {
            pos=ftell(f1);
            // FILE IS LESS THAN 10 LINES
            if(pos<begin)
                break;
            ch=fgetc(f1);
            if(ch=='
')
                count++;
            fputc(ch,f2);
            fseek(f1,pos-1,end);
        }
    return 0;
}

UPD 1:

changed code: it has just 1 error now - if input has lines like

3enil
2enil
1enil

it prints 10 lines only

line1
line2
line3?ine1
line2
line3?ine1
line2
line3?ine1
line2
line3?ine1
line2

PS:
1. working on windows in notepad++

1. this is not homework

2. also i want to do it without using any more memory or use of STL.

3. i am practicing to improve my basic knowledge so please don't post about any functions (like tail -5 tc.)

please help to improve my code.

Answer 1

Comments in the code

#include <stdio.h>
#include <stdlib.h>

int main(void)
{
    FILE *in, *out;
    int count = 0;
    long int pos;
    char s[100];

    in = fopen("input.txt", "r");
    /* always check return of fopen */
    if (in == NULL) {
        perror("fopen");
        exit(EXIT_FAILURE);
    }
    out = fopen("output.txt", "w");
    if (out == NULL) {
        perror("fopen");
        exit(EXIT_FAILURE);
    }
    fseek(in, 0, SEEK_END);
    pos = ftell(in);
    /* Don't write each char on output.txt, just search for '
' */
    while (pos) {
        fseek(in, --pos, SEEK_SET); /* seek from begin */
        if (fgetc(in) == '
') {
            if (count++ == 10) break;
        }
    }
    /* Write line by line, is faster than fputc for each char */
    while (fgets(s, sizeof(s), in) != NULL) {
        fprintf(out, "%s", s);
    }
    fclose(in);
    fclose(out);
    return 0;
}