Implement your own tail (Read last n lines of a huge file)

Given a huge file having dynamic data, write a program to read last n lines from the file at any point without reading the entire file. The problem is similar to tail command in linux which displays the last few lines of a file. It is mostly used for viewing log file updates as these updates are appended to the log files.

Source : Microsoft Interview

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The problem mainly focuses on below things –

1. The program should not read entire file.
2. The program should handle incoming dynamic data and returns last n lines at any point.
3. The program should not close input stream before reading last n lines.

Below is its C++ implementation





// C++ program to implement your own tail
#include <bits/stdc++.h>
using namespace std;
#define SIZE 100
// Utility function to sleep for n seconds
void sleep(unsigned int n)
    clock_t goal = n * 1000 + clock();
    while (goal > clock());
// function to read last n lines from the file
// at any point without reading the entire file
void tail(FILE* in, int n)
    int count = 0;  // To count '\n' characters
    // unsigned long long pos (stores upto 2^64 – 1
    // chars) assuming that long long int takes 8 
    // bytes
    unsigned long long pos;
    char str[2*SIZE];
    // Go to End of file
    if (fseek(in, 0, SEEK_END))
        perror("fseek() failed");
        // pos will contain no. of chars in
        // input file.
        pos = ftell(in);
        // search for '\n' characters
        while (pos)
            // Move 'pos' away from end of file.
            if (!fseek(in, --pos, SEEK_SET))
                if (fgetc(in) == '\n')
                    // stop reading when n newlines
                    // is found
                    if (count++ == n)
                perror("fseek() failed");
        // print last n lines
        printf("Printing last %d lines -\n", n);
        while (fgets(str, sizeof(str), in))
            printf("%s", str);
// Creates a file and prints and calls tail() for 
// 10 different values of n (from 1 to 10)
int main()
    FILE* fp;
    char buffer[SIZE];
    // Open file in binary mode
    // wb+ mode for reading and writing simultaneously
    fp = fopen("input.txt", "wb+");
    if (fp == NULL)
        printf("Error while opening file");
    // Dynamically add lines to input file
    // and call tail() each time
    for (int index = 1; index <= 10; index++)
        /* generate random logs to print in input file*/
        for (int i = 0; i < SIZE - 1; i++)
            buffer[i] = rand() % 26 + 65; // A-Z
        buffer[SIZE] = '\0';
        /* code to print timestamp in logs */
        // get current calendar time
        time_t ltime = time(NULL);
        // asctime() returns a pointer to a string
        // which represents the day and time
        char* date = asctime(localtime(<ime));
        // replace the '\n' character in the date string
        // with '\0' to print on the same line.
        date[strlen(date)-1] = '\0';
        /* Note in text mode '\n' appends two characters,
        so we have opened file in binary mode */
        fprintf(fp, "\nLine #%d [%s] - %s", index,
                                    date, buffer);
        // flush the input stream before calling tail
        // read last index lines from the file
        tail(fp, index);
        // sleep for 3 seconds
        // note difference in timestamps in logs
    /* close the file before ending program */
    return 0;


Some points to Note –
1. This code won’t work on online compiler as it requires file creation permissions. When run local machine, it produces sample input file “input.txt” and dynamically write data to it 10 times and calls tail() function every time.

2. We should avoid using fseek() and ftell() for huge files(in GBs) as they operate on file positions of type long int. Use _fseeki64(), _ftelli64() instead.

3. unsigned long has max allowed value of 232 – 1 (Assuming that unsigned long takes 4 bytes). It can be used for files size of less than 4 GB file.

4. unsigned long long has max allowed value of 264 – 1 (Assuming that unsigned long long takes 8 bytes). It can be used for files size over 4 GB.

This article is contributed by Aditya Goel. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above

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