// 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");
else
{
// 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)
break;
}
else
perror("fseek() failed");
}
// print last n lines
printf("Printing last %d lines -\n", n);
while (fgets(str, sizeof(str), in))
printf("%s", str);
}
printf("\n\n");
}
// 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");
exit(EXIT_FAILURE);
}
srand(time(NULL));
// 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(<time>));
// 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
fflush(fp);
// read last index lines from the file
tail(fp, index);
// sleep for 3 seconds
// note difference in timestamps in logs
sleep(3);
}
/* close the file before ending program */
fclose(fp);
return 0;
}
