This example implements the option n of tail command.

#define _FILE_OFFSET_BITS 64
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <errno.h>
#include <unistd.h>
#include <getopt.h>

#define BUFF_SIZE 4096

FILE *openFile(const char *filePath)
{
  FILE *file;
  file= fopen(filePath, "r");
  if(file == NULL)
  {
    fprintf(stderr,"Error opening file: %s\n",filePath);
    exit(errno);
  }
  return(file);
}
 
void printLine(FILE *file, off_t startline)
{
  int fd;
  fd= fileno(file);
  int nread;
  char buffer[BUFF_SIZE];
  lseek(fd,(startline + 1),SEEK_SET);
  while((nread= read(fd,buffer,BUFF_SIZE)) > 0)
  {
    write(STDOUT_FILENO, buffer, nread);
  }
}

void walkFile(FILE *file, long nlines)
{
  off_t fposition;
  fseek(file,0,SEEK_END);
  fposition= ftell(file);
  off_t index= fposition;
  off_t end= fposition;
  long countlines= 0;
  char cbyte;

  for(index; index >= 0; index --)
  {
    cbyte= fgetc(file);
    if (cbyte == '\n' && (end - index) > 1)
    {
      countlines ++;
      if(countlines == nlines)
      {
    break;
      }
     }
    fposition--;
    fseek(file,fposition,SEEK_SET);
  }
  printLine(file, fposition);
  fclose(file);
}

int main(int argc, char *argv[])
{
  FILE *file;
  file= openFile(argv[2]);
  walkFile(file, atol(argv[1]));
  return 0;
}

Note: take in mind that i not wrote code to parse input options and arguments, neither code to check if the lines number argument is really a number.

This example implements the -n option of the tail command.

#define _FILE_OFFSET_BITS 64
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <errno.h>
#include <unistd.h>
#include <getopt.h>

#define BUFF_SIZE 4096

FILE *openFile(const char *filePath)
{
  FILE *file;
  file= fopen(filePath, "r");
  if(file == NULL)
  {
    fprintf(stderr,"Error opening file: %s\n",filePath);
    exit(errno);
  }
  return(file);
}
 
void printLine(FILE *file, off_t startline)
{
  int fd;
  fd= fileno(file);
  int nread;
  char buffer[BUFF_SIZE];
  lseek(fd,(startline + 1),SEEK_SET);
  while((nread= read(fd,buffer,BUFF_SIZE)) > 0)
  {
    write(STDOUT_FILENO, buffer, nread);
  }
}

void walkFile(FILE *file, long nlines)
{
  off_t fposition;
  fseek(file,0,SEEK_END);
  fposition= ftell(file);
  off_t index= fposition;
  off_t end= fposition;
  long countlines= 0;
  char cbyte;

  for(index; index >= 0; index --)
  {
    cbyte= fgetc(file);
    if (cbyte == '\n' && (end - index) > 1)
    {
      countlines ++;
      if(countlines == nlines)
      {
    break;
      }
     }
    fposition--;
    fseek(file,fposition,SEEK_SET);
  }
  printLine(file, fposition);
  fclose(file);
}

int main(int argc, char *argv[])
{
  FILE *file;
  file= openFile(argv[2]);
  walkFile(file, atol(argv[1]));
  return 0;
}

Note: keep in mind that i didn't write code to parse input options or arguments, nor code to check if the argument is really a number.