Exercise 5-13. Write the program tail, which prints the last n lines of its input. By default, n is set to 10, let us say, but it can be changed by an optional argument so that 

tail -n 

prints the last n lines. 

The program should behave rationally no matter how unreasonable the input or the value of n. Write the program so it makes the best use of available storage; lines should be stored as in the sorting program of Section 5.6, not in a two-dimensional array of fixed size.

/* tail */

#include <stdio.h>

#include <string.h>

#include <stdlib.h>

#define MAXLINES 5000 //max lines 

#define BUFSIZE 5000

#define reserve 10

char *lineptr[MAXLINES]; //pointers to text lines

int readlines(char *lineptr[], char *buf, int nlines);

void writelines(char *lineptr[], int n, int nlines);

/* prints the last n lines */

int main(int argc, char **argv)

{

    int nlines, n; //number of input lines read && last lines

    char buf[BUFSIZE];  

    

    if(argc > 2)

    {

        printf("ERROR: invalid parametrs\n");

        return 1;

    }

    if(argc == 2 && atoi(*++argv) < 0)

        n = -(atoi(*argv));

    else

    {

        printf("wrong parameters, to use default\n");

        n = reserve;

    }

    if((nlines = readlines(lineptr, buf, MAXLINES)) >= 0)

    {

        printf("\n");

        writelines(lineptr, n, nlines);

        return 0;

    }

    else

    {

        printf("error: input too big to sort\n");

        return 1;

    }

}

#define MAXLEN 1000 //max lenght of any input line

int getlin(char *, int);

/* readlines: read input lines */

int readlines(char *lineptr[], char *buf, int maxlines)

{

    int len, nlines;

    char line[MAXLEN];

    char *p = buf;     

    char *bufstop = buf + BUFSIZE;

    

    nlines = 0;

    while((len = getlin(line, MAXLEN)) > 0)

    {

        if(nlines >= maxlines || p + len > bufstop)

            return -1;

        else

        {

            line[len - 1] = '\0'; //delete newline

            strcpy(p, line);

            lineptr[nlines++] = p;

            p += len;

        }

    }

    return nlines;

}

/* writelines: write output lines */

void writelines(char *lineptr[], int n, int nlines)

{

    int i;

    

    if(n > nlines)

    {

        n = nlines;

        printf("not enough lines\n");

    }

    for(i = nlines - n ; n > 0; n--)

        printf("%s\n", lineptr[i++]);

}

int getlin(char *l, int lim)

{

    int c;

    char *tmp = l;

    

    for(; --lim > 0 && (c=getchar()) != EOF && c!='\n'; l++)

        *l = c;

    if(c=='\n')

        *l++ = c;

    *l = '\0';

    return l - tmp;

}