C - Write the program tail which prints the last n lines of its input-蒲公英云

C - Write the program tail which prints the last n lines of its input

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/*
 * 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.
 *
 * Tail.c - by FreeMan
 */
#include <stdio.h>
#include <stdlib.h>
#define MAXLINES        1024
#define MAXINPUT        10240
#define DEF_NUM_LINES   10
int  getlines(char *);
void parse_args(int, char **);
char  linestr[MAXINPUT];
char *lineptr[MAXLINES];
int num_of_lines = DEF_NUM_LINES;
int main(int argc, char *argv[])
{
    int lines;
    int ltp = 0;
    int i;
    int c;
    *lineptr = linestr;
    /* Parse the argument strings passed to the program */
    if (argc > 1) {
        parse_args(argc, argv);
    }
    /* Get the input from the user */
    lines = getlines(linestr);
    if (num_of_lines == 0) {
        num_of_lines = 10;
    }
    ltp = lines < num_of_lines ? lines : num_of_lines;
    printf("\n>>> Printing %d line(s):\n", ltp);
    if (ltp == DEF_NUM_LINES) {
        printf(">>> The default number of lines to print is ");
        printf("%d\n", DEF_NUM_LINES);
    }
    printf("\n");
    for (i = lines; i > 0; i--)
    {
        while ((c = *lineptr[0]++) != '\n')
        {
            if (i <= ltp)
            {
                printf("%c", c);
            }
        }
        if (c == '\n' && i <= ltp)
        {
            printf("\n");
        }
    }
    return 0;
}
int getlines(char *buffer)
{
    int i, count = 0;
    char c;
    for (i = 0; (c = getchar()) != EOF && i < MAXINPUT; i++) {
        *buffer++ = c;
        if (c == '\n') {
            lineptr[++count] = buffer;
        }
    }
    *buffer++ = '\0';
    return count;
}
void parse_args(int argc, char **argv)
{
    char c;
    while (--argc > 0 && (*++argv)[0] == '-') {
        c = *++argv[0];
        switch (c) {
        case 'n':
            num_of_lines = atoi(*(argv + 1));
            break;
        }
    }
}