#include<stdio.h>
int  getline(void);
void copy(void);

/* print longest input line; specialized version */
main()
{
    int len;
    extern int max;
    extern char longest[];

    max = 0;
    while ((len = getline()) > 0)
        if(len > max) {
            max = len;
            copy();
        }
        if (max > 0)  /* there was a line */
            printf ("%s", longest);
        return 0;
}

/* getline: specialized version */
int getline(void)
{
    int c, i;
    extern char line[];
    for (i = 0; i < MAXLINE - 1
        && (c=getchar) != EOF && c != '\n'; ++i)
        line[i] = c;
    if (c == '\n') {
        line[i] = c;
        ++i;
    }
    line[i] = '\0';
    return i;
}

/* copy: specialized version */
void copy(void)
{
    int i;
    extern char line[],longest[];

    i = 0;
    while ((longest[i] = line[i]) != '\0')
        ++i;
}

很感谢