K&R(1): Character Arrays and External Variables

#include    "stdio.h"

#define MAXLINE 1000    /* maximum input line length */

    int max;    /* maximum length seen so far */
    char line[MAXLINE]; /* current input line */
    char longest[MAXLINE]; /* longest line saved here */
    int getline1(void);
    void copy(void);

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

    max = 0;
    while ((len = getline1()) > 0)
      if (len > max) {
          max = len;
          copy();
      }
    if (max > 0) /* there was a line */
      printf("%s", longest);
    return 0;
}

/* getline: read a line into s, return length */
int getline1()
{
    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: copy 'from' into 'to'; assume to is big enouth */
void copy(void)
{
    int i;
    extern char line[], longest[];
    i = 0;
    while ((longest[i] = line[i]) != '\0')
      ++i;
}