External Variables and Scope

#include <stdio.h>

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

int max;               /* maxumum length seen so far */
char line[MAXLINE];    /* current input line */
char longest[MAXLINE]; /* longest line saved here */

int getline(void);
void copy(void);

/* print longest input line; specialized version */
int main(void)
{
  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;
}

$\bullet$

Concepts: Automatic vs external variables