Extern. first file is from book, second is mine

extern.c

#include <stdio.h>
#define    MAX    1000

int i;
char line[MAX];
char longest[MAX];

int length(void);
void copy(void);

int main(void)
{
  int max;

  while (length()) {
    if (i > max) {
      max = i;
      copy();
    }
  }
  

  printf("%s", longest);
    
}

int length(void)
{
  int c;

  for (i = 0; i < MAX - 1 && (c = getchar()) != EOF && c != '\n'; ++i)
    line[i] = c;

  if (c == '\n')
    line[i++] = c;

  line[i] = '\0';
  
}

void copy(void)
{
  int j = 0;

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

main.c

#include <stdio.h>
#define    MAX    1000

int max;
char longest[MAX];
char line[MAX];

int length(void);
void copy(void);

int main(void)
{
  int len;
  extern int max;
  extern char longest[];

  while (len = length()) {
    if (len > max){
      max = len;
      copy();
    }
  }
    

  printf("%s", longest);
  return 0;
}

int length(void)
{
  int i, c;

  for (i = 0; i < MAX - 1 && (c = getchar()) != EOF && c != '\n'; ++i)
    line[i] = c;

  if (c == '\n')
    line[i++] = c;

  line[i] = '\0';

  return i;
}

void copy(void)
{
  int i = 0;
  extern char line[];
  extern char longest[];
  
  while ((longest[i] = line[i]) != '\0')
    ++i;
  
}

reverse.c

#include <stdio.h>
#define    MAX    1000

int max;
char longest[MAX];
char line[MAX];

int length(void);
void copy(void);
void reverse(void);

int main(void)
{
  int len;
  extern int max;
  extern char longest[];

  max = 0;
  while (len = length()) {
    if (len > max) {
      max = len;
      copy();
    }
  }

  printf("%s", longest);

  reverse();

  return 0;
}

int length(void)
{
  int i, c;
  extern char line[];

  for (i = 0; i < MAX - 1 && (c = getchar()) != EOF && c != '\n'; ++i)
    line[i] = c;

  if (c == '\n')
    line[i++] = c;

  line[i] = '\0';

  return i;
}

void copy(void)
{
  int i = 0;

  extern char line[], longest[];

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

void reverse(void)
{
  extern int max;
  extern char longest[];
  char reverse[MAX];
  int j, i;
  for (i = max - 1, j = 0; j < max; ++j, --i)
    reverse[j] = longest[i];

  printf("%s", reverse);
}