C语言全局变量

/* global.c  -- uses an external variable */
#include <stdio.h>
int units = 0;         /* an external variable      */
void critic(void);
int main(void)
{
    extern int units;  /* an optional redeclaration */

    printf("How many pounds to a firkin of butter?\n");
    scanf("%d", &units);
    while ( units != 56)
        critic();
    printf("You must have looked it up!\n");

    return 0;
}

void critic(void)
{
    /* optional redeclaration omitted */
    printf("No luck, my friend. Try again.\n");
    scanf("%d", &units);
}