《C Primer Plus》程序清单12.4无法运行

Question

请各位帮忙看一下为什么不能运行，编译也没有警告和错误

#include <stdio.h>

int units = 0;
void critic(void);
int main(void)
{
    extern int units;

    printf("How many pounds to a firkin of butter?\n");
    scanf("%d", &units);
    while (units != 56);  //程序会卡在这里无法进行
    {
        critic();
    }
    printf("You must have looked it up!\n");

    return 0;
}

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

Answer 1

while (units != 56);

多了一个分号，造成死循环