scanf()函数读取类型不匹配问题

一般情况下scanf("%d",&num)会读取一个字符，如果该字符是数字,它便保存该数字并读取下一个字符，知道遇到第一个空白字符为止。或者遇到非数字字符，它也会停止读入，然后scanf()函数把非数字字符放回输入。这意味着在改程序下一次读入时，首先读取到的是上一次丢弃的非数字字符。

在linux系统下，如果scanf()函数使用的是%d转换说明符，那么当程序读取的第一个字符为非数字字符比如‘A’时，scanf()将会停留在那里，并把字符‘A’放回输入，然后继续读取，scanf()一直无法越过A读到下一个字符，一直反复读入，并陷入死循环。

下面就是程序代码和示例（Linux系统在命令行模式下执行的结果）：

#include<stdio.h>

#define SEC_PER_MIN 60

int main(void)

{
int sec,min,left;

printf("Convert seconds to minutes and seconds!\n");
printf("Enter the number of seconds (<=0 to quit):\n");
scanf("%d",&sec);
while(sec>0)
{
min = sec / SEC_PER_MIN;
left = sec % SEC_PER_MIN;
printf("%d seconds is %d minutes,%d seconds.\n",sec,
min,left);
printf("Enter next value (<=0 to quit):\n");
scanf("%d",&sec);
}
printf("Done!\n");

return 0;

}

(以上代码选自C Primer Plus)

在Linux命令行执行结果如下所示：

$ ./min_sec

Convert seconds to minutes and seconds!

Enter the number of seconds (<=0 to quit):

30

30 seconds is 0 minutes,30 seconds.

Enter next value (<=0 to quit):

20

20 seconds is 0 minutes,20 seconds.

Enter next value (<=0 to quit):

70

70 seconds is 1 minutes,10 seconds.

当我输入字符q时：

Enter next value (<=0 to quit):

q

70 seconds is 1 minutes,10 seconds.

Enter next value (<=0 to quit):

70 seconds is 1 minutes,10 seconds.

Enter next value (<=0 to quit):

70 seconds is 1 minutes,10 seconds.

Enter next value (<=0 to quit):

70 seconds is 1 minutes,10 seconds.

Enter next value (<=0 to quit):

70 seconds is 1 minutes,10 seconds.

Enter next value (<=0 to q
4000
uit):

70 seconds is 1 minutes,10 seconds.

Enter next value (<=0 to quit):

70 seconds is 1 minutes,10 seconds.

Enter next value (<=0 to quit):

70 seconds is 1 minutes,10 seconds.

Enter next value (<=0 to quit):

70 seconds is 1 minutes,10 seconds.

Enter next value (<=0 to quit):

70 seconds is 1 minutes,10 seconds.

Enter next value (<=0 to quit):

70 seconds is 1 minutes,10 seconds.

Enter next value (<=0 to quit):

70 seconds is 1 minutes,10 seconds.

Enter next value (<=0 to quit):

70 seconds is 1 minutes,10 seconds.

Enter next value (<=0 to quit):

70 seconds is 1 minutes,10 seconds.

Enter next value (<=0 to quit):

70 seconds is 1 minutes,10 seconds.

Enter next value (<=0 to quit):

70 seconds is 1 minutes,10 seconds.

Enter next value (<=0 to quit):

^Z

[2]+  已停止               ./min_sec

程序就会陷入死循环，并且执行的值是上一次的值(因为sec赋值失败，并且保留的是上一次所读入的值)，我按下Ctrl+Z强行终止进程。

当然，输入小于零的数就会退出循环并终止程序，O(∩_∩)O。