在linux ctrl + d（EOF）中三种情况下的行为 [英] In linux ctrl + d(EOF) behavior in three different situation

问题描述

//first example
void readInWBW()
{
    int ch; 
    while((ch=getchar()) != EOF)
        putchar(ch);
}

当我输入 qweCTRL + D时，第一次输入ctrl + z刷新缓冲区，然后重新输入
ctrl + d就像 qweCTRL + DCTRL + D，然后EOF工作，程序终止。
结果是

when I input "qweCTRL+D", the first time Where I input ctrl+z just flush the buffer, and I re-input "ctrl+d" so it's like "qweCTRL+DCTRL+D", then the EOF work, program terminate. result is

$ ./a.out
qweqwe$

---

---

//second example
void guess()
{
    int guess = 1;  
    printf("Pick an integer from 1 to 100. I will try to guess ");
    printf("it.\nRespond with a y if my guess is right and with");
    printf("\nan n if it is wrong.\n");
    printf("Uh...is your number %d?\n", guess);
    while (getchar() != 'y'){      //<---------------------------
        printf("Well, then, is it %d?\n", ++guess); //<----------
        sleep(1);
    }
    printf("I knew I could do it!\n");
}

在此示例中，我输入 qweCTRL + D，它将显示三倍好吧……，但如果我再次输入CTRL + D，程序将进入无限循环。
reuslt是：

In this example, I input "qweCTRL+D", it will shows three times "Well, then...", but if I input CTRL+D again, the program will get into infinite loop. reuslt is:

Pick an integer from 1 to 100. I will try to guess it.
Respond with a y if my guess is right and with
an n if it is wrong.
Uh...is your number 1?
qweWell, then, is it 2? //<--------------------------
Well, then, is it 3?
Well, then, is it 4?
Well, then, is it 5?
Well, then, is it 6?
Well, then, is it 7?
Well, then, is it 8?
Well, then, is it 9?
Well, then, is it 10?
Well, then, is it 11?
^C

---

---

//third example    
void test()
{
    char ch;
    while((ch = getchar()) != '#')
        putchar(ch); 
}

我尝试输入 qweCTRL + D；与其他示例类似，但是在刷新缓冲区之后， CTRL + D不再响应，即使我输入＃，它也不会终止。
结果是：

I tried to input "qweCTRL+D" like other example, but after flush buffer, "CTRL+D" not response anymore, even if I input "#", it still not terminate. result is:

$ ./a.out
qweqwe
#
#
^C
$

我不明白为什么example2和example3中有无限循环并且可以t终止程序。谁能解释它，谢谢。

I don't understand why in example2 and example3 have infinite loop and can't terminate the program. can anyone explains it, thank you.

推荐答案

让我们关注第三个示例。

Let's focus on your third example.

void test()
{
    char ch;
    while((ch = getchar()) != '#')
        putchar(ch); 
}

请记住，输入Cd表示文件结束，因此程序停止自然是很自然的响应输入。您已经说过没有更多输入。这就是EOF的意思。

Remember that inputting C-d means "end of file", so it's very natural that the program stops responding to input. You have said that there is no more input. That's what EOF means.

因此，当您输入Cd时，将发生 ch = getchar（）不断读取EOF的情况。 。在C-d之后键入的任何内容都不会进入输入流。因此，为什么要进入无限循环的简单答案是 EOF！='＃'与 getchar（）的事实相结合的简单事实一次读取EOF总是会读取它。

So when you have inputted C-d, what will happen is that ch = getchar() will constantly read EOF. Whatever you type after C-d will not go into the input stream. So the simple answer to why you enter an infinite loop is the simple fact that EOF != '#' in combination with the fact that getchar() always will read EOF after it has read it once.

在这里可能要提到的另一件事是您正在使用缓冲输入。因此， getchar（）只会等到 stdin 中有东西。当您输入几个字符时，除非您按Enter或Cd，否则它们不会刷新到 stdin 。

Another thing that might be good to mention here is that you are using buffered input. So getchar() will simply wait until there is something in stdin. When you enter a few characters, they will not be flushed to stdin until you press enter OR C-d.

此外，请记住 EOF 不适合放在 char 中。函数 getchar 返回 int ，该数字可以是 char的数字或 EOF 。变量 ch 应该声明为int并正确检查 EOF 。

Also, remember that EOF does not fit in a char. The function getchar returns an int which is either a number that fits in a char or EOF. The variable ch should be declared as an int and properly checked for EOF.

可以重置 stdin ，尽管我不确定是否可以通过便携式方式进行重置。但这是一个问题：如何在Ctrl + D之后重新启动stdin？

It is possible to reset stdin, although I'm not sure if it's possible in a portable way. But here is a question about that: How to restart stdin after Ctrl+D?

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