scanf格式字符串中“％c”之后的空格将忽略最后一个字符

Question

#include <stdio.h>

int main(void)
{
    char c;
    while (scanf(" %c ", &c) != EOF)
        printf("you typed: %c \n", c);

    return 0;
}

I can just put blank space before %c like " %c" to let scanf read a single character and skip any white spaces, but if I put another blank space after %c there comes problems:

a s d f
you typed: a
you typed: s
you typed: d
1 2 3
you typed: f
you typed: 1
you typed: 2

so why does it leave last character in input stream? Even though I actually typed "asdf " with tailing space.

In fact scanf leaves "f \\n" unread, right?

btw in the case of " %c" there's no problem as expected.

Answer 1

From the draft of the C11 standard :

7.21.6.2 The fscanf function

[...]

5. A directive composed of white-space character(s) is executed by reading input up to the first non-white-space character (which remains unread), or until no more characters can be read. The directive never fails.

This means that a whitespace character in the format string of *scanf will read and discard all whitespace characters until a non-whitespace character is encountered .

So, even if you input "asdf " , your scanf , after consuming the f , would read and discard the final space and would wait for further input as it hasn't encountered a non-whitespace character yet.

You can close the stream by sending in an EOF signal so that the scanf will stop scanning and return EOF . Or you could remove that space and revert it to " %c" which you originally had.