C while loop does not stop running once input is made

Question

I am trying to create a program that repeatedly asks the user for input. However, in this case, the program just keeps printing "Enter your input" until it crashes after I enter something for the first time.

#include <stdio.h>

int main() {

    while (1) {
        char input[100];

        printf("Enter a input: ");
        scanf("%[^\n]s", input);
    }
}

If you check the return value from `scanf`, you'll see that it's not reading anything, which could be a good clue. — Nate Eldredge, Sep 29 '20 at 23:18
Note that you almost certainly meant `"%[^\n]"` instead of `"%[^\n]s"`, unless you really want to match a literal `s` in the input. — William Pursell, Sep 29 '20 at 23:24
Does this answer your question? [Read a string as an input using scanf](https://stackoverflow.com/questions/35103745/read-a-string-as-an-input-using-scanf) — , Sep 29 '20 at 23:28
This may also be relevant: https://stackoverflow.com/questions/42265038/how-to-check-if-user-enters-blank-line-in-scanf-in-c — , Sep 29 '20 at 23:37

score 1 · Answer 1 · answered Sep 29 '20 at 23:23

You are asking scanf() to read everything up to, but not including, the line break that ends the user's input.

So, the line break stays in the input buffer. Then on subsequent calls to scanf(), the line break is still not read from the input buffer, preventing scanf() from reading any new input.

You need to read that line break from the input buffer so that a subsequent call to scanf() can read the user's next input.

score 1 · Accepted Answer · answered Sep 29 '20 at 23:39

In:

scanf("%[^\n]s", input);

There are 2 problems:

The s is not part of that specific specifier, it should be only %[^\n].
That specifier leaves the \n newline character in the input buffer.

Your cycle will enter an infinite loop because there is always something to read there, but it's something that must not be parsed and should remain in the buffer.

A simple way to get rid of it is to place a space before the specifier:
```
scanf(" %[^\n]", input);
       ^
       |
```

C while loop does not stop running once input is made

2 Answers2