C cannot read string after I read an integer

Question

Below is my source code. After I read the integer, the program should wait until I type a string and then press enter. However, as soon as I enter the integer, the program exits. Can you tell me where is my fault?

#include <stdio.h>

#include <string.h>

int main()
{

    int n;
    char command[255];

    scanf("%d", &n);
    fgets(command, 255, stdin);

    return 0;
}

I mention that I also tried to use gets(command), but I get the same result.

scanf doesn't consume the newline char ' \n' and fgets stops at that. — Magisch, Dec 01 '15 at 15:16
Also [fgets doesn't work after scanf](http://stackoverflow.com/questions/5918079/fgets-doesnt-work-after-scanf) — nnn, Dec 01 '15 at 15:20

score 13 · Accepted Answer · edited May 23 '17 at 12:24

13

There is a trailing newline, since you press ENTER after the integer. Eat it by changing this:

scanf("%d", &n);

to this:

scanf("%d ", &n); // this will eat trailing newline

As chux said, scanf("%d ", &n) will not return until the user enters the number and some following non-white-space.

Relevant question: C: Multiple scanf's, when I enter in a value for one scanf it skips the second scanf

Also I have in mind my example: Caution when reading char with scanf .

Also, as Marco stated, you could use: scanf("%d\n", &n);, which targets the newline specifically.

There is also this possibility:

#include <stdio.h>
#include <string.h>

int main()
{

    int n;
    char command[255], newline;

    scanf("%d", &n);
    scanf("%c", &newline); // eat trailing newline
    fgets(command, 255, stdin);

    return 0;
}

However, I would personally use two fgets() and no scanf(). :)

edited May 23 '17 at 12:24

Community

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1

answered Dec 01 '15 at 15:16

gsamaras

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Why not `scanf("%d\n", &n);`? Looks clearer. – Marco Bonelli Dec 01 '15 at 15:20
That would work too @MarcoBonelli, grazie. – gsamaras Dec 01 '15 at 15:22
`scanf("%d ", &n);` will not return until the user enters the number and some following non-white-space. – chux - Reinstate Monica Dec 01 '15 at 19:00
That's true @chux, I am updated the answer, if you think we can improve it further, please let me know. – gsamaras Dec 01 '15 at 20:58
1

For learner code, now it is fine. The basic issue with `scanf("%d ", &n)` is with many SO posts where that trailing `" "` causes issues. In the end, `scanf()` is the weak link and certainly problematic to mix with `fgets()`. I would have used 2 `fgets()` calls - but the issues that it helps are likely well beyond the scope of this post. – chux - Reinstate Monica Dec 01 '15 at 21:03
@chux I agree with your last statement and that's how I was taught, so please allow me to update. – gsamaras Dec 01 '15 at 21:59

score 2 · Answer 2 · answered Dec 01 '15 at 15:17

2

The scanf() leaves newline character after the integer, and fgets() will read it and exit.

Try reading all input with fgets().

#include <stdio.h>
#include <string.h>

int main()
{

    char buffer[255];
    int n;
    char command[255];

    fgets(buffer, sizeof(buffer), stdin);
    sscanf(buffer, "%d", &n);
    fgets(command, sizeof(command), stdin);

    return 0;
}

answered Dec 01 '15 at 15:17

MikeCAT

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Yes! - Good to not mix `scanf()` with `fgets()` (Best to not use `scanf()` as this answer did not). – chux - Reinstate Monica Dec 01 '15 at 19:05

score 1 · Answer 3 · edited Dec 01 '15 at 19:02

1

Typing "3c" and pressing enter in your console will make your input buffer stdin look like this: {'3','c','\n'} and would work, since scanf consumes the 3, fgets consumes the c, and the \n would be where fgets stops.

But if you type "3" and press enter, scanf will consume the 3, and the newline character will be left, causing fgets to consume no characters.

edited Dec 01 '15 at 19:02

chux - Reinstate Monica

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answered Dec 01 '15 at 15:18

Magisch

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The input buffer would be `{'3', 'c', '\n'}` in the first scenario, right? I think you accidentally typed 'd' instead of 'c' ? – Polb Dec 01 '15 at 15:26
@Polb yah, sorry about that – Magisch Dec 01 '15 at 16:48

C cannot read string after I read an integer

3 Answers3