console input C

Question

I am a beginner in C and would like to know what's the problem about my code here :

#include "stdio.h"

int main(void)
{ 
    int a;

    printf("Please input an integer value: ");
    scanf("%d", &a);
    printf("You entered: %d\n", a);

    return 0;
}

My problem is that I have to type a value before having any consol output, for example if I type 7, I get this console output : Please input an integer value: You entered: 7

I tried the exact same code in another computer and it worked pretty well, I guess it's a buffer problem ? but I have no idea how to fix it.. Any ideas please ?

you need to either print a newline character, or flush `stdout` since it is buffered: `fflush(stdout);` — Christian Gibbons, Feb 18 '20 at 19:35
Thank you Christian, it works ! But can I have more details about it ? Why does it work in my friends computer and not in mine without flushing ? Will I have to do it each time ? — Mehdi Zayene, Feb 18 '20 at 19:39
In some environments stdio buffers are flushed automatically on input functions like `scanf()` or `fgets()`, but this is nothing you can depend on. — Ctx, Feb 18 '20 at 19:42
Some systems will flush the buffer when a newline is output, but you don't write one before the `scanf`. — Weather Vane, Feb 18 '20 at 19:44
I believe you can force the stream to be unbuffered by way of `setvbuf` if you really wanted to. — Christian Gibbons, Feb 18 '20 at 19:50
https://stackoverflow.com/questions/1716296/why-does-printf-not-flush-after-the-call-unless-a-newline-is-in-the-format-strin — yano, Feb 18 '20 at 20:34

myradio · Answer 1 · 2020-02-18T20:26:17.503

1

As other already mentioned, to guarantee that that line is going to be printed at that point in your code you can flush the standard output like this,

#include "stdio.h"

int main(void)
{ 
    int a;

    printf("Please input an integer value: ");
    fflush(stdout);
    scanf("%d", &a); 
    printf("You entered: %d\n", a);

    return 0;
}

you can read this for more details, Why does printf not flush after the call unless a newline is in the format string?

updated thanks to @Osiris comments

edited Feb 18 '20 at 20:26

answered Feb 18 '20 at 20:12

myradio

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Well, it doesn't really matter in this case. – myradio Feb 18 '20 at 20:17
No. That is not the case, the your input from `scanf()` you will see it anyways, to hide the input you have to use a function with masking. – myradio Feb 18 '20 at 20:21
The output itself you won't see until the `printf()`. Maybe what might be useful is to put another `fflush()` after the second printf inc ase the code grows. Or as mentioned in the link, just set the buffer to 0: `setbuf(stdout, NULL);` – myradio Feb 18 '20 at 20:23
You are totally right. Potentially the first printf could have occured after the `scanf()` asked for input. – myradio Feb 18 '20 at 20:27
Since my comments do not apply to the updated answer i will delete them. – Osiris Feb 18 '20 at 20:29

console input C

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