C/C++ printf() before scanf() issue

Question

I'm using Eclipse to code in C/C++ and I'm struggling with what might be something pretty easy. In my code below I use printf() and after scanf(). Althougth printf is written before scanf() the output differs. I was able to find out something about similar issue here. But I wasn't able to solve it. Any ideas?

Code:

#include <stdio.h>

int main()
{
    int myvariable;

    printf("Enter a number:");
    scanf("%d", &myvariable);
    printf("%d", myvariable);

    return 0;
}

Expected output:

Enter a number:1
1

Instead I get:

1
Enter a number:1

Answer 1

Your output is being buffered. You have 4 options:

1. explicit flush

   fflush after each write to profit from the buffer and still enforce the desiredbehavior/display explicitly.

   fflush( stdout );
   

2. have the buffer only buffer lines-wise

   useful for when you know that it is enough to print only complete lines

   setlinebuf(stdout);
   

3. disable the buffer

   setbuf(stdout, NULL);
   

4. disable buffering in your console through what ever options menu it provides

---

Examples:

Here is your code with option 1:

#include <stdio.h>
int main() {

    int myvariable;

    printf("Enter a number:");
    fflush( stdout );
    scanf("%d", &myvariable);
    printf("%d", myvariable);
    fflush( stdout );

    return 0;
}

---

Here is 2:

#include <stdio.h>
int main() {

    int myvariable;

    setlinebuf(stdout);    

    printf("Enter a number:");
    scanf("%d", &myvariable);
    printf("%d", myvariable);

    return 0;
}

---

and 3:

#include <stdio.h>
int main() {

    int myvariable;

    setbuf(stdout, NULL);     

    printf("Enter a number:");
    scanf("%d", &myvariable);
    printf("%d", myvariable);

    return 0;
}

Answer 2

Ok, so finally I used something similar to what @zsawyer wrote as an option labelled 3. In my code I inserted this line:

setvbuf(stdout, NULL, _IONBF, 0);

As a first line in main():

#include <stdio.h>

int main()
{
    setvbuf(stdout, NULL, _IONBF, 0);

    int myvariable;

    printf("Enter a number:");
    scanf("%d", &myvariable);
    printf("%d", myvariable);

    return 0;
}

I got it from here.

Answer 3

Fast and painless, I've just defined a macro named "eprint" to add a flush() after calling printf(...) and I just use the eprint macro below:

#define eprintf(...) printf(__VA_ARGS__); \
                 fflush(stdout);      \

---

Sample code here:

#include <stdio.h>
#include <stdlib.h>

#define eprintf(...) printf(__VA_ARGS__); \
                     fflush(stdout);      \

int main(void) {
    int a;

    eprintf("a=");

    scanf("%d",&a);

    eprintf("I've read value %d.\n",a);

    return EXIT_SUCCESS;
}

---

Eclipse console output:

a=5
I've read value 5.

---

PS: I've just wasted 30 minutes looking up this eclipse console issue and possible fixes, and this one seems to be the most straight forward and easy to understand for anyone looking for such a thing.