Accepting empty strings in C

Question

My program can either accept a number of any length or empty input. However, if the input is empty (space or newline), the program continues to wait for an input. I also tried fgets but if space/newline is pressed, it still waits for more input that is not a space/newline before closing.

Simpified code:

#include <stdio.h>
main()
{
    int num;
    scanf("%i",&num);
    printf("%i",num);
}

Input:

363792 

Output:

363792

Desired:

Input:

Output:

I'm new to C and am having a very hard time accomplishing this.

What tried using fgets:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
main()
{
    int n;
    char s[20];

    fgets(s,20,stdin);
    n = atoi(s);

    printf("%i",n);
}

Edit: Turns out I was not compiling the code right. So every time I tried to make changes, it just looked at the original code using scanf.

Answer 1

I also tried fgets but if space/newline is pressed, it still waits for more input that is not a space/newline before closing.

Firstly fgets will work in this case. If you had shown us what exactly you tried to do using fgets() then answer to this question would have much narrow or very specific.

I tried to do the same using fgets() and below is the code snippet.

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

int main(int argc, char *argv[])
{  
    char string[100];
    printf("Enter a number: ");

    fgets (string, 100, stdin);
    /* Here remove  the trailing new line char; not required unless you are trying to print the string */
    string[strlen(string) - 1] = '\0';

    printf("Num is %d\n", atoi(string));

    return 0;
}

If there is no input or just enter or space and enter, then number printed will be zero and fgets() will not wait until you enter a valid number.

Also check this on why you shouldn't use gets. Do not use gets()