Question

I’ve got a question about how the scanf function works. In this program:

#include <stdio.h>

int main(void) {

    int x;
    char y;

    printf("Write a number: ");
    scanf("%d", &x);

    printf("Write a character: ");
    scanf(" %c", &y);

    printf("%d", x);
    printf("%c", y);

    return 0;
}

If the user writes a number then hit enter then the scanf function would see from the format string that is should expect a number. When the user taps enter it would not read the new-line character and put it back in the buffer. Then since I am reading a character next I need to have a whitespace in the format string because the scanf function would otherwise take the new-line character and put in the y-variable.

However I am just curious about what happened if the user wrote something like

j344lk4fjk388

Would it put everything back in the buffer? And everything that gets “put-back” in the buffer would it automatically be read by the next scanf function in my program?

I’m reading: C Programming A Modern Approach 2nd Edition

Answer 1

You should always check the return value from scanf() to ensure that all the conversions you expected were successful.

Given the sample input:

j344lk4fjk388

The first scanf() would fail because j cannot be converted to an integer – returning 0 for the number of successful conversions. The second call would succeed, returning ‘j’ as the character.

If you need the string to be treated as an integer, use fgets() or a relative (perhaps POSIX getline()) to read the string, then strtoul() (or the appropriate relative – strtol(), strtoll(), strtoull(), strtoimax() or strtoumax()), specifying a base of at least 21 to convert the ‘j’, or 23 to take the ‘l’ and ‘k’.