c - What is the behavior of scanf when using "%s" format specifier for reading in single char?

Question

#include <stdio.h>

int main(void){

char a;
scanf("%s", &a);

return 0;
}

Works for me, but I wonder if in other context there might be any unexpected results.

Answer 1

Take this code for example -

#include <stdio.h>

int main(void){

    char a;
    char b = '1';
    char c = '1';
    scanf("%s", &a);

    printf("%c = %c
", b, c);
    return 0;
}

You would expect it to print 1 = 1, but just now when I ran it, it printed = 1 (at least in my compiler, don't expect anything stable from it)

scanf writes a string to the address of a, expecting enough space was allocated there (which is wrong in this case), this string has the input char and the null terminator. The null terminator overwrites some other memory, in my case - that of b. This is undefined behavior - don't do that (at least not while expecting it to make any sense).