Question

int x;
scanf("%d",&x);
printf("%d",x);

Input: . (just a period)
Output: 4096

Why does it output 4096 here. Here is what I am thinking: So the ASCII value of a period is 46. Upon input, it reads in x as the bit pattern for 46? And when it prints, does it print out four bytes at the memory location of x, so only the first byte is filled with the bit pattern corresponding to 46, and the rest is random stuff that makes up the 4096? But this is incorrect, because look what happens when I do this–

int x;
scanf("%d",&x);
printf("%c",x);

Input: . (period)
Output: (nothing)

Input: 46
Output: . (period)

Even more confusing is what is going on when I do this:

int x;
scanf("%c",&x);
printf("%d",x);

Input: . (period)
Output: 4142

Input: 46
Output: 4148

Input: 47
Output: 4148

Answer 1

You need to check the return value of scanf (it’s the count of successfully read values):

int x;

if (scanf("%d",&x) == 1) {
    printf("%d", x);
} else {
    printf("Invalid input.");
}

If the parsing failed, the value of x is not defined.

Once scanf has failed, further calls will also fail because the invalid input is not removed from the stream. Because of this it is not a good idea to use scanf.