A problem with the behaviour of scanf when data type is unsigned char

Question

So I have no idea why this behavior is observed:

#include <stdio.h>

int main()
{
    unsigned char c;
    char d;
    scanf("%hhu", &d);
    printf("%c\n", d);

    return 0;
}

If I remember correctly %hhu is for unsigned char. If I input almost any character like a, or b the answer is nothing and d gets the value 0. But if I input 97 then d prints the value a (so ASCII is converted to char). This clearly not the case when the data type is char. I can just input the character directly and it is stored.

Also in the slight modification of this code

#include <stdio.h>

int main()
{
    unsigned char c;
    char d;
    int g;
    scanf("%hhu", &d);
    printf("%c\n", d);
    scanf("%d", &g);
    printf("%d is g\n", g);

    return 0;
}

If I give first input as a character like w or a then it just skips the second scanf, but if the input is a number then it works normally.

Answer 1

a is not a valid input for %hhu. An integer (in the Math sense) is expected. If we add error checking (code below), we get:

$ gcc -Wall -Wextra -pedantic a.c -o a && ./a
a
Number of assigned variables: 0
Can't read: Invalid input

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

int main(void) {
    unsigned char c;

    int rv = scanf("%hhu", &c);
    if (rv == EOF) {
       if (ferror(stdin)) {
          perror("Can't read");
       } else {
          fprintf(stderr, "Can't read: Premature EOF\n");
       }

       exit(1);
    }

    printf("Number of assigned variables: %d\n", rv);

    if (rv < 1) {
       fprintf(stderr, "Can't read: Invalid input\n");
       exit(1);
    }

    printf("%hhu\n", c);

    return 0;
}

---

Invalid input stays in the handle's buffer to be obtained by future reads. So an error in a early scanf can cause later scanf to fail.

If you wish to proceed after an error, you could simply read until you get a LF to clear any "junk" in the buffer.

void read_stdin_to_eol(void) {
   while (1) {
      int ch = getchar();
      if (ch == '\n' || ch == EOF)
         return ch;
   }
}