man scanf
       c      Matches a sequence of width count characters  (default  1);  the
              next pointer must be a pointer to char, and there must be enough
              room for all the characters (no terminating NUL is added).   The
              usual  skip of leading white space is suppressed.  To skip white
              space first, use an explicit space in the format.

#include <stdio.h>

typedef unsigned char BYTE;

void print_memory(BYTE *start, int size)
{
        int i;

        for(i = size - 1; i >= 0; --i) {
                printf("%p:%02x: %c\n", start + i, *(start + i), *(start + i));
        }
}

int main(void)
{
        char ch[7] = "XXXXXXX";

        print_memory(ch, sizeof(ch));
        scanf("%3c", ch);
        print_memory(ch, sizeof(ch));

        return 0;
}

0xbffff536:58: X
0xbffff535:58: X
0xbffff534:58: X
0xbffff533:58: X
0xbffff532:58: X
0xbffff531:58: X
0xbffff530:58: X
abcde
0xbffff536:58: X
0xbffff535:58: X
0xbffff534:58: X
0xbffff533:58: X
0xbffff532:63: c
0xbffff531:62: b
0xbffff530:61: a

#include <stdio.h>

typedef unsigned char BYTE;

void print_memory(BYTE *start, int size)
{
        int i;

        for(i = size - 1; i >= 0; --i) {
                printf("%p:%02x: %c\n", start + i, *(start + i), *(start + i));
        }
}

int main(void)
{
        char ch[7] = "XXXXXXX";

        print_memory(ch, sizeof(ch));
        scanf("%3c%2c", ch, ch + 1);
        print_memory(ch, sizeof(ch));
        printf("%c, %c\n", *ch, *(ch + 1));

        return 0;
}

0xbfffe5b6:58: X
0xbfffe5b5:58: X
0xbfffe5b4:58: X
0xbfffe5b3:58: X
0xbfffe5b2:58: X
0xbfffe5b1:58: X
0xbfffe5b0:58: X
abcde
0xbfffe5b6:58: X
0xbfffe5b5:58: X
0xbfffe5b4:58: X
0xbfffe5b3:58: X
0xbfffe5b2:65: e
0xbfffe5b1:64: d
0xbfffe5b0:61: a
a, d

#include <stdio.h>

typedef unsigned char BYTE;

void print_memory(BYTE *start, int size)
{
        int i;

        for(i = size - 1; i >= 0; --i) {
                printf("%p:%02x: %c\n", start + i, *(start + i), *(start + i));
        }
}

int main(void)
{
        char ch1 = 'X', ch2 = 'Y';

        print_memory(&ch2, 5);
        scanf("%3c%2c", &ch1, &ch2);
        print_memory(&ch2, 5);
        printf("%c, %c\n", ch1, ch2);

        return 0;
}

0xbfffdf4a:ff: ÿ
0xbfffdf49:df: ß
0xbfffdf48:68: h
0xbfffdf47:58: X
0xbfffdf46:59: Y
abcde
0xbfffdf4a:ff: ÿ
0xbfffdf49:63: c
0xbfffdf48:62: b
0xbfffdf47:65: e
0xbfffdf46:64: d
e, d

man scanf
...
       c      Matches a sequence of width count characters  (default  1);  the
              next pointer must be a pointer to char, and there must be enough
              room for all the characters (no terminating NUL is added).   The
              usual  skip of leading white space is suppressed.  To skip white
              space first, use an explicit space in the format.
----------
#include <stdio.h>

typedef unsigned char BYTE;

void print_memory(BYTE *start, int size)
{
        int i;
        for(i = size - 1; i >= 0; --i)
        {
                printf("%p:%02x: %c\n",
                        start + i,
                        *(start + i),
                        *(start + i));
        }
}

int main(void)
{
        char a[] = "XXXXXXX";

        print_memory(a, sizeof(a));
        scanf("%3c", a);
        print_memory(a, sizeof(a));

        return 0;
}

0xbfffe4a7:00:
0xbfffe4a6:58: X
0xbfffe4a5:58: X
0xbfffe4a4:58: X
0xbfffe4a3:58: X
0xbfffe4a2:58: X
0xbfffe4a1:58: X
0xbfffe4a0:58: X
abcde
0xbfffe4a7:00:
0xbfffe4a6:58: X
0xbfffe4a5:58: X
0xbfffe4a4:58: X
0xbfffe4a3:58: X
0xbfffe4a2:63: c
0xbfffe4a1:62: b
0xbfffe4a0:61: a
----------
#include <stdio.h>

typedef unsigned char BYTE;

void print_memory(BYTE *start, int size)
{
        int i;
        for(i = size - 1; i >= 0; --i)
        {
                printf("%p:%02x: %c\n",
                        start + i,
                        *(start + i),
                        *(start + i));
        }
}

int main(void)
{
        char a[] = "XXXXXXX";

        print_memory(a, sizeof(a));
        scanf("%3c%2c", a, a + 1);
        print_memory(a, sizeof(a));

        return 0;
}

0xbffff7a7:00:
0xbffff7a6:58: X
0xbffff7a5:58: X
0xbffff7a4:58: X
0xbffff7a3:58: X
0xbffff7a2:58: X
0xbffff7a1:58: X
0xbffff7a0:58: X
abcde
0xbffff7a7:00:
0xbffff7a6:58: X
0xbffff7a5:58: X
0xbffff7a4:58: X
0xbffff7a3:58: X
0xbffff7a2:65: e
0xbffff7a1:64: d
0xbffff7a0:61: a
----------
#include <stdio.h>

typedef unsigned char BYTE;

void print_memory(BYTE *start, int size)
{
        int i;
        for(i = size - 1; i >= 0; --i)
        {
                printf("%p:%02x: %c\n",
                        start + i,
                        *(start + i),
                        *(start + i));
        }
}

int main(void)
{
        char ch1 = 'X', ch2 = 'Y';

        print_memory(&ch2, 4);
        scanf("%3c%2c", &ch1, &ch2);
        print_memory(&ch2, 4);

        return 0;
}

0xbfffe629:e6: æ
0xbfffe628:48: H
0xbfffe627:58: X
0xbfffe626:59: Y
abcde
0xbfffe629:63: c
0xbfffe628:62: b
0xbfffe627:65: e
0xbfffe626:64: d