Thread: Issue with pointer

I am still learning the C programming language and trying out various
things with pointers, so please bare with me.

I have written a program which allows me to view the memory contents
using a pointer, I first started with this program.

Code:

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

#define  MAX_CHARS  4

typedef unsigned char BYTE;

void dump(BYTE *ptr);


int main(void) {

    int offset = 0;
    BYTE *ptr = NULL;
    char *memory = NULL;

    memory = (char *)malloc(sizeof(char) * MAX_CHARS);

    *(memory+0) = 'T';
    *(memory+1) = 'e';
    *(memory+2) = 's';
    *(memory+3) = 't';

    for (offset=0; offset<MAX_CHARS; offset++) {
        ptr = (BYTE *)memory+offset;
        dump(ptr);
    }

    free(memory);

    return 0;

}


void dump(BYTE *ptr) {

    printf("%x %x (%03d) ",(unsigned int)ptr,*ptr,*ptr);

    if (isprint(*ptr)) {
        printf("[%c]\n",*ptr);
    } else {
        printf("[.]\n");
    }

}

Output:

80a2008 54 (084) [T]
80a2009 65 (101) [e]
80a200a 73 (115) [s]
80a200b 74 (116) [t]

This works great, just how I expected, however when I change the
prgoram to use integers instead of chars:

Code:

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

#define  MAX_INTS  4

typedef unsigned char BYTE;

void dump(BYTE *ptr);


int main(void) {

    int offset = 0;
    BYTE *ptr = NULL;
    int *memory = NULL;

    memory = (int *)malloc(sizeof(int) * MAX_INTS);

    *(memory+0) = 1;
    *(memory+1) = 1;
    *(memory+2) = 4;
    *(memory+3) = 9;

    for (offset=0; offset<MAX_INTS; offset++) {
        ptr = (BYTE *)memory+offset;
        dump(ptr);
    }

    free(memory);

    return 0;

}


void dump(BYTE *ptr) {

    printf("%x %x (%03d) ",(unsigned int)ptr,*ptr,*ptr);

    if (isprint(*ptr)) {
        printf("[%c]\n",*ptr);
    } else {
        printf("[.]\n");
    }

}

Output:

8f2e008 1 (001) [.]
8f2e009 0 (000) [.]
8f2e00a 0 (000) [.]
8f2e00b 0 (000) [.]


As you can see this does not work in the same manner, I understand that
the char is 1-byte and the int is 4-bytes, but I thought when you increment
a pointer it incremented based on its data type.

I'm sure I'm doing something really dumb, can anyone please help?

Any assistance with this problem would be much appreciated.

Thanks

As per your comment about the ascii chart (thank you), I have changed the code as follows:

Code:

    *(memory+0) = 49;
    *(memory+1) = 57;
    *(memory+2) = 55;
    *(memory+3) = 49;

Output:

93ec008 31 (049) [1]
93ec009 0 (000) [.]
93ec00a 0 (000) [.]
93ec00b 0 (000) [.]

Actually, since you have a typedef you can just change it to int. The cast in your for loop is then redundant but it should work.

I can see what I've done now. I've changed as you said and it works, many thanks for your assistance.

I'm not paying enough attention! There is another issue here, it really should work to reach the lower byte of the int since the numbers are so low.

And if you look at the output in the first example you see that the first one is correct. It prints 1.

The problem is that the offset of memory is also four bytes so you are copying the first byte from int 1, then from int 2 etc.

if you try to make the cast like this: (BYTE*)(memory+offset) it will work even for the unsigned char.