Pointers in C, integer pointer to character pointer

Question

When I run this code:

int arr[3] = {2,3,4};
char *p;
p = (char*)arr;
printf("%d", *p);
p = p+1;
printf("%d", *p);

The output is 2 and 0. The second result is slightly confusing. Could someone explain why this is happening?

Answer 1

Let's break this down:

int arr[3] = {2,3,4};

Creates an array of 3 integers. Assuming your system is 32bit little endian this is how it looks in memory:

02 00 00 00 03 00 00 00 04 00 00 00

char *p; p = (char*)arr;

p now points to arr but is a pointer to char*. In other words, p points to the 02.

printf("%d", *p);

You are printing as an int the location referenced by p. So when you dereference p (by writing *p) you are accessing the char (since p is of type char*) referenced by p. Which is 02.

p = p+1;

p now points to the 00 just after 02, because p is char*. So when you add 1, it will move by 1 * sizeof(char) = 1 * 1 = 1 byte in memory.

printf("%d", *p);

You are printing as an int the location referenced by p. So when you dereference p (by writing *p) you are accessing the char (since p is of type char*) referenced by p. Which is 00.

If you wanted to print 3 instead of 0 you have to change your pointer type to int* instead of char*, making the pointer move by 1 * sizeof(int) = 1 * 4 = 4 bytes in memory.

Answer 2

The result you get will depend on the size of int on your implementation and its endianness.

Assuming 32bit ints, 8bit chars and a litte-endian environment (say x86), arr will be like this in memory:

<  arr[0]  > <  arr[1]  > <  arr[2] >
02 00 00 00  03 00 00 00  04 00 00 00
^  ^         ^
p  p+1 ...   p+4

If you take a char pointer to the start of that memory, and print out the first element, 2 should be output. If you increment that pointer, 0 will be output next. You'll need to increment it a few times more to 'see' 3.

Note that on a big-endian environment with the same type sizes, your program would have output two zeros, because the layout would have been:

<  arr[0]  > <  arr[1]  > <  arr[2] >
00 00 00 02  00 00 00 03  00 00 00 04
^  ^         ^
p  p+1 ...   p+4