C - distance between 2 pointers

Question

How can I know the distance in bytes between 2 pointers? For example:

I would like to know how many bytes there are between p2 and p1 ( in this case 3) because to reach p2 with p1 I have to do 3 steps...
step1 p1 is in B
step2 p1 is in C
step3 p1 is in D

so i need that return to me 3
I'm asking this type of question because I'm implementing the lz77 algorithm

Answer 1

You could try with:

ptrdiff_t bytes = ((char *)p2) - ((char *)p1);

But this only works as expected if the pointers you subtract point to the same single piece of memory or within it. For example:

This will not work as expected:

char *p1 = malloc(3); // "ABC", piece 1
char *p2 = malloc(3); // "DEF", piece 2
char *p3 = malloc(3); // "GHI", piece 3

ptrdiff_t bytes = p3 - p1; // ABC ... DEF ... GHI
                           // ^               ^
                           // p1              p3
                           // Or:
                           // GHI ... ABC ... DEF
                           // ^       ^
                           // p1      p3

// Gives on my machine 32
printf("%td\n", bytes);

Because:

• The malloc implementation could allocate some additional bytes for internal purposes (e.g. memory barrier). This would effect the outcome bytes.
• It is not guaranteed that p1 < p2 < p3. So your result could be negative.

However this will work:

char *p1 = malloc(9);  // "ABCDEFGHI", one piece of memory
char *p2 = p1 + 3;     // this is within the same piece as above
char *p3 = p2 + 3;     // this too

ptrdiff_t bytes = p3 - p1; // ABC DEF GHI
                           // ^       ^
                           // p1      p3



// Gives the expected 6
printf("%td\n", bytes);

Because:

• The allocated 9 Bytes will always be in one piece of memory. Therefore this will always be true: p1 < p2 < p3 and since the padding/additional bytes are on the start/end of the piece subtraction will work.