String length between pointers

Question

Hi, I ran into a little problem and need some help:

If I have an allocated buffer of chars and I have a start and end points that are somewhere inside this buffer and I want the length between these two point, how can I find it?

i.e

char * buf; //malloc of 100 chars
char * start; // some point in buff
char * end; // some point after start in buf

int length = &end-&start? or &start-&end? 
//How to grab the length between these two points.

Thanks

Answer 1

It is just the later pointer minus the earlier pointer.

int length = end - start;

Verification and sample code below:

int main(int argc, char* argv[])
{
    char buffer[] = "Its a small world after all";
    char* start = buffer+6;  // "s" in SMALL
    char* end   = buffer+16; // "d" in WORLD

    int length = end - start;

    printf("Start is: %c\n", *start);
    printf("End is: %c\n", *end);
    printf("Length is: %d\n", length);
}

Answer 2

Just

length = end - start;

without ampersands and casts. C pointer arithmetics allows this operation.

Answer 3

simply

int length = (int)(end - start);

Answer 4

There's even a type, ptrdiff_t, which exists to hold such a length. It's provided by 'stddef.h'