Index of element pointed to by pointer - c

Question

I have a 2D char array declared as
char A[100][100]
I also have a pointer that points to one element of that array
char *ptr;
ptr = A[5]
This pointer gets passed to another function, which needs to be able to know the index of that pointer (in this case, 5)
void func(void *ptr) {
int index = ???
}
How can I do this? Is this even possible?

Answer 1

Yes it is possible if you can see A in func (A is just a doubly-indexed 1D array, not a pointer on arrays, that's why it is possible).
#include <stdio.h>
char A[100][100];
void func(void *ptr) {
char *ptrc = (char*)ptr;
printf("index %d\n",int((ptrc-A[0])/sizeof(A[0])));
}
int main()
{
char *ptr = A[5];
func(ptr);
}
result:
index 5
of course, if you pass an unrelated pointer to func you'll have undefined results.
Note: it is required to cast the incoming void * pointer to char * or the compiler won't let us diff pointers of incompatible types.
EDIT: as I was challenged by chqrlie to compute both indexes, I tried it and it worked (also added safety to prevent the function being called with an unrelated pointer):
#include <stdio.h>
#include <assert.h>
char A[100][100];
void func(void *ptr)
{
char *ptrc = (char*)ptr;
ptrdiff_t diff = (ptrc-A[0]);
assert(0 <= diff);
assert(diff < sizeof(A));
printf("index %d %d\n",(int)(diff/sizeof(A[0])),(int)(diff % sizeof(A[0])));
}
int main()
{
char *ptr = &(A[5][34]);
func(ptr);
}
result:
index 5 34

Answer 2

You can compute the offset of ptr from the beginning of the array and derive the coordinates:
#include <stdio.h>
#include <stdlib.h>
char A[100][100];
void func(void *ptr) {
if (ptr == NULL) {
printf("ptr = NULL\n");
return;
}
ptrdiff_t pos = (char *)ptr - A[0];
if (pos < 0 || pos > (ptrdiff_t)sizeof(A)) {
printf("ptr points outside A: ptr=%p, A=%p..%p\n",
ptr, (void*)A, (void*)&A[100][100]);
} else {
printf("ptr = &A[%d][%d]\n",
(int)((size_t)pos / sizeof(A[0])), // row number
(int)((size_t)pos % sizeof(A[0]))) // column number
}
}
int main(void) {
char *ptr = &A[5][3];
func(ptr);
return 0;
}