You are freeing something not returned by malloc (q is never initialized). Also I can see you are doing

p++

In doing this, you are losing p and you can't free it anymore. Just in case you meant q=p, that's not valid either. You can only free what malloc returned.

EDIT

In light of comment it seems the OP does intend q = p. You can do this:

char *p;char *save_p;p = malloc(10); /* stop casting malloc */save_p = p; /* save the original value of p BEFORE altering it *//* use p to your heart's content */free(save_p); /* it's legal to free this */

I see you are asking something about char versus integer. It's the same:

You can only free the exact values returned by malloc.

You never initialize that q pointer you're freeing, that's why your program segfaults: it tries to free a random uninitialized pointer.

You do not initialize q to the area you malloc-ed, and then you trash p (doing p = q), too. You need to save your pointer to malloc-ed area in order to be able to free it. So something like

p = q = malloc(...);// play with p, or q, but not with bothfree( /* the one you have not played with */ );