yea, i got your point but let me make my self a little more clear,if i have one application say(filebrowser) which uses an extension(basicly a virtual table in C) both will share the same heap space(or process space as the extension is always created in respect to the parent application filebrowser in this case)...now say this extension has a feature via which it can save a supplied buffer into any where in the filesystem with the desired filename.
This buffer is basically created by the application via calloc/realloc ,and the pointer is supplied to the extension, now in the extension i want to know the size(number of bytes) of the memory chunk it is pointing to.

i Hope i made myself a little more clear.