why am i such a novice

offset / blocksize = blocks into the archive (rounded)
offset % blocksize = null bytes in the file

in c, do i have to do two math operations to get these (fmod?) or can i do one
or do i have to use some non-standard math operation

There isn't a built in operator but the standard headers contain a function or macro that puts both results into a structure, ideally using its own machine-specific implementation. Can't remember what it is though.