More edit: It turns out that files in "/proc" or "/sys" are not really files. Not on disk, but not even in memory. Their contents is generated by the kernel on the fly. So, there is no way to tell their size and so fstat() returns zero. Only way to read them is to just read() them in until EOF.

Maybe they could be read up to the buffer (page) size and if the EOF is encountered, the filesize becomes determined...?