Returns 0 on stream end or if buffer has no space available for data.
See also:
readerpub fn readPositional(file: File, io: Io, buffer: []const []u8, offset: u64) ReadPositionalError!usize
pub fn readPositional(file: File, io: Io, buffer: []const []u8, offset: u64) ReadPositionalError!usize {
return io.vtable.fileReadPositional(io.userdata, file, buffer, offset);
}