Purpose of parameter in sysfs binary read

["Rick L. Vinyard, Jr." <rvinyard@cs.nmsu.edu>]

The read function pointer of the sysfs bin_attribute structure has this
signature:

ssize_t (*read)(struct kobject *kobj, struct bin_attribute *attr,
                        char *buf, loff_t offset, size_t size);

I've figured out the purpose of all the parameters except the loff_t
parameter.

Obviously it's an offset of some sort, but what is the meaning of the offset?

If I have binary data in a char* named bindata of size bsize, should it be
copied into buf+offset in something like:
  memcpy(buf+offset, bindata, bsize);

Or, is it an offset from both buf and bindata in something like:
  memcpy(buf+offset, bindata+offset, bsize-offset);

Thanks,

Rick