linux-kernel - Purpose of parameter in sysfs binary read

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Message-ID: <9f97a27ccc13bd63ce99be04018b6bde.squirrel@intranet.cs.nmsu.edu>
Date:	Tue, 18 Aug 2009 08:55:29 -0600
From:	"Rick L. Vinyard, Jr." <rvinyard@...nmsu.edu>
To:	"Linux USB" <linux-usb@...r.kernel.org>,
	"LKML" <linux-kernel@...r.kernel.org>
Subject: Purpose of parameter in sysfs binary read

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

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