If you follow the directions in Documentation/DocBook/procfs-guide.tmpl
then your file read will not work correctly unless the whole file is
read in one call.
After examining (and experimenting with) fs/proc/generic.c I now see
what you have to do: in addition to writing bytes in the page
parameter, setting eof as appropriate and returning the number of
bytes written, you also need to do:
*start = page;
At minimum the documentation should be corrected.
Actually I think this is a bug in proc_file_read.
The problem seems to be in this code, executed after the read function:
if (!start) {
/*
* For proc files that are less than 4k
*/
start = page + *ppos;
n -= *ppos;
if (n <= 0)
break;
if (n > count)
n = count;
}
Can anyone explain what it's supposed to do?
What does it have to do with files < 4k ?
The addition of "*start = page;" both sets start to the correct value
and avoids executing the problematic code above.
[Note: The "start" in your read function is actually the address of
the "start" in proc_file_read.]
Therefore I think that code should be
if (!start) start=page;
The current code normally works for the first read because *ppos is 0.
Otherwise:
- start is given the wrong value (should be page)
- n is the number of bytes read, and it makes no sense to reduce that
by ppos, which is the file position at which you were supposed to
start reading.
The behavior I saw was that on the second buffer count and ppos were
both 512 (asked to read bytes 512-1023). My function returned 512,
so n was set to 0, which was then interpreted as eof.
Please cc me on replies.
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