On Mon, Mar 29, 2004 at 20:55:06 +0530, Amith wrote:
> hi all,
> i'am new to module programming and was reading lkmpg .I
> read some stuff
> relating to the /proc filesystem.i have some doubts here.
>
> 1) struct proc_dir_entry {
> ....
> ....
> struct file_operations * proc_fops;
> ....
> void *data;
> read_proc_t *read_proc;
> write_proc_t *write_proc;
> .....
> .....
> };
>
> 1) Now why do we need read_proc and write_proc here ? proc_fops can be
> initialise to read,write ...etc function to read and write from the
> proc file right ? then why read_proc and write_proc ? I'am sure ive
> missed something here ..
IIRC it's just a convenience. You can choose. Either implement your own
read and write in file_ops, or use the standart ones and
read_proc/write_proc.
Which way you should choose is a matter of what data you have.
read_proc/write_proc is convenient when you have a buffer, that is used
as a variable on kernel side and you need to provide access to it, which
is what most of /proc happens to do.
> 2) I initialise read_proc as below .
> struct proc_dir_entry *temp_proc_file;
> temp_proc_file->read_proc = procfile_read;
>
>
> int procfile_read(char *buffer,
> char **buffer_location,
> off_t offset,
> int buffer_length,
> int zero) {
>
> what does char **buffer_location and char *buffer mean here ?
> i tried :
>
> *buffer_location = my_buffer; /* as given in the example */
>
> it works fine when i do a cat /proc/myfile - it displays
> whatever is
> there in my_buffer (i.e set by me ).
>
> But where is **buffer_location actually ? and please let me
> know what char *buffer means up there .
buffer -- I'd guess you are setting it up somewhere else and it just
gets passed back to you as a convenience (so you can use one function
for several similar proc files). (But I didn't look in the code, actualy
-- you should).
buffer_location -- the read_proc method is NOT a read method, NOR is it
simply invoked by read method. It is only called when user STARTS
reading. You set this to the star of data, that should be (eventualy)
passed to userspace. It may take several read requests to pass them and
each will move this pointer to what remains. The buffer_location actualy
points somewhere to the file structure or a structure associated with
it.
-------------------------------------------------------------------------------
Jan 'Bulb' Hudec <bulb@ucw.cz>
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