Re: read_proc issue

[Randy.Dunlap]

On Tue, 26 Feb 2002, Laurent wrote:

| I'm developping a module which uses an entry in /proc (read-only)
| Currently my (*read_proc) function just write a integer in my /proc entry
| and increments it. Here is the code:
|
| static int number_read_procmem(char *buf, char **start, off_t offset, int
| count, int *eof, void *data)
| {
|         int len = 0;
|
|         len = sprintf(buf, "%d\n", number_current++);
|         *eof = 1;
|         return len;
| }
|
| the function is registered in init_module with this:
| create_proc_read_entry("number", 0, NULL, number_read_procmem, NULL);
|
| Problem is:
| when I 'cat /proc/number' multiple times, instead of getting
| 0
| 1
| 2
| 3
| ...
|
| I get
| 0
| 2
| 4
| 6
| ...

I was thinking that some part(s) of the proc-fs code might call
fs/proc/generic.c::proc_file_read() multiple times (internally),
but no, it's just the application (or libc) doing it AFAICT.

app. calls read() [Key data: with offset=0], which calls sys_read(),
which calls proc_file_read(), which returns N bytes, and then
proc_file_read() and sys_read() and read() return to the app.

When the app. gets N bytes of actual data on its read(), it
doesn't know whether it was at EOF or not, so it tries another
read() [with offset=N], and sys_read() calls proc_file_read()
again, both of which return 0, so the app. knows that there
is no more data.

-- 
~Randy