* strace on cat /proc/my_file gives results by calling read twice why?
@ 2005-03-05 7:04 cranium2003
0 siblings, 0 replies; 2+ messages in thread
From: cranium2003 @ 2005-03-05 7:04 UTC (permalink / raw)
To: kernel newbies, kernerl mail
hello,
I have a /proc file kernel module with its proc
read function as
static char mybuf[1024];
ssize_t my_file_read(struct file *filp, char *buf,
size_t count, loff_t *ppos)
{
printk("Reading a from a /proc file....\n");
static int b_read = 0;
int len = strlen(mybuf);
if (len == 0 || b_read) {
b_read = 0;
return 0;
}
if (len > count - 1)
len = count - 1;
copy_to_user(buf, mybuf, len);
put_user(0, &buf[len]);
++len;
b_read = 1;
return len;
}
when i strace cat /proc/my_file i found message
printing twice
Reading a from a /proc file....
Reading a from a /proc file....
Why that happening?
Also i check by writing to it as cat >
/proc/my_file
and got single result for write function call not
twice as for read.
regards,
cranium
__________________________________
Celebrate Yahoo!'s 10th Birthday!
Yahoo! Netrospective: 100 Moments of the Web
http://birthday.yahoo.com/netrospective/
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: strace on cat /proc/my_file gives results by calling read twice why?
[not found] <no.id>
@ 2005-03-05 7:43 ` Alan Curry
0 siblings, 0 replies; 2+ messages in thread
From: Alan Curry @ 2005-03-05 7:43 UTC (permalink / raw)
To: cranium2003; +Cc: kernel newbies, kernerl mail
cranium2003 writes the following:
>
>when i strace cat /proc/my_file i found message
>printing twice
>Reading a from a /proc file....
>Reading a from a /proc file....
> Why that happening?
The first read returns some data and returns the number of bytes, and the
second one indicates that EOF has been reached by returning 0. A single read
call can't do both of those things, so cat needs to do 2 reads.
This has nothing to do with /proc or Linux; it is a logical consequence of
the way read() is defined, and the job cat is supposed to do: it copies
entire files to stdout, so it has to read until EOF. Only if the input file
is empty will it see the EOF on the first read.
^ permalink raw reply [flat|nested] 2+ messages in thread
end of thread, other threads:[~2005-03-05 7:44 UTC | newest]
Thread overview: 2+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2005-03-05 7:04 strace on cat /proc/my_file gives results by calling read twice why? cranium2003
[not found] <no.id>
2005-03-05 7:43 ` Alan Curry
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).