strace on cat /proc/my_file gives results by calling read twice why?

[cranium2003]

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/
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to ***@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

[Alan Curry]

Post by cranium2003
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.
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to ***@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/