On 29-Jul-2014 5:54 PM, "Anh Le" <anhlq2110@gmail.com> wrote:

Thanks, Jonathan, for the quick reply.

On Tue, Jul 29, 2014 at 3:36 PM, Jonathan Neuschäfer <j.neuschaefer@gmx.net> wrote:

...

Which "write operation" is called twice in your scenario?

It's the "write" method of the "struct file_operations" structure. I define it like this (no synchronizatin yet): static ssize_t sample_write(struct file *f, const char __user *buf, size_t len, loff_t *ppos) { printk(KERN_DEBUG "[sample] buf len: %u, *ppos: %u\n", len,

*ppos);

return simple_write_to_buffer(kbuf, 2048, ppos, buf, len); }

...

If a userspace program writes 1000 bytes at first, how can you know that it wants to perform another write later on?

I use printk to debug the module. More details in the answer below.

...

If a userspace program wants to write a chunk of data atomically, it should use just one call to write(2). (On Linux, one can save some copying by using writev(2), which writes data from multiple buffers in one atomic step.)

I tried the following command: echo $(perl -e "print 'a'x2000") >

/dev/sample

and get the following messages from dmesg: [30884.066433] [sample] buf len: 1008, *ppos: 0 [30884.066451] [sample] buf len: 993, *ppos: 1008

So as I understand my 2001 bytes has been split into 2 chunks, the

Who do you think did this split? I would say write a simple program and do only a single write syscall for your buffer size. Just see if that split happens again.

first one with 1008 bytes and the second one with 993 bytes, and therefore the write operation is called 2 times to consume the whole input.

-- Le Quoc Anh

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