How to implement a driver's read and write operations with synchronization properly
Jonathan Neuschäfer
j.neuschaefer at gmx.net
Tue Jul 29 08:47:58 EDT 2014
On Tue, Jul 29, 2014 at 07:15:08PM +0700, Anh Le wrote:
[...]
> 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
> 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.
I've tried this out myself, and it seems to be an issue with bash:
$ cat /tmp/aaaa.sh
#!/bin/sh
echo aaaaaaa...[trimmed to fit in an e-mail]...aaaaaaa
$ strace -e trace=write /tmp/aaaa.sh > /dev/null
write(1, "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"..., 2001) = 2001
+++ exited with 0 +++
$ strace -e trace=write bash /tmp/aaaa.sh > /dev/null
write(1, "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"..., 1008) = 1008
write(1, "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"..., 993) = 993
+++ exited with 0 +++
[ My default shell, /bin/sh, is Debian's dash. ]
As you can see, I observed the same pattern of 1008 and 993 bytes.
Greetings,
Jonathan Neuschäfer
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