From mboxrd@z Thu Jan 1 00:00:00 1970 From: krzaq Subject: Re: open() and read() a file from sysfs Date: Tue, 1 Mar 2005 09:58:20 +0100 Message-ID: <2ab8d39a05030100582e47df30@mail.gmail.com> References: <2ab8d39a050228061825dda7fe@mail.gmail.com> <4223C2C2.9070101@laposte.net> Reply-To: krzaq Mime-Version: 1.0 Content-Transfer-Encoding: 7bit In-Reply-To: <4223C2C2.9070101@laposte.net> Sender: linux-c-programming-owner@vger.kernel.org List-Id: Content-Type: text/plain; charset="us-ascii" To: linux-c-programming@vger.kernel.org On Tue, 01 Mar 2005 02:17:54 +0100, simon wrote: > hello > > krzaq wrote: > > Hi all! > > > > I have a basic question with file opening. > > I would like to make use of the data accessed from i2c sensors through sysfs. > > > > Here's the code: > > > > fd = open(/sys/bus/i2c/.../temp2_input,O_RDONLY); > > > > while(1) { > > char[64] buf; > > read(fd,buf,siezof(buf)); > > ... > > do_something > > ... > > sleep(5); > > } > > > > The thing is I always get the same reading in each read() :(. > > When I do: > > # cat /sys/bus/i2c.../temp2_input > > I see that the temperature is changing, but my code still displays the > > same reading. > ok... i suggest you to read the return value of read... > in your code, you can't see if an error append... > reinit your buffer with memset can be a good idea... > in your programm, we can imagine a first correct read, a second who > return an error... you don't detect it... and you read an unchanged > buffer... > ok that's not your only problem... This was kinda 'pseudo code'. The real program checks all errors including what read() returns and OFCOURSE it has lseek() (sorry about the confusion). Anyway the code runs without errors, but still it doesn't get updated values. Now for the fun part. There are two softwares displaying system information (including i2c reads): - torsmo - gkrellm I looked at the code of both. Torsmo open()'s i2c devices and then sequentially read()'s them lseek()'ing to the begining (exaclty what I was trying to do). With linux - 2.6.10 it doesn't update the values! It's been reported as a bug on sf homepage of this project. Gkrellm, which is a much more advanced project, displays updated readings. After poking around with source I saw, that they are re-open()ing each file (sensor) they want to read() :). Since I do not have 2.6.9 anymore, I will have to download it and check if read()'ing and lseek()'ing on one opened file is working there. -- Regards Karol Krzak