open() and read() a file from sysfs

open() and read() a file from sysfs

[krzaq]

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.

So what's the deal? Why is it cached?
Am I supposed to open() and close() the file inside 'while' loop??
What's the proper way to do it?

-- 
Regards
Karol Krzak

Re: open() and read() a file from sysfs

[simon]

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...

try something like that :

char buf[SIZE];
int fd;

if ((fd = open(/sys/bus/i2c/.../temp2_input,O_RDONLY)) == -1)
{
	perror ("open ()");
	return (-1);
}
while (1)
{
	memset (buf, 0, SIZE);
	if (read (fd, buf, SIZE) != SIZE)
	{
		fprintf (stderr, "bad read size... bla bla\n");
		return (-1);
	}
	...
	do what you want
	...
	lseek (buf, 0, SEEK_SET);
}

lseek is possible as files in /sys are specials... open, read and write 
methods are directly handle by drivers...
don't expect to do that with classic files

simon

Re: open() and read() a file from sysfs

[krzaq]

On Tue, 01 Mar 2005 02:17:54 +0100, simon <simon.guinot@laposte.net> 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