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

simon Mon, 28 Feb 2005 17:20:43 -0800

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

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