Linux device files

Hi,

Is there a limit to how many times you can open or write to a device file? I have a program which is a continuous loop and is always writing or reading an I2C device file. After about the 1000th time it fails to open the device.

Anybody have an idea?

Here's the code:

Code:

---

/************************************************************************/
/* read_ad                                                                */
/* inputs: I2C address to write to (addr)                                */
/* outputs: Reading from the A/D in hex (ad_output)                        */
/* descritpion: Reads I2C A/D chip                */
/************************************************************************/
int read_ad(char addr){

        sprintf(filename, "/dev/i2c-%d", adapter_nr);
        if ((file = open(filename,O_RDWR)) < 0)
        {
                printf("Error openning device file in read_ad()\n");
                exit(1);
        }
                else{
                //printf("Device file opened\n");
                }
       
        if (ioctl(file,I2C_SLAVE,addr) < 0)
        {
                printf("Error with ioctl function in read()\n");
                exit(1);
        }
               
          if (read(file,ad_output,2) != 2)
        {
                    printf("Error reading A/D chip\n");
          }
                else{
                printf("Device file (A/D chip) reads %x\n", ad_output[0]);
                }

}

---

What is the declaration for filename -- does it have enough room for the text of "/dev/i2c-1000" (at least 14 characters)?

And are you close()ing the open file every time through the loop? There's a limit to how many open files you can have:

Code:

---

itsme@dreams:~$ ulimit -a
...
open files                  1024
...

---

Also, why don't you use errno to see why open() is failing? Then you'd know if it's hitting that limit. open() sets errno to:

Code:

---

      EMFILE The process already has the maximum number of files
              open.

---

...if it fails because of that.

Code:

---

ulimit -aH

---

will show you the current hard limit for open files (file descriptors).

If error checking shows that is the problem you can :

Edit /etc/security/limits.conf and add the lines:

Quote:

---

* soft nofile 1024
* hard nofile 65535

---

Edit /etc/pam.d/login, adding the line:

Quote:

---

session required /lib/security/pam_limits.so

---

The system file descriptor limit is set in
/proc/sys/fs/file-max.

This will increase the limit to 65535:

Quote:

---

echo 65535 > /proc/sys/fs/file-max

---

Then increase the file descriptor limits for the process

[quote[ulimit -n unlimited[/quote]

(--- after you log out and then back in.)

The command will set the limits to the hard limit in /etc/security/limits.conf.

You really should consider a different program design - maybe forking some new processes, or closing and reusing fd's. Using 65335 for file limit is usually not a good idea system-wide.

Thanks for the reply guys,

I went back and closed the file I opened and it seems to work.

Thanks for the help.