Best way to update driver data from userspace

Question

I have a userspace application which interacts with a kernel-space driver in a Linux environment. The driver, in this case, is an LED driver. In typical *nix fashion, the driver exposes a file in /sys/class/leds/actled1:green/ called value. When the value of value exceeds a threshold, the LED turns on.

So, my question is, what is the best-practice way of updating value from user-space in C?

My current approach is to simply write to the file, with a hardcoded filepath:

int value = 0;
FILE *fp;

/* update value here... */

fp = fopen("/sys/class/leds/actled1:green/value", "w");
if(fp != NULL) fprintf(fp, "%d", value);
fclose(fp);

Is there a better way to do this? It seems that there should be a standard POSIX C api for this, perhaps one that allows for driver lookup (so that I don't need to hard-code the filepath).

Answer 1

There are lots of Linux kernel APIs. E.g. to name a few:

• system calls
• char and block devices (under /dev/)
  • write and read data
  • call ioctl() on the open device
• netlink sockets
• sysfs
• procfs
• configfs

Sometimes objectives determine the suitable API to use. Sometimes it's a bit arbitrary which one to pick. At some point an API was picked by one or more developers who created the API (probably after some discussion on Linux kernel mailing lists).

In the case of Linux kernel LEDs, /sys/class/leds/ in the sysfs filesystem is the API for userspace applications to control LEDs. So your app will control LEDs by writing to a file in sysfs.

I just recommend you make the LED's name configurable in your application. You could just make the name configurable (e.g. actled1:green), or you could make the entire path configurable (e.g. /sys/class/leds/actled1:green/brightness). That might allow the option of using some sort of userspace pseudo-LED API (e.g. a userspace app that controls LEDs over a network connection, providing a pseudo-LED API under /var/run/led-net/actled1:green/brightness).