We tried to write a simple class driver framework which will create a directory in /sys/class, we were able to compile the code but got the following error while trying to install the driver as,
$ sudo insmod hello.ko insmod: ERROR: could not insert module hello.ko: Unknown symbol in module
$ dmesg [ 7514.340407] hello: Unknown symbol __class_register (err 0) [ 7514.340436] hello: Unknown symbol class_unregister (err 0)
The function __class_create is exported only for GPL modules (exported with EXPORT_SYMBOL_GPL). So, you need to use a GPL license with MODULE_LICENSE macro to make use of that function.
Hence if you append your driver code with
and recompile driver and try insmod again, it will definately work and you will not get such errors.