IS_ERR() macro in Linux

Be careful for pitfall:

#define IS_ERR_VALUE(x) unlikely((x) >= (unsigned long)-MAX_ERRNO)
#define MAX_ERRNO       4095

This covers -1 to -4095, which represents error code, not number below 4096, nor NULL (0). Every value from 0 to 4294963201 (0xfffff001) is considered no error. Do not use it to cover NULL checking.

If you want to know what the macro expands to, just compile your file using the -E option of gcc, which will only do pre-processing. It will include all headers and expand all macros.

The macro does not "get executed" per se, it's just a "search and replace" type of thing.

Tests if the supplied pointer should be considered an error value.

It does not check if the pointer is valid.

In your code IS_ERR is used to check if class_create succeded creating ebbcharClass. If an error occurs unregister the char driver and signal the error.

You can find MACROs and inline funcs in err.h