Cannot understand what "error: initializer element is not constant" means

Question

I'm writing a small kernel module designed solely for accessing a particular key using the kernel's key retention service. Since I couldn't find a simple function to return a key's contents given its ID, I had to resort to request_key as outlined in the kernel documentation.

This line is what gives the error:

struct key *my_key = request_key(&key_type_user, "test key", NULL);

I'm referencing the proper key type and I still get this error. The function in security/keys/request_key.c that my code should be calling is declared:

 struct key *request_key(struct key_type *type,
                         const char *description,
                         const char *callout_info)

I don't see anything there about *type need to be a constant. I've read that the error message might have to do with false advertising on the part of C as to what makes something "constant", but I can't see how that could relate to the above function. Any help?

Answer 1

If that line is in a declaration section, it has the syntax of an initializer and that may be the compiler's complaint. Move any declarations which immediately follow it to be above it. Or split it:

struct key *my_key;
my_key = request_key(&key_type_user, "test key", NULL);

Answer 2

You are missing to show us the context of your declaration, but from the error you get I'd guess that it is in file scope, declaring a global. Statically allocated objects like that are initialized at compile time, therefore you can't use function calls and other run-time constructs to initialize it.