Discovering x-macros

I found this weird pattern I did not understand in the kernel:

static const struct bpf_verifier_ops * const bpf_verifier_ops[] = {
#define BPF_PROG_TYPE(_id, _name, prog_ctx_type, kern_ctx_type) \
	[_id] = & _name ## _verifier_ops,
#define BPF_MAP_TYPE(_id, _ops)
#define BPF_LINK_TYPE(_id, _name)
#include <linux/bpf_types.h>
#undef BPF_PROG_TYPE
#undef BPF_MAP_TYPE
#undef BPF_LINK_TYPE
};

I found out that these are X-Macros along with C99-Designated-Initializers. So, X Macros are a pattern of using programming language macros to generate list like structures.

An X macro application consists of two parts:

1. The definition of the list’s elements.
2. Expansion(s) of the list to generate fragments of declarations or statements.

The list is defined by a macro or header file (named, LIST) which generates no code by itself, but merely consists of a sequence of invocations of a macro (classically named “X”) with the elements’ data. Each expansion of LIST is preceded by a definition of X with the syntax for a list element. The invocation of LIST expands X for each element in the list.

So here, it gets the stuff form bpf_types.h and runs these multitudes of macros on it and it turns into an initializer list that shows all the programs.

The double hashtag (##) is a token pasting operator in the C preprocessor. Used to merge two tokens into one during preprocessing. its like <name>_verifier_ops that comes about by concatenating <name> and _verifier_ops.

This is honestly some C syntax that I never knew about.