Having trouble with creating a isolated implementation for a General Allocator for my GC

[rapid musk]

I am currently trying to implement a general allocator for my garbage collector project so you can create different allocators for different purposes, and have multiple allocators in one allocator (eg. a slab allocator). Eventually still have the garbage collector be able to manage it.

I've had a bunch of trouble with function pointers and trying to isolate the internal routines of warena.c, only exposing them through a allocator struct.

And currently im facing some issues with my arena_t not being avalible for linking outside of warena.c which I need for what im doing right now. Writing tests. But I am just not sure how to expose it.

But the big problem right now is i'm a little lost in errors and having tried a few things that didn't work like:

• Declaring the struct as extern (Current situation errors below)
• Trying to use typedef in different ways but getting an error trying to use an incomplete typedef

make tests - to run exactly what im running

https://github.com/seaaldev/WallyGC/tree/main

[robust frigate]

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[rapid musk]

Related Files

• warena.c & warena.h
• test-warena-api.c

[rose cedar]

To my knowledge typedefing something extern doesn't really make much sense.
Try removing the extern and that error should disappear.

As for the error in warena.c, that's just because of the useless extern again.
So if you remove those two externs, those two errors should be gone (maybe new ones appear?)

Ah, but actually you maybe want to do:

typedef struct arena_t arena_t;
struct arena_t;
```in the `.h` file

But

extern struct arena_t {
  size_t      size;
  size_t allocated;
};
```really doesn't make much sense. It's not `extern`. You define it right here.

[rapid musk]

Would you mind explaining why it doesn't make sense?

As far as my understanding of extern isn't it to change it's scope to be accessed throughout all linked files?

I had tried something similar as what you suggested and it failed. Now I'm getting the same errors with your suggestion

[rose cedar]

extern means external linkage which is the opposite of internal linkage.
Internal linkage essentially means that each translation unit (i.e. each .c file that you compile), has it's own copy of that part, but external linkage means that all files share a single variable.

If you want to read up on it yourself, try this here: https://en.cppreference.com/w/c/language/storage_class_specifiers.html and scroll down to "Linkage and libraries". I do have to admit that it's probably very confusing to read though, but it does provide an example which may help

[rose cedar]

is the code on GitHub updated?

[rapid musk]

Not with your new suggestion no I can quickly do that though!
Not that good a github sorry

But yeah I will definitely read up on it

[rapid musk]

it is now

[rose cedar]

I'm not on Linux rn, but what if you do extern struct arena_t; in your warena.h instead of struct arena_t;?

[rapid musk]

I'll try 😄

[rapid musk]

Nope, the compiler just throws a empty declaration error at me 😅

[rose cedar]

ah, that error should disappear if you switch the order of the typedef struct arena_t arena_t; and the extern struct arena_t;

Let's see what error it spits out now

[rapid musk]

Is it the extern specifier it sees as useless?

Also does the structs implementation in warena.c also have to have the extern specifier?

[rose cedar]

lemme experiment a bit

[rapid musk]

Let it rip i will keep reading that reference article 😄

[rose cedar]

Okay, so I completely overlooked that this is not a variable but a type definition doesn't need an extern because it's really compile-time only and afterwards doesn't really matter anymore, whereas for variables it's important where they live.
You would only need extern if you wanted to declare a global struct variable.

As for your test code, you can either just move your struct arena_t definition to inside the .h file in order to expose the struct with its fields, or you can add functions to the warena.h file that would act as a proxy/middleman for these operations, if you want to keep the struct definition completely hidden

So if you write

typedef struct arena_t arena_t;
struct arena_t {
    size_t      size;
    size_t allocated;
};
```in your `.h` file and remove the 
```c
struct arena_t {
    size_t      size;
    size_t allocated;
};
```part from your `.c` file, then you will get different errors

[rapid musk]

Thank you! I think I can figure something out from here 😄

[rose cedar]

good luck

[robust frigate]

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[rapid musk]

!solved