[thradams]

mempool does not solve double free, use after free (at least at compile time) or fopen sample. But mempool and ownership can be complementary.

[ActorNightly]

If you are talking about a very naive version of mempool, then you are correct, but thats why I said a good implementation.

The whole point of a good mempool is that you malloc once, and only call free when you exit the program. The data structures for memory allocation will never get corrupted. And the memory pool will never release chunk twice cause it keeps tracks of allocated chunks.

User after free is mitigated in the same way. When you allocate, you get a struct back that contains a pointer to the data. When you release, that pointer is zeroed out.

[lmm]

> If you are talking about a very naive version of mempool, then you are correct, but thats why I said a good implementation.

No true Scotsman.

> The whole point of a good mempool is that you malloc once, and only call free when you exit the program. The data structures for memory allocation will never get corrupted. And the memory pool will never release chunk twice cause it keeps tracks of allocated chunks.

Then you've just moved the same problem one layer up - "use after returned to mempool" takes the place of "use after free" and causes the same kind of problems.

> When you allocate, you get a struct back that contains a pointer to the data. When you release, that pointer is zeroed out.

And the program - or, more likely, library code that it called - still has a copy of that pointer that it made when it was valid?

[ActorNightly]

Its not about comparing implementations, its about the fact that a correct mempool implementation solves the problem without need for complex borrow checkers.

For example, in that implementation, you request memory from a mempool, it returns a chunk-struct with the pointer to allocated memory, the size of the chunk, and optionally some convenience functions for safe access (making sure that the pointer is not incremented or decremented beyond the limits). It also keeps its own pointer to the chunk-struct, along with the chunk that it was allocated. When you release the chunk, it zeros out the pointer in the chunk-struct. Now any access to it will cause a segfault.

You can of course write code that bypasses all those checks, but in Rust, thats equivalent to using unsafe when you wanna be lazy. Also you could argue that Rust is better because instead of segfaulting, the check will be caught during compile time, which is true but only for fairly simple programs. Once you start using RefCells, you cannot guarantee everything during compile time.

[lmm]

> You can of course write code that bypasses all those checks, but in Rust, thats equivalent to using unsafe when you wanna be lazy.

The difference is that most of the Rust ecosystem is set up to allow you to not use unsafe. Whereas whenever you use a library in C, you need to pass it a pointer, so bypassing these checks has to be routine. (Note that the article claims as a key merit that it's possible to add annotations to existing libraries)

> When you release the chunk, it zeros out the pointer in the chunk-struct. Now any access to it will cause a segfault.

Only if you're very lucky. Null pointer dereference is undefined behaviour, so it may cause a different thread to segfault on a seemingly unrelated line, or your program may silently continue with subtly corrupted state in memory, or...

> Also you could argue that Rust is better because instead of segfaulting, the check will be caught during compile time, which is true but only for fairly simple programs. Once you start using RefCells, you cannot guarantee everything during compile time.

Using RefCells should be (and, idiomatically, is) the exception rather than the rule. And incorrect use of RefCell results in a safe panic rather than undefined behaviour.

[ActorNightly]

Null pointer dereference in the vast majority of cases will segfault. In the cases where it doesn't, thats fully on you for running some obscure os on some obscure hardware.

>Whereas whenever you use a library in C, you need to pass it a pointer,

When it comes to developing with Rust, any performance oriented project is necessarily going to have lots of unsafe for interacting with C libraries in the linux kernel in the same way that C code does.

As for comparison to fully safe Rust code outside the unsafes, you can largely accomplish analogous behavior in C with good mempool implementation. Or if you don't need to pass around huge amount of data, you can also do it by simply just never mallocing and using stack variables. There is still some things you have to worry about (using safe length bounded memory copy/move functions, using [type]* const pointer values to essentially make them act like references for function parameters, some other small things).

The point is Rust isn't the defacto standard for memory safety, and while it can exist as its own project, porting its semantics to other languages is not worth it.

[lmm]

> Null pointer dereference in the vast majority of cases will segfault.

Attempting access to a zero address will segfault on most hardware, but unfortunately common C compilers in common configurations will not reliably compile a null pointer dereference to an access to the zero address. Look up why the Linux kernel builds with -fno-delete-null-pointer-checks (sadly, most applications and libraries don't).

> When it comes to developing with Rust, any performance oriented project is necessarily going to have lots of unsafe for interacting with C libraries in the linux kernel in the same way that C code does.

I'm not talking about performance oriented projects. I'm talking about regular use of libraries e.g. I need to talk to PostgreSQL so I'll call libpq, I need to uncompress some data so I'll use zlib, I need to make a HTTP call so I'll use libcurl...

> The point is Rust isn't the defacto standard for memory safety

It absolutely is though. It's got clear, easy-to-assess rules for whether a project is memory-safe or not, and a substantial ecosystem that follows them; so far it's essentially unique in that unless you include GCed languages.

[thradams]

Cake is not porting Rust semantics. It works on classical C code, like the first sample using fopen.

    #include <ownership.h>
    #include <stdio.h>

    int main()
    {
      FILE *owner f = fopen("file.txt", "r"); 
      if (f)
        fclose(f);
    }

But comparisons are inevitable, and I also think there are lessons learned in Rust.

C programmers uses contracts, these contracts are part of documentation of some API. For instance, if you call fopen you must call fclose.

All we need is to create contracts that the compiler can read and verify automatically.

[jart]

> The whole point of a good mempool is that you malloc once, and only call free when you exit the program

So you're describing fork() and _exit(). That's my favorite memory manager. For example, chibicc never calls free() and instead just forks a process for each item of work in the compile pipeline. It makes the codebase infinitely simpler. Rui literally solved memory leaks! No idea what you're talking about.

[thradams]

One issue I see with this approach (compiler leaking memory) is, for instance, if the requirements change and you need to utilize the compiler as a lib or service. For example, if the Cake source is used within a web browser compiled with Emscripten, leaking memory with each compilation would lead to a continuous increase in memory usage.

Additionally, compilers often offer the option to compile multiple files. Therefore, we cannot afford to leak memory with each file compilation.

Initially I was planning a global allocator for cake source. It had a lot of memory leaks that would be solved in the future.

When ownership checks were added it was a perfect candidate for fixing leaks. (actually I also had this in mind)

[jart]

True, but with some stuff you just ain't gonna need it. For example, chibicc forks a process for each input file. They're all ephemeral. So the fork/_exit model does work well for chibicc. You could compile a thousand files and all its subprocesses would just clean things up. Now needless to say, I have compiled some juicy files with chibicc. Memory does get a bit high. It's manageable though. I imagine it'd be more of an issue if it were a c++ compiler.

[thradams]

(I think preprocessor is the place where memory is used and released all the time while expanding macros.)

[jart]

It is.