Handling C pointer in Rust

Hello, I am new to Rust and currently working on developing a Rust library functions intended to interface with C code. This function is supposed to take input buffer provided by C code, process it, and then provide the processed output back to the C code, which will be stored in an output buffer.

Could you please provide guidance or examples on how to implement this in Rust with minimal unsafe code? Specifically, I'm looking for information on how to properly handle buffer management, memory allocation, and interfacing between Rust and C code to ensure efficient and safe data exchange

The nomicon FFI guide is pretty comprehensive and should be enough to get you started.

Sure, Thanks for the link.

The nomicon is a great resource as mentioned. When working across Foreign Function Interface (FFI) boundaries, some of the guarantees provided by the Rust compiler with safe Rust cannot be ensured by the compiler alone. A primary goal in developing an FFI interface is to maintain soundness, meaning the checks that safe Rust would normally guarantee must be manually ensured by the developer at the FFI boundary.

Consider lifetimes as an example: The Rust borrow checker ensures via lifetimes that a reference cannot outlive its referent. However, across FFI boundaries, references are often passed as raw pointers, which can lead to unbounded lifetimes. These unbounded lifetimes must be carefully managed to prevent memory safety issues (see Unbounded Lifetimes in the Rustonomicon). For instance, if a C function provides Rust with a pointer and helper functions to an iterator over a buffer (since the buffer may be an opaque structure, so Rust cannot directly iterate over it), the iterator could become invalidated if the underlying buffer is mutated or freed. Despite this, nothing inherently prevents the Rust code from retaining a copy of the pointer beyond the intended duration of the iterator function call, leading to potential unsoundness. Avoiding these could be thought of as upholding a series of "gentleman's agreements," where both sides agree to uphold certain invariants to ensure the soundness of the FFI.

Ownership and the dropping of pointers to heap-allocated values is another example. Consider the following function signature which creates a pointer to an arbitrary structure (e.g. a buffer):

#[no_mangle]
pub extern "C" fn create_structure() -> *const c_void;

In the implementation of this function, a Box, Vec, or another heap-allocating type would used to allocate the structure. Typically, in Rust, the owned heap value would be dropped automatically at the end of the function's scope. However, to allow a raw pointer to be returned from create_structure(), functions like Box::into_raw() and std::mem::forget are used to prevent Rust from automatically dropping the owned heap-allocated value at the end of the scope, allowing the raw pointer to safely exist beyond the function's scope.

To ensure proper memory management, it is necessary to provide complementary functions that take these raw pointers, convert them back into owned values of the correct type, and then properly drop them when they are no longer needed. It is also important that pointers are freed by the same allocator that allocated them.

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