Malleum Knowledge Base

c_void

5 min read

In Rust, std::ffi::c_void is an equivalent to C’s void *, a type used for pointers to unspecified data types.

It is part of Rust’s foreign function interface (FFI) utilities,which are designed to facilitate the integration of Rust code with other programming languages, especially C.

The c_void type is used in Rust when you need to work with raw pointers that point to memory locations without a specified type.

Why Use std::ffi::c_void?

When interfacing with C libraries or system APIs from Rust,you often encounter functions that require or return pointers to data of an unspecified type,

i.e., void *. Since Rust is strongly typed, you need a way to handle these untyped pointers safely within the constraints of Rust’s type system.

This is where std::ffi::c_void comes in.

The concepts of as_mut_ptr() and *mut c_void in Rust pertain to handling pointers, but they serve different roles and are used in distinct contexts:

as_mut_ptr()

Functionality: as_mut_ptr() is a method available on various Rust collections, most notably on Vec<T>, which returns a raw pointer (*mut T) to the first element of the collection.

This pointer can be used to manipulate the contents of the collection directly, albeit unsafely.

Use Case: You would use as_mut_ptr() when you need a mutable raw pointer to the data held by a collection.

This is often necessary when interfacing with C code that requires direct memory access, or when performing certain low-level memory operations.

Safety: The use of as_mut_ptr() is inherently unsafe because it allows for unchecked memory manipulation.

Consequently, accessing the pointer returned by as_mut_ptr() must be wrapped in an unsafe block.

*mut c_void

Type: *mut c_void is a type in Rust used to represent a mutable raw pointer to any type, without specifying what that type is. It is the Rust equivalent of C’s void *, which is used for generic pointers.

Use Case: *mut c_void is used in Rust when you need a pointer type that can point to any kind of data, typically when interfacing with C functions that expect a void* parameter. This type is used to indicate that the specific nature of the data is not important, or to allow for data type flexibility.

Safety: Like as_mut_ptr(), *mut c_void is used in contexts where safety cannot be guaranteed by Rust’s borrowing rules, and thus its manipulation must also occur within an unsafe block.

Key Differences

Context: as_mut_ptr() is a method that gives you a pointer from a Rust collection. *mut c_void is a type used for compatibility with C interfaces or for generic pointer operations.

Purpose: as_mut_ptr() is used to get a pointer to data managed by Rust, while *mut c_void is used when the type of the data does not need to be specified or when interfacing with C code that uses void*.

While both as_mut_ptr() and *mut c_void deal with raw pointers in Rust, they serve different purposes and have different type semantics, which is why they can’t be used interchangeably without explicit casting. Let’s delve into the specifics:

Type-Specificity vs. Type-Agnosticism

  1. Output of as_mut_ptr():

    • This method returns a pointer with a specific type, *mut T, where T represents the type of elements in the collection (like Vec<T>). This type information is crucial in Rust, even for raw pointers, because it aligns with Rust’s strong type safety and correctness principles—even in unsafe code.
    • For example, if you have a Vec<u8>, as_mut_ptr() returns a *mut u8.

  2. *mut c_void:

    • Conversely, *mut c_void is a type-agnostic pointer, which means it points to some memory location but does not specify what type of data resides there. It’s analogous to void* in C, which is used for generic pointer operations and interfacing.
    • This type is used primarily for interfacing with C APIs that expect a generic pointer or when the data type cannot be determined at compile time.

Why Not Interchangeable Directly?

  • Type Safety: In Rust, every pointer type carries information about the data it points to, except for *mut c_void. This type information is used by the compiler to ensure operations on the data are valid and make sense according to the data’s type.

  • Memory Safety and Operations: Operations on *mut T can take into account the size of T, which is important for performing pointer arithmetic. For instance, incrementing a *mut u32 implicitly knows to move four bytes forward to the next u32. However, *mut c_void does not carry information about the size of the data it points to, so any arithmetic or similar operations would require explicit sizing, making it inherently more error-prone and less safe.

Conversion with Casting

  • You can convert *mut T to *mut c_void explicitly when needed (usually when interfacing with C code). This is done with casting and is common in FFI scenarios:
let mut data = vec![1, 2, 3]; unsafe {
let ptr: *mut c_void = data.as_mut_ptr() as *mut c_void;
}
// Now ptr can be passed to a C function expecting a void pointer
  • Converting *mut c_void back to *mut T is also possible but should be done carefully to ensure that the pointer indeed points to the type T.

Graph View