Creating a static C struct containing strings

Question

I\'m trying to create a dynamic library in Rust that exports a struct as a symbol that will be loaded into a C program via dlopen().

However, I\'m was running into s

Answer 1

The layout of a slice (&[T] or &str) is a pointer followed by a length, as documented by the Slice struct of the std::raw module. That's why reading the version field from your C code shows the length of the name field's value. (Note, however, that the exact memory layout of slices is not considered stable, so it might change in a later version. In any case, you should not pass Rust-specific data types to C; only pass primitive types – which includes raw pointers – and types annotated with #[repr(C)].)

EDIT: Unfortunately, there seems to be no way to do this in Rust for now. There are functions to get raw pointers from slices, but function calls are not allowed in static initializers. As suggested by sellibitze in the comments, you should define that variable in a C source file.

Answer 2

The short answer is that you can't statically allocate such a struct. Future Rust will probably gain this ability.

What you can do, is statically allocate a struct that contains null pointers, and set those null pointers to something useful when you call the function. Rust has static mut. It requires unsafe code, is not threadsafe at all and is (to the best of my knowledge) considered a code smell.

Right here I consider it a workaround to the fact that there is no way to turn a &[T] into a *const T in a static.

static S: &'static [u8] = b"http://example.org/eg-amp_rust\n\0";
static mut desc: LV2Descriptor = LV2Descriptor {
    amp_uri: 0 as *const libc::c_char, // ptr::null() isn't const fn (yet)
};

#[no_mangle]
pub extern fn lv2_descriptor(index: i32) -> *const LV2Descriptor {
     let ptr = S.as_ptr() as *const libc::c_char;
     unsafe {
        desc.amp_uri = ptr;
        &desc as *const LV2Descriptor
     }
}

answer copied from duplicate question