Passing a Rust function pointer to C for logging library's stderr output (libxml FFI)

help
1

Having successfully implemented the FFI to be able to parse XML docs, I am now trying to redirect libxml's stderr output into my own log function inside my Rust application (wrapping the log crate macros) so that I can keep any errors together with the rest of the debug logging.

According to the libxml docs, I need to use initGenericErrorDefaultFunc (xmlGenericErrorFunc * handler) to specify the handler for errors, which I think takes a pointer to my Rust function, but I am unsure how to pass my Rust function to C via the FFI. Using the first edition of The Book, I ended up with:

pub enum XmlGenericErrorFunc {}

#[link(name = "xml2")]
extern "C" {
    pub fn initGenericErrorDefaultFunc(handler: XmlGenericErrorFunc);
    pub fn xmlGenericErrorFunc(
        ctx: *mut XmlValidCtxtPtr,
        msg: *const c_char,
    ) -> XmlGenericErrorFunc;
}

#[no_mangle]
extern fn error_func_redirect(msg: *const c_char) {
    error!("libxml error output: {:?}", msg);
}

As soon as I've setup my context, I'm then trying to tell libxml about my Rust function (which I thought I had to pass the function signature in via):

let cvp_ptr: *mut XmlValidCtxtPtr = create_context_ptr()?;
unsafe {
    let error_function_name = std::ffi::CString::new("error_func_redirect(msg: *const c_char)").unwrap();
    let error_func = xmlGenericErrorFunc(cvp_ptr, error_function_name.as_ptr());
    initGenericErrorDefaultFunc(error_func);
}

But I get a linker error:

Undefined symbols for architecture x86_64:                                                                              
            "_xmlGenericErrorFunc", referenced from:                                                                              
                validator::XmlValidator::new::h3155275e1d619a12 in libvalidator-206fd62ca886041b.rlib(validator-206fd62ca886041b.4pmkvih2bq1byoha.rcgu.o)
          ld: symbol(s) not found for architecture x86_64                                                                         
          clang: error: linker command failed with exit code 1 (use -v to see invocation)

I'm now in a tangle trying to work out how I correctly pass a pointer to my Rust function through into libxml. Any tips most appreciated!

2

You need to define a function pointer type, not a function and an unrelated enum.

extern "C" {
    pub fn initGenericErrorDefaultFunc(handler: xmlGenericErrorFunc);
}
pub type xmlGenericErrorFunc = Option<extern "C" fn(ctx: *const libc::c_void, msg: *const libc::c_char, ...)>;

Also, your Rust-side function has to match that signature; you can't just ignore the ctx argument. I'm not sure you can implement such a function, though. Last I checked, Rust doesn't support defining C ABI functions with variadics, and I couldn't find anything to contradict that (heck, the new edition of the book doesn't mention them at all).

1 Like
3

There's an approved RFC to enable definition of variadic C ABI functions, but the implementation hasn't landed yet: https://github.com/rust-lang/rust/pull/49878

1 Like
4

Thank you @DanielKeep and @sfackler - I'm going to redirect the stderr output to file for now, and keep an eye on the RFC for future.

5

You can use the cc create to add your own varargs wrapper function in C.

6

You can use the cc create to add your own varargs wrapper function in C.