Rust bindgen cannot find platform specific library? - c

Question

I am trying to port my simple application from C to Rust. It was running only on my Mac, with a library on Mac only. Here is a simplified version of the failed part in C code
// myLog.h
#include <os/log.h> // macOS header
void debug(const char *str);
//************************************
// myLog.c
#include "myLog.h"
void debug(const char* str) {
// call the macOS log function
os_log_debug(OS_LOG_DEFAULT, "%{public}s", str);
}
This code can be compiled simply calling gcc debug.c, and it works fine.
Then I added the .h and .c to my rust project with bindgen specified like below
fn main() {
println!("cargo:rerun-if-changed=myLog.h");
let bindings = bindgen::Builder::default()
.header("myLog.h")
.parse_callbacks(Box::new(bindgen::CargoCallbacks))
.generate()
.expect("Unable to build bindgen");
let out_path = PathBuf::from(env::var("OUT_DIR").unwrap());
bindings
.write_to_file(out_path.join("mylog_bindings.rs"))
.expect("Couldn't write bindings!");
}
And the main function has no other functions, but testing the log for now:
#![allow(non_upper_case_globals)]
#![allow(non_camel_case_types)]
#![allow(non_snake_case)]
use std::ffi::CString;
include!(concat!(env!("OUT_DIR"), "/mylog_bindings.rs"));
fn main() {
let log_infomation = CString::new("Log from Rust").expect("Failed to create c string");
let c_pointer = log_infomation.as_ptr();
unsafe {
debug(c_pointer);
}
}
The program failed with following error:
error: linking with `cc` failed: exit code: 1
|
= note: "cc" "-m64" "-arch" "x86_64" "-L" ......
= note: Undefined symbols for architecture x86_64:
"_debug", referenced from:
bindgen_test::main::hc0e5702b90adf92c in bindgen_test.3ccmhz8adio5obzw.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)
error: aborting due to previous error; 2 warnings emitted
error: could not compile `bindgen_test`.
I am not sure why this failed, but I found if I remove the whole unsafe block (without calling the function), the compilation will work. But can someone explain to me what I did wrong? Is there something I need to add to make it compile?
Thank you very much!

Answer 1

The problem is that you are not including the myLog.c file anywhere, only the myLog.h header. This is what bindgen does: it converts a C header file into Rust code, but it does not compile the C code itself.
For that you need the cc crate. You have to use both cc and bindgen together in your build.rs file:
use std::env;
use std::path::PathBuf;
fn main() {
println!("cargo:rerun-if-changed=myLog.h");
println!("cargo:rerun-if-changed=myLog.c"); // new line here!!
let bindings = bindgen::Builder::default()
.header("myLog.h")
.parse_callbacks(Box::new(bindgen::CargoCallbacks))
.generate()
.expect("Unable to build bindgen");
let out_path = PathBuf::from(env::var("OUT_DIR").unwrap());
bindings
.write_to_file(out_path.join("mylog_bindings.rs"))
.expect("Couldn't write bindings!");
//Compile and link a static library named `myLog`:
cc::Build::new()
.file("myLog.c")
.compile("myLog");
}
And do not forget to add the cc crate to your build-dependencies.