How to handle static inline functions

[pvdrz]

Before v0.64.0 was released, the only way to handle static inline functions on bindgen was using the --generate-inline-functions option which generated rust bindings for these functions. However, that meant that the input C library should still expose those function symbols somehow, most likely by compiling the library without inlining enabled which could be a serious performance issue.

With the new bindgen version there is another alternative, the --wrap-static-fns-* flags which generate external wrapper functions for these static inline functions, requiring the user to only compile these generated wrappers against the headers file being used as an input. For me, the clearest way to explain how this works is by doing an example.

Let's say we have the following input.h header file:

static inline int inc(int x) {
    return x + 1;
}

static int dec(int x) {
    return x - 1;
}

If we passed this file to bindgen without any flags we would get an empty output:

$ bindgen input.h 
/* automatically generated by rust-bindgen 0.64.0 */

However, if we pass the --wrap-static-fns flag we get the following:

$ bindgen --experimental --wrap-static-fns input.h
/* automatically generated by rust-bindgen 0.64.0 */

extern "C" {
    #[link_name = "inc__extern"]
    pub fn inc(x: ::std::os::raw::c_int) -> ::std::os::raw::c_int;
}
extern "C" {
    #[link_name = "dec__extern"]
    pub fn dec(x: ::std::os::raw::c_int) -> ::std::os::raw::c_int;
}

We need to pass this --experimental flag because this feature is not complete and prone to change. However, the good news is that now we got rust bindings for both inc and dec. Additionally a new c source file should be created under the bindgen directory inside your temporal folder (/tmp/bindgen/ if you're on unix-like systems):

$ cat /tmp/bindgen/extern.c
#include "input.h"

// Static Wrappers

int inc__extern(int x) { return inc(x); }
int dec__extern(int x) { return dec(x); }

These __extern functions are wrappers for the static functions we defined in our input. Now the only thing we need to do is to compile this new extern.c file into a library and include input.h:

$ clang -O -c -o extern.o /tmp/bindgen/extern.c -include input.h
$ objdump -d extern.o

extern.o:     file format elf64-x86-64


Disassembly of section .text:

0000000000000000 <inc__extern>:
   0:	8d 47 01             	lea    0x1(%rdi),%eax
   3:	c3                   	ret
   4:	66 66 66 2e 0f 1f 84 	data16 data16 cs nopw 0x0(%rax,%rax,1)
   b:	00 00 00 00 00 

0000000000000010 <dec__extern>:
  10:	8d 47 ff             	lea    -0x1(%rdi),%eax
  13:	c3                   	ret

As we can see, the extern.o object file includes two symbols: inc__extern and dec__extern. These symbols are the ones that will replace inc and dec in our Rust bindings, and that's why both function declarations in the bindings have the #[link_name] attribute overriding the linking name.

We could take different approaches from here, one of them would be turning this object file into a static library:

$ ar rcs libextern.a extern.o

or if you're on windows:

$ LIB extern.o /OUT:extern.lib

And now we can link our bindings against this libextern static library with rust. This same procedure could be done in a build script:

use bindgen::{Builder, CargoCallbacks};  
use std::path::PathBuf;

fn main() {
    let input = "input.h";
    
    // Tell bindgen to generate wrappers for static functions
    let bindings = Builder::default()
        .header(input)
        .parse_callbacks(Box::new(CargoCallbacks))
        .wrap_static_fns(true)
        .generate()
        .unwrap();

    let output_path = PathBuf::from(std::env::var("OUT_DIR").unwrap());
    // This is the path to the object file.
    let obj_path = output_path.join("extern.o");
    // This is the path to the static library file.
    let lib_path = output_path.join("libextern.a");

    // Compile the generated wrappers into an object file.
    let clang_output = std::process::Command::new("clang")
        .arg("-O")
        .arg("-c")
        .arg("-o")
        .arg(&obj_path)
        .arg(std::env::temp_dir().unwrap().join("bindgen").join("extern.c"))
        .arg("-include")
        .arg(input)
        .output()
        .unwrap();
        
    if !clang_output.status.success() {
        panic!(
            "Could not compile object file:\n{}",
            String::from_utf8_lossy(&clang_output.stderr)
        );
    }

    // Turn the object file into a static library
    #[cfg(not(target_os = "windows"))]
    let lib_output = Command::new("ar")
        .arg("rcs")
        .arg(out_dir_path.join("libextern.a"))
        .arg(obj_path)
        .output()
        .unwrap();
    #[cfg(target_os = "windows")]
    let lib_output = Command::new("LIB")
        .arg(obj_path)
        .arg(format!("/OUT:{}", out_dir_path.join("libextern.lib").display())
        .output()
        .unwrap();
    if !lib_output.status.success() {
        panic!(
            "Could not emit library file:\n{}",
            String::from_utf8_lossy(&ar_output.stderr)
        );
    }

    // Tell cargo to statically link against the `libextern` static library.
    println!("cargo:rustc-link-lib=static=extern");

    // Write the rust bindings.
    bindings
        .write_to_file(output_path.join("bindings.rs"))
        .expect("Cound not write bindings to the Rust file");
}

In either case, you should be able to call inc and dec from rust without issue now!

Using LTO optimizations

If you made it up to this point you might have noticed that using the wrappers for static function is going to be less performant just because those functions are not being inlined by the Rust compiler. To illustrate this. We will edit the src/lib.rs file so it has the following contents:

mod bindings {
    #![allow(non_upper_case_globals)]
    #![allow(non_camel_case_types)]
    #![allow(non_snake_case)]

    include!(concat!(env!("OUT_DIR"), "/bindings.rs"));
}

#[inline(never)]
#[no_mangle]
pub fn increase(x: ::std::os::raw::c_int) -> ::std::os::raw::c_int {
    unsafe { bindings::inc(x) }
}

#[inline(never)]
#[no_mangle]
pub fn decrease(x: ::std::os::raw::c_int) -> ::std::os::raw::c_int {
    unsafe { bindings::dec(x) }
}

and we will add a src/main.rs file with the following contents:

fn main() {
    assert_eq!(1, playground_bindgen::increase(0));
    assert_eq!(0, playground_bindgen::decrease(1));
}

where playground_bindgen is the name of our crate.

If we compile this crate using cargo build --release and then disassemble the resulting binary using objdump we will find this

0000000000008550 <increase>:
    8550:       ff 25 ba 45 04 00       jmp    *0x445ba(%rip)        # 4cb10 <_GLOBAL_OFFSET_TABLE_+0x1d8>
    8556:       66 2e 0f 1f 84 00 00    cs nopw 0x0(%rax,%rax,1)
    855d:       00 00 00 

0000000000008560 <decrease>:
    8560:       ff 25 8a 44 04 00       jmp    *0x4448a(%rip)        # 4c9f0 <_GLOBAL_OFFSET_TABLE_+0xb8>
    8566:       66 2e 0f 1f 84 00 00    cs nopw 0x0(%rax,%rax,1)
    856d:       00 00 00 

Basically increase and decrease are just jumping to someplace else instead of doing lea as inc__extern and dec__extern do.

In order to solve this, we can enable LTO optimizations for our crate. First we need to change the clang invocation so it uses "thin" LTO:

  let clang_output = std::process::Command::new("clang")
        .arg("-flto=thin")
        .arg("-O")
        .arg("-c")
        .arg("-o")
        .arg(&obj_path)
        .arg(std::env::temp_dir().unwrap().join("bindgen").join("extern.c"))
        .arg("-include")
        .arg(input)
        .output()
        .unwrap();

We must also change the ar invocation (if someone knows the windows equivalent of this, please let me know):

    let lib_output = Command::new("ar")
        .arg("crus")
        .arg(out_dir_path.join("libextern.a"))
        .arg(obj_path)
        .output()
        .unwrap();

Then we must change the Cargo.toml manifest to enable "thin" LTO from the rust side by adding the following:

[profile.release]
lto = "thin"

Finally we can compile our project with the following RUSTFLAGS:

$ env RUSTFLAGS="-Clinker-plugin-lto -Clinker=clang -Clink-arg=-fuse-ld=lld" cargo build --release

Now if we check the generated machine code using objdump we will find this

000000000004ac10 <increase>:
   4ac10:       8d 47 01                lea    0x1(%rdi),%eax
   4ac13:       c3                      ret
   4ac14:       cc                      int3
   4ac15:       cc                      int3
   4ac16:       cc                      int3
   4ac17:       cc                      int3
   4ac18:       cc                      int3
   4ac19:       cc                      int3
   4ac1a:       cc                      int3
   4ac1b:       cc                      int3
   4ac1c:       cc                      int3
   4ac1d:       cc                      int3
   4ac1e:       cc                      int3
   4ac1f:       cc                      int3

000000000004ac20 <decrease>:
   4ac20:       8d 47 ff                lea    -0x1(%rdi),%eax
   4ac23:       c3                      ret

Where increase and decrease just do lea and then return!

Customizing the wrappers

There are additional flags/methods to customize the behavior of this feature:

• You can change the path of the wrapper functions file by using the --wrap-static-fns-path. You should not try to set the extension of this file as bindgen will infer automatically if this should be a C or C++ source code file.
• You can change the __extern suffix used for wrapper functions by using the --wrap-static-fns-suffix. This is useful if for some reason there are name collisions with the default suffix.

Where's the catch?

The weakest point of this feature is the C/C++ code generation. As of today, we can only generate a subset of C code and we know that this subset is good enough to compile some real-life libraries. However, C++ support is lacking (PRs are welcome!).

If you have any issues with this feature you can open a new issue or discussion and tag me.

Thanks to @JMS55 for the windows instructions and to @DemiMarie for the LTO suggestion!

[JMS55]

Some fixes I discovered:

• Without adding a link search path, cargo cannot find the generated extern library
• On MSVC, you need to use the lib command instead of ar

fn link_static_fns(out_dir_path: PathBuf) {
    let obj_path = out_dir_path.join("extern.o");

    let clang_output = Command::new("clang")
        .arg("-O")
        .arg("-c")
        .arg("-o")
        .arg(&obj_path)
        .arg(env::temp_dir().join("bindgen").join("extern.c"))
        .arg("-include")
        .arg("src/wrapper.h")
        .output()
        .unwrap();

    if !clang_output.status.success() {
        panic!(
            "Could not compile object file:\n{}",
            String::from_utf8_lossy(&clang_output.stderr)
        );
    }

    #[cfg(not(target_os = "windows"))]
    let lib_output = Command::new("ar")
        .arg("rcs")
        .arg(out_dir_path.join("libextern.a"))
        .arg(obj_path)
        .output()
        .unwrap();
    #[cfg(target_os = "windows")]
    let lib_output = Command::new("lib").arg(&obj_path).output().unwrap();

    if !lib_output.status.success() {
        panic!(
            "Could not emit library file:\n{}",
            String::from_utf8_lossy(&lib_output.stderr)
        );
    }

    println!(
        "cargo:rustc-link-search=native={}",
        out_dir_path.to_string_lossy()
    );
    println!("cargo:rustc-link-lib=static=extern");
}

[pvdrz]

Hi! Thanks for figuring out the different steps that must be taken to do this on windows. I don't have a windows setup so I didn't thought about it until you mentioned it.

[tgross35]

@pvdrz I think with this you could remove the (if someone knows the windows equivalent of this, please let me know) part of your (amazing) post 🙂

[pvdrz]

Oh! That's because I don't know what's the LIB invocation to enable thin-lto 😁

[tgross35]

Oh whoops, I thought it was just the lib command at all (my bad!)

[DemiMarie]

Would it be possible to generate Rust code instead, perhaps via c2rust? That would allow the code to be inlined, which could be a big performance win.

[pvdrz]

I'm not sure about how feasible is this. The main issue I see is that bindgen would need to extract the body of a function and not just their signature. I'm not sure if that's something bindgen can handle right now or not

[DemiMarie]

Is cross-language LTO an option?

[pvdrz]

Yes I tried to use LTO and I think it works correctly.

I followed the rustc docs about LTO and adapted the build-script so it runs clang and ar with the right arguments. I had to use -flto which i guess is fat LTO instead of -flto=thin so the functions are actually inlined. But it works! I'll update the main post with this

Edit: nevermind I was eagerly excited. It does not work 😞

[DemiMarie]

I wonder if Rust should include an internal C compiler, as both Zig and D do.

[pvdrz]

Maybe it is possible to use LTO with the right invocations, I just have never done it.

Edit: Ok thin-lto works. I'll update the post with instructions.

[pustekuchen91]

First of all thanks for this guide.

Where is out_dir_path and ar_output defined in your example?

[pvdrz]

hey sorry, it is an error, out_dir_path should be output_path and ar_output should be lib_output.

[pustekuchen91]

What needs to be considered when cross-compiling?
I've attached the arg --sys-root=/usr/lib/arm-none-eabi like the same bindgen::Builder clang_args

    // Compile the generated wrappers into an object file.
    let clang_output = std::process::Command::new("clang")
        .arg("-O")
        .arg("-c")
        .arg("-o")
        .arg(&obj_path)
        .args(clang_args.clone())
        .arg(std::env::temp_dir().join("bindgen").join("extern.c"))
        .arg("-include")
        .arg(input)
        .output()
        .unwrap();

But i still get this error:

= note: rust-lld: error: /workspaces/xxx/target/thumbv7em-none-eabihf/debug/deps/libxxx-ec0a2b471f8932aa.rlib(extern.o) is incompatible with /tmp/rustcdvnfTo/symbols.o

any advices?

[pustekuchen91]

the probem was, that the target parameter was missing.

        .arg("-target")
        .arg("thumbv7em-none-eabihf")

[hi-glenn]

This discussion is very great. Could it be summarized as an example project ?

In order to help others to use inline functions easily with bindgen.

[lenianiva]

What is the purpose of the crus flag on ar as opposed to rcs? The manpage of ar says

GNU ar allows you to mix the operation code p and modifier flags mod in any order, within the first command-line argument.

...

       u   Normally, ar r... inserts all files listed into the archive.  If you would like to insert only those of the files you list that are newer than existing members of the same names, use this modifier.  The u modifier is allowed only for the operation r (replace).  In particular, the combination qu is not allowed, since checking the
           timestamps would lose any speed advantage from the operation q.

[pvdrz]

u forces ar to only replace files if the existing files are older.

[lenianiva]

so its not required for this example? I had the impression that it was needed for thin LTO to work because it wasn't in the first example.

[pvdrz]

I don't remember exactly but I think it shouldn't matter. Maybe I should do a template as @hi-glenn mentioned.

[emarteca]

Thanks for adding this feature! The functionality seems to have been stable for a while -- are there any plans to move it out of --experimental soon?

[pvdrz]

I don't see why not 😃

[Bendrien]

What needs to be done to make this happen 😃

[pvdrz]

It is done already 🚀

[alistair23]

Is there interest in an issue being opened for C code that this doesn't handle yet?

I'm trying to get it working with Rust for Linux, but there are a few errors popping up

[pvdrz]

yes

[JMS55]

It's much easier to use the cc crate:

bindgen::Builder::default()
    .header(format!("{}/src/wrapper.h", env!("CARGO_MANIFEST_DIR")))
    .parse_callbacks(Box::new(bindgen::CargoCallbacks::new()))
    .wrap_static_fns(true)
    .wrap_static_fns_path(out_dir.join("wrap_static_fns"))
    .clang_arg(format!("-I{dlss_sdk}/include"))
    .clang_arg(format!("-I{vulkan_sdk}/{vulkan_sdk_include}"))
    .allowlist_item(".*NGX.*")
    .generate()
    .unwrap()
    .write_to_file(out_dir.join("bindings.rs"))
    .unwrap();

cc::Build::new()
    .file(out_dir.join("wrap_static_fns.c"))
    .includes([
        format!("{}/src/wrapper.h", env!("CARGO_MANIFEST_DIR")),
        format!("{dlss_sdk}/include"),
        format!("{vulkan_sdk}/{vulkan_sdk_include}"),
    ])
    .compile("wrap_static_fns");

[lenianiva]

Is RUSTFLAGS="-Clinker-plugin-lto -Clinker=clang -Clink-arg=-fuse-ld=lld" still required with the cc crate?

[JMS55]

I don't know, I didn't look at LTO stuff at all.

[FedericoMenegoz]

I am working on a MacOS.

 $ rustc --version --verbose
rustc 1.85.0 (4d91de4e4 2025-02-17)
binary: rustc
commit-hash: 4d91de4e48198da2e33413efdcd9cd2cc0c46688
commit-date: 2025-02-17
host: x86_64-apple-darwin
release: 1.85.0
LLVM version: 19.1.7

After running RUSTFLAGS="-Clinker-plugin-lto -Clinker=clang -Clink-arg=-fuse-ld=lld" cargo build I get these errors:

error: linking with `clang` failed: exit status: 1
  |
  = note: env -u IPHONEOS_DEPLOYMENT_TARGET -u TVOS_DEPLOYMENT_TARGET -u XROS_DEPLOYMENT_TARGET LC_ALL="C" PATH="/Users/user/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/x86_64-apple-darwin/bin:/Users/user/.nvm/versions/node/v22.11.0/bin:/Users/user/.sdkman/candidates/java/current/bin:/bin:/bin::./Library/Python/3.9/lib/python/site-packages:./Library/Python/3.9/lib/python/site-packages:/Users/user/opt/anaconda3/condabin:/Library/Frameworks/Python.framework/Versions/3.12/bin:/Library/Frameworks/Python.framework/Versions/3.9/bin:/usr/local/bin:/System/Cryptexes/App/usr/bin:/usr/bin:/bin:/usr/sbin:/sbin:/Users/user/Library/Python/3.9/bin:/opt/X11/bin:/Library/Apple/usr/bin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/local/bin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/bin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/appleinternal/bin:/Users/user/.cargo/bin:/Users/user/bin:/Users/user/Documents/go/bin" VSLANG="1033" ZERO_AR_DATE="1" "clang" "/var/folders/py/4xm9lwps4237tps_dyzs8m0c0000gn/T/rustcImaCGt/symbols.o" "<5 object files omitted>" "/Users/user/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/x86_64-apple-darwin/lib/{libstd-4342b9106d0415dd.rlib,libpanic_unwind-0a79a6ecf5b18530.rlib,libobject-84e437265aa7041b.rlib,libmemchr-30ebf9d2fcf0bfc8.rlib,libaddr2line-dbfe74fe37aade7e.rlib,libgimli-85a50da1374c89c1.rlib,librustc_demangle-5db1266bfa58bba4.rlib,libstd_detect-e9afbee47cd520d6.rlib,libhashbrown-959071037b1ad0aa.rlib,librustc_std_workspace_alloc-2a434012e00a54bc.rlib,libminiz_oxide-2a0f0a883febe6c2.rlib,libadler-be50c1a262537f4d.rlib,libunwind-07189d75869ed30f.rlib,libcfg_if-696a5bc7b60e0977.rlib,liblibc-4b8d08ad7d2ea874.rlib,liballoc-358cac0c3ed5a5a5.rlib,librustc_std_workspace_core-f56758809865f8ac.rlib,libcore-c79541fdd52d93c7.rlib,libcompiler_builtins-ee44a15358384055.rlib}" "-lSystem" "-lc" "-lm" "-arch" "x86_64" "-mmacosx-version-min=10.12.0" "-Wl,-plugin-opt=O0,-plugin-opt=mcpu=penryn" "-o" "/path/to/repo/target/debug/build/libc-52fc387f25eb33c9/build_script_build-52fc387f25eb33c9" "-Wl,-dead_strip" "-nodefaultlibs" "-fuse-ld=lld"
  = note: some arguments are omitted. use `--verbose` to show all linker arguments
  = note: ld64.lld: error: unknown argument '-plugin-opt=O0'
          ld64.lld: error: unknown argument '-plugin-opt=mcpu=penryn'
          clang: error: linker command failed with exit code 1 (use -v to see invocation)

Does anyone know how can I fix this?

[madsmtm]

This is an upstream rustc issue, see rust-lang/rust#60059