FAQ - The bindgen User Guide
Description: `bindgen` automatically generates Rust FFI bindings to C and C++ libraries.

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# Frequently Asked Questions

- Why isn’t `bindgen` generating methods for this allowlisted class?
- Why isn’t `bindgen` generating bindings to inline functions?
- Does `bindgen` support the C++ Standard Template Library (STL)?
- How to deal with bindgen generated padding fields?
- How to generate bindings for a custom target?
- Why isn’t `bindgen` generating documentation for system headers?

### Why isn’t `bindgen` generating methods for this allowlisted class?

Are the methods `inline` methods, or defined inline in the class? For example:

    class Dooder {
      public:
        // Function defined inline in the class.
        int example_one() { return 1; }

        // `inline` function whose definition is supplied later in the header, or in
        // another header.
        inline bool example_two();
    };

    inline bool Dooder::example_two() {
        return true;
    }

If so, see “Why isn’t `bindgen` generating bindings to inline functions?”

If not, consider filing an issue!

### Why isn’t `bindgen` generating bindings to inline functions?

These functions don’t typically end up in object files or shared libraries with symbols that we can reliably link to, since they are instead inlined into each of their call sites. Therefore, we don’t generate bindings to them, since that creates linking errors.

However, if you are compiling the C/C++ yourself (rather than using a system shared library, for example), then you can pass `-fkeep-inline-functions` or `-fno-inline-functions` to `gcc` or `clang`, and invoke `bindgen` with either the `bindgen::Builder::generate_inline_functions` method or the `--generate-inline-functions` flag.

Note that these functions and methods are usually marked inline for a reason: they tend to be hot. The above workaround makes them an out-of-line call, which might not provide acceptable performance.

As an alternative, you can invoke `bindgen` with either the `bindgen::Builder::wrap_static_fns` method or the `--wrap-static-fns` flag. Which generates a C source file that can be compiled against the input headers to produce Rust headers for `static` and `static inline` functions. See How to handle `static inline` functions for further information.

### Does `bindgen` support the C++ Standard Template Library (STL)?

Sort of. A little. Depends what you mean by “support”.

Most functions, methods, constructors, and destructors are inline in the STL. That ties our hands when it comes to linking: “Why isn’t `bindgen` generating bindings to inline functions?”

As far as generating opaque blobs of bytes with the correct size and alignment, `bindgen` can do pretty well. This is typically enough to let you use types that transitively contain STL things. We generally recommend marking `std::.*` as opaque, and then allowlisting only the specific things you need from the library you’re binding to that is pulling in STL headers.

### How to deal with bindgen generated padding fields?

Depending the architecture, toolchain versions and source struct, it is possible that bindgen will generate padding fields named `__bindgen_padding_N`. As these fields might be present when compiling for one architecture but not for an other, you should not initialize these fields manually when initializing the struct. Instead, use the `Default` trait. You can either enable this when constructing the `Builder` using the `derive_default` method, or you can implement this per struct using:

    impl Default for SRC_DATA {
        fn default() -> Self {
            unsafe { std::mem::zeroed() }
        }
    }

This makes it possible to initialize `SRC_DATA` by:

    SRC_DATA {
        field_a: "foo",
        field_b: "bar",
        ..Default::default()
    }

In the case bindgen generates a padding field, then this field will be automatically initialized by `..Default::default()`.

### How to generate bindings for a custom target?

To generate bindings for a custom target you only need to pass the `--target` argument to `libclang`. For example, if you want to generate bindings for the `armv7a-none-eabi` target using the command line, you need to invoke `bindgen` like so:

    $ bindgen <input_headers> -- --target=armv7a-none-eabi

If you are using `bindgen` as a library, you should call `builder.clang_arg("--target=armv7a-none-eabi")` on your `builder`.

### Why isn’t `bindgen` generating documentation for system headers?

By default, Bindgen does not generate documentation for system headers because `libclang` does not provide this information. To address this, you should call `builder.clang_arg("-fretain-comments-from-system-headers")` on your `builder`.