Frequently Asked Questions

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 {
    // 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.

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:

    field_a: "foo",
    field_b: "bar",

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