Functional C macros are not expanded.

[nicokoch]

So I'm generating bindings to <linux/uinput>. This works pretty well, but two c macros in particular do not produce any bindings in the output. Reduced input below:

Input C/C++ Header

#define UI_DEV_CREATE		_IO(UINPUT_IOCTL_BASE, 1)
#define UI_DEV_DESTROY		_IO(UINPUT_IOCTL_BASE, 2)

Bindgen Invocation

let bindings = bindgen::Builder::default()
        .no_unstable_rust()
        .header("wrapper/wrapper.h")
        .expect("Unable to generate bindings").

wrapper.h looks like this:

#include <linux/uinput.h>

Actual Results

No constant UI_DEV_CREATE or UI_DEV_DESTROY generated in output.

Expected Results

Constants UI_DEV_CREATE and UI_DEV_DESTROY generated in output.

I'm not really that familiar with the kernel macro _IO and not sure if this is technically even possible, but I figured to report just to get a more professional opinion. If it's not possible, workarounds welcome.

[emilio]

How is _IO defined? Grepping define _IO in my /usr/include/linux dir didn't yield anything.

[emilio]

As in... If we don't know how _IO is defined, how could we generate a sensible constant?

[emilio]

That being said, even with that it doesn't work (huh, I'd swear we knew how to deal with macro expansions):

#define _IO(a_) (a_)

#define UI_DEV_CREATE _IO(1)
#define UI_DEV_DESTROY _IO(2)

[emilio]

This is probably a bug for rust-cexpr. Indeed, it's a dupe of jethrogb/rust-cexpr#3.

[emilio]

I guess I can try to add to libclang a way to get whether the macro definition belongs to a functional macro...

[nicokoch]

If it helps any:
_IO is defined in uapi/asm-generic/ioctl.h as:
#define _IO(type,nr) _IOC(_IOC_NONE,(type),(nr),0)
and _IOC in turn is defined as:

#define _IOC(dir,type,nr,size) \
      (((dir)  << _IOC_DIRSHIFT) | \
      ((type) << _IOC_TYPESHIFT) | \
      ((nr)   << _IOC_NRSHIFT) | \
      ((size) << _IOC_SIZESHIFT))

[emilio]

Ok, so libclang provides a clang_Cursor_isMacroFunctionLike API. Perhaps we could use that.

[emilio]

I've asked @jethrogb about what the best API for this would be. It shouldn't be a big deal to support it in bindgen once cexpr support is there.

[nicokoch]

Meanwhile, I used the following in my wrapper.h to workaround the issue:

#include <linux/uinput.h>

const __u64 _UI_DEV_CREATE = UI_DEV_CREATE;
#undef UI_DEV_CREATE
const __u64 UI_DEV_CREATE = _UI_DEV_CREATE;

const __u64 _UI_DEV_DESTROY = UI_DEV_DESTROY;
#undef UI_DEV_DESTROY
const __u64 UI_DEV_DESTROY = _UI_DEV_DESTROY;

[emilio]

Yeah, note that that would only work in libclang 3.9+ though.

[jethrogb]

Current state: C preprocessor needed. @emilio can you add "help wanted" label?