make _cgo_export.h available to Cgo C files

[eliasnaur]

TinyGo supports calling a Go function from C in the usual Cgo way of //export'ing the Go function and adding a C extern declaration matching the Go function signature. To minimize risk of function and declaration mismatch, the Go standard implementation generates a _cgo_export.h header file containing all extern declarations for exported Go functions.

However, TinyGo doesn't seem to offer _cgo_export.h, which adds friction to using otherwise portable Cgo packages.

[aykevl]

Yes, sounds like a feature we should implement eventually.
After a bit of testing I found that _cgo_export.h is only present in separate C files, not in the import "C" header. Which makes sense: otherwise you're in a chicken-and-egg situation (you only have the types of the exported function after typechecking, and CGo processing happens before typechecking). So basically, to implement this we'd need to add a temporary directory to the C header search path which includes _cgo_export.h generated from the //export declarations of that package.

[haikyuu]

Very interesting. Sorry to jump on the issue @eliasnaur but is it possible to call esp32 libraries from c where they are not supported by tinygo? (e.g the Bluetooth module on esp32)?

[eliasnaur]

I don't know the answer, sorry.