The shared functionality for the export tool is written in go to take advantage of existing libraries.
To make sure it can be easily integrated with a myriad of other toolkits, a C interface is exported.
Due to the nature of CGo, Go pointers can't be shared across the C boundary. We instead use cgo.Handle type to pass
cgo handle to the C code as struct pointers.
It's recommended to free all allocated memory by this shared library(strings, arrays, etc..) using the etFree function
or other equivalents, to prevent issues on windows, where the CGO shared library is built with mingw and the remaining
code is built with MSVC.
Since CGO can't call function pointers, if you need/want to simulate callback, you to write a C function that accepts your vtable/function pointer and calls the function pointer in question. See this header for an example.
Since there's no "special" cancelled state in C, this situation needs to be handled internally and communicated via a special return value.