How to combine Swift and C code in a static library - c

Question

I am trying to create a static library that contains Swift and C code, and exposes only a Swift interface. So the C code side should ideally not be exposed to the outside world. I can't seem to figure out the right way to do this without causing warnings or errors.
For the sake of simplification, I tried setting up a minimal Swift+C static library with these files:
Test.swift
class Test {
func test() -> CInt {
return testFromC()
}
}
Test.c
#include "Test.h"
int testFromC(void) {
return 0xdeadbeef;
}
Test.h
int testFromC(void);
What I want is a static library that exposes the Swift class Test, but not the C function testFromC(). I can live with it being exposed if there is no alternative, though.
What I have tried is:
Add a bridging header. This almost works, but a) lots of people say this is not supported or recommended, b) it exposes testFromC() to the importer of the module, and c) under some circumstances it throws warnings about implicitly importing the bridging header (which is true, but what I want is specifically to NOT do that).
Add a modulemap. I can't figure out how to do this. No examples online seem to work for me, and if anyone knows how to do this correctly, I would love to see an example for these three files.
Add a private modulemap - I gather this is what I want if I want to avoid exposing the C functions but I had even less success with this.
Is anyone able to put together the exact steps to make this trivial example build?

Answer 1

You can indeed create a private module map so that the C symbols are not exported, see https://clang.llvm.org/docs/Modules.html#private-module-map-files.
The bridging header would not work because you want to create a framework and not an application.
There are certainly several ways to set up the project, one of which would be: create a project with the corresponding module map settings in XCode.
The .modulemap file could look something like this:
framework module SwiftOnlyAPI {
umbrella header "SwiftOnlyAPI.h"
export *
module * { export * }
}
framework module SwiftOnlyAPI_Private {
header "Test.h"
}
Your Test.swift would rather look something like this:
import SwiftOnlyAPI_Private
public class Test {
public init() {
//perform some initializations
}
public func test() -> CInt {
return testFromC()
}
}
Please note a few small changes: the import of SwiftOnlyAPI_Private, this is necessary for the C functions that are not exported, but of course are called locally within the framework. Classes and exported methods must be public to be accessible from outside the framework.
Instructions
<File/New Project...> choose Framework, choose Language Swift name it SwiftOnlyAPI
add you files (.c,.h,.swift) to the SwiftOnlyAPI folder
select Test.h in the project navigator and switch the target membership in the inspector from Project to Private
choose SwiftOnlyAPI in the project navigator, select the SwiftOnlyAPI target
now click on Build Settings and for Mach-O Type select: Static Library
still in Build Settings: for Module Map File enter: SwiftOnlyAPI/SwiftOnlyAPI.modulemap
select the folder SwiftOnlyAPI in the SwiftOnlyAPI project, choose New File... from the context menu and, choose Empty template from the Other section and name it SwiftOnlyAPI.modulemap (uncheck target membership)
add the modulemap content shown above
A test produces the desired result, just as shown for the alternative solution for multiple targets below.
Static Library
Frameworks are really just either dynamic or static libraries (depending on the setting) with additional bundled files. Convenient for the framework users.
You can test this yourself with the file command:
Alternative
If you want to have the C part separate (e.g. for other consumers), you can also split it into two subprojects in an Xcode workspace, one for a static C lib and one for a static Swift-based framework:
create a XCode workspace <File/New Workspace...>, e.g. name it SwiftCWorkspace
<File/New Project...> choose Library, choose under Framework None (Plain C/C++ Library), name it MyCLib and choose type Static
in the dialog choose for Add to and Group: SwiftCWorkspace
select the MyCLib node in project navigator, choose New File... from the context menu and choose C file template, name it Test
replace the content with your Test.c and Test.h content
<File/New Project...> choose Framework, choose Language Swift name it SwiftOnlyAPI
in the dialog choose for Add to and Group: SwiftCWorkspace
select the folder SwiftOnlyAPI in the SwiftOnlyAPI project, choose New File... from the context menu and, choose Swift template and replace it with your ccontent
choose SwiftOnlyAPI in the project navigator, select the SwiftOnlyAPI target
now click on Build Settings and for Mach-O Type select: Static Library
still in Build Settings: for Module Map File enter: SwiftOnlyAPI/SwiftOnlyAPI.modulemap
now click on General and in the Frameworks and Library section add libMyCLib.a
select the folder SwiftOnlyAPI in the SwiftOnlyAPI project, choose New Group... from the context menu and name it MyCLib
drag & drop Test.h from MyCLib project to the MyCLib folder in SwiftOnlyAPI, uncheck Copy items if needed
with the newly linked file Test.h selected switch the target membership in the inspector from Project to Private
select the folder SwiftOnlyAPI in the SwiftOnlyAPI project, choose New File... from the context menu and, choose Empty template from the Other section and name it SwiftOnlyAPI.modulemap (uncheck target membership)
add the modulemap content shown above
Notes
If Xcode displays error messages even though you have set everything up correctly, it is occasionally necessary to exit Xcode after the project setup tasks, delete the ~/Library/Developer/Xcode/DerivedData folder, and then restart the build.
Test
import SwiftOnlyAPI
...
let result = SwiftOnlyAPI.Test()
print(result.test())
You can test this in a separate application using the SwiftOnlyAPI framework: the correct result will be output.
Also the C function testFromC is private and not externally accessible as desired, see screenshot: