cpp-swift-mwe

A minimal example that investigates how to build a Swift project that depends on a compiled, non-system C++ library.

Built from @aciidb0mb3r's blog post.

All of this is executed with

Apple Swift Package Manager - Swift 3.0.2 (swiftpm-11750)

This is the latest Toolchain available through Apple's channels at the time of this writing.

Starting Point

Following the blog post closely, the initial project (commit 565752f6e27633b41b6c62c1ead700ba4e8d7d95) is structured as follows: the Swift module depends on a C wrapper around a C++ module with sources.

swift build compiles all three modules; .build/debug/swift prints 5.

Goal

The ultimate goal is to use a C++ library -- let's call it C -- in Swift.

• C is open source.
• We can not expect C to be available on target systems.
• We want to use a custom (minimal, platform specific) build of C.

Thus, as long as SwiftPM does not allow us to specify custom build instructions for dependencies, we have to supply the library as binary file, with headers to compile against. (In settings where sharing the sources of C is not an option, the need is even more immediate.)

Ideally, we want to receive a build result that can be easily referenced in other Swift projects, in particular such developed in XCode.

In case that is relevant, the library we want to build is to be used by iOS apps.

Attempt 1: No C++ sources, binary library

Add libcpplib.dylib built with the Starting Point configuration. Remove Sources/cpplib/cpplib.cpp. Add build.sh for convenience.
(Forgot to exclude the non-module stuff, but does not matter here; see Attempt 2.)

Commit 85700aa9aacca73b13e2082bba68853a13b37add

Build with command:

swift build -Xlinker -L/path/to/Dependencies \
            -Xlinker -lcpplib

Output:

error: the module at /path/to/cpp-swift-mwe/Sources/cpplib
       does not contain any source files
fix:   either remove the module folder, or add a source file to the module

Attempt 2: Dummy C++ sources, binary library

Add an empty file Sources/cpplib/empty.cpp. Update Package.swift to exclude Dependencies, build.sh.

Commit 7db39811f72128db9cc6cf77ee15b5c70c19b32a

Build with the same command as Attempt 1. Output:

Compile cpplib empty.cpp
Linking cpplib
Linking cwrapper
Undefined symbols for architecture x86_64:
  "cpplib::five()", referenced from:
      _cwrapperfive in cwrapper.cpp.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
<unknown>:0: error: build had 1 command failures

Apparently, the folder provided by -L does not take precedence as the documentation promises.

Interlude: A Workaround

The problem is with who looks for libraries where.

@aciidb0mb3r proposes a workaround in swiftpm.slack:

Run these two commands once:

mv Dependencies/libcpplib.dylib Dependencies/libcpplibVendored.dylib
install_name_tool -id @executable_path/../../Dependencies/libcpplibVendored.dylib \
                  Dependencies/libcpplibVendored.dylib

Then use option -Xlinker -lcpplibVendored instead of -Xlinker -lcpplib.

This fixes the build but does not generate a shippable product, since in production the paths will be different.

I filed a feature request towards support of non-system binary dependencies.

Attempt 3: A Makefile

Needed to do this, but as .cpp.

Then, it is mostly copying .build/debug.yaml (for now; release later) from Starting Point into another build tool and replace compilation of the C++ library with some copying instructions.

In order to make the process less arduous the next time around, I have built a configure script that should deal with other projects that use the same structure.

This now works:

$ make clean
$ ./configure.rb
$ make
$ .build/debug/swift
5 -- all the way from C++!

Open Problems

• Does this work with libswift.a just as well?
• Generated modulemap files are probably broken if there is more than one C header.
• Will need adapting for building Swift libraries.
• Will probably not work properly if the (Swift) source folders contain subfolders.
• Will need adapting for building for iOS.
• configure.rb has its parameters hard-coded for now.

Thoughts

It seems clear that the current design of SwiftPM does not consider dependencies that are provided as non-system binaries. Maybe there should be a protocol Dependency -- currently Package.dependencies is of type [Package]! -- with a new subtype Binary with initializer

Binary(binary: String, header: String)

or similar. Note that the file may have to depend on the target platform and architecture.
(Would we need parameters for the header language and whether to integrate the binary into the package binary?)

Could this be implemented by SwiftPM by creating pkgconfig files?