rockfordwei/cswift

Introduction

There are two C files: CSwift.c and CSwift.h. The objective is to build a CSwift library and export it to Swift.

Quick Start

Please compile this project with Swift 4.0.3 toolchain.

Build & Test

$ git clone https://github.com/RockfordWei/CSwift.git
$ cd CSwift
$ swift build
$ swift test

The sources are written in C while the test program is written in Swift. So if all tests passed, then congratulations! You've already mastered the process to call C api in a Swift source.

Walk Through

However, even without the sources above, you can still start everything from blank:

Start From An Empty Folder:

Assume the objective library is still CSwift, then find an empty folder and try these commands in a terminal:

mkdir CSwift && cd CSwift && swift package init
mkdir Sources/CSwift/include && rm Sources/CSwift/CSwift.swift

The above commands will set up the empty project template

C Header File

Now edit the header file Sources/CSwift/include/Swift.h:

extern int c_add(int, int);
#define C_TEN 10

C Source

Finishing the implementation of the C body Sources/CSwift/CSwift.c:

#include "include/CSwift.h"
int c_add(int a, int b) { return a + b ; }

Module Map

Next, we will setup an umbrella file for swift: Sources/CSwift/include/module.modulemap

module CSwift [system] {
  header "CSwift.h"
  export *
}

Call C API in Swift

Now let's check if the library works by editing a test script: Tests/CSwiftTests/CSwiftTests.swift

import XCTest
@testable import CSwift
class CSwiftTests: XCTestCase {
  func testExample() {
    let three = c_add(1, 2)
    XCTAssertEqual(three, 3)
    XCTAssertEqual(C_TEN, 10)
  }
  static var allTests : [(String, (CSwiftTests) -> () throws -> Void)] {
      return [  ("testExample", testExample)  ]
  }
}

Test

The final step is the easiest one - build & test:

$ swift build
$ swift test

If success, then perfect!

Swift as Script and Call C lib dynamically

Beside the above classic static build & run, Swift also provide an interpreter to execute swift source as scripts, just like a playground in a terminal. This project also makes an example for swift script, and even more, introduces how to call the same C api dynamically in such a script.

Dynamic Link Library

The default linking object of Swift 4 is static, so it needs a bit modification to turn it into a dynamic one.

To do this, edit the Package.swift file, and add a line:

.library(
    name: "CSwift",
    type: .`dynamic`,  // <------------ Insert the dynamic type right here!
    targets: ["CSwift"]),

Please check a swift script dll.swift.script, actually it is a common swift with no difference to any other swift sources:

// First thing first, make sure your dll path is an dynamic library in an ABSOLUTE path.
// on Mac, the suffix is ".dylib"; on Linux, it is ".so"
guard let lib = dlopen(dllpath,  RTLD_LAZY) else {
  exit(0)
}

// declare the api prototype to call
typealias AddFunc = @convention(c) (CInt, CInt) -> CInt

// look up the function in the library
guard let c_add = dlsym(lib, "c_add") else {
  dlclose(lib)
  exit(0)
}

// attache the function to the real API address
let add = unsafeBitCast(c_add, to: AddFunc.self)

// call the C method, dynamically
let x = add(1, 2)
print(x)

// release resources
dlclose(lib)

Run the Swift Script

This project also provides a bash script dll.sh to run the swift script above.

# step one, build the C library
swift build

# then test what OS it is: .dylib for apple and .so for linux
if swift --version|grep apple
then
  SUFFIX=dylib
else
  SUFFIX=so
fi

# generate the full path of new library.
DLL=$PWD/.build/debug/libCSwift.$SUFFIX

# run the swift script and call the libray.
swift dll.swift.script $DLL

More Info

If Xcode is preferred, then try command swift package generate-xcodeproj before building.

Package Metadata

Repository: rockfordwei/cswift

Default branch: master

README: README.md