Try mixing more C and Swift in one project (OS X, Linux)

Overview

• Last time only called the C function from Swift, but this time I will call the Swift function from C as well.

wrap up

• About the same as last time.
• Swift functions are mangled, so if you want to call them from C ... Which one:

1. Swift suppresses mangling
2. Call Swift's mangled function from C

• This time, 1. is adopted. The reason is as follows:
• Because 1. is easy.
• Swift's ABI is still unstable [^ Summary-1].

[^ Summary-1]: ABI is stable from Swift 3.0 ... It should have been postponed further [^ Summary-1-Note-1]. [^ Summary-1-Note-1]: Winding down the Swift 3 release

environment

Same as last time.

• OS X = OS X El Capitan + Xcode 7.3 (Swift 2.2)
• Linux = CentOS 7 + Swift 2.2.1

OS X

$ clang --version
Apple LLVM version 7.3.0 (clang-703.0.29)
Target: x86_64-apple-darwin15.5.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

$ swift --version
Apple Swift version 2.2 (swiftlang-703.0.18.1 clang-703.0.29)
Target: x86_64-apple-macosx10.9

Linux

$ clang --version
clang version 3.8.0 (tags/RELEASE_380/final)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/local/bin

$ swift --version
Swift version 2.2.1 (swift-2.2.1-RELEASE)
Target: x86_64-unknown-linux-gnu

source file

This time I prepared 4 files.

$ ls
c.c  c.h  swift.h  swift.swift

c.h

#include <stdio.h>
void c_function(const char *string);
void call_swift_function(void);

swift.h

void swift_function(const char *string);

c.c

#include "c.h"
#include "swift.h"
void c_function(const char *string) {
  printf("%s", string);
}
void call_swift_function(void) {
  swift_function("Hello, I'm C.\n");
}

swift.swift

@_silgen_name("swift_function")
func swift_function(cString:UnsafePointer<CChar>) {
  print(String.fromCString(cString)!) 
}
c_function("Hello, I'm Swift.\n")
call_swift_function()

• For Swift 2.2 and below, use @ asmname instead of @ _silgen_name.

Compilation procedure

Create object file from C source

OS X, Linux common

$ clang -c c.c -oc.o
$ ls
c.c  c.h  c.o  swift.h  swift.swift

Compile Swift source with object files

OS X

$ xcrun --sdk macosx swiftc swift.swift c.o -import-objc-header c.h
ld: warning: object file (c.o) was built for newer OSX version (10.11) than being linked (10.9)
$ ls
c.c		c.h		c.o		main		swift.h		swift.swift
$ ./main
Hello, I'm Swift.
Hello, I'm C.

Linux

$ swiftc swift.swift c.o -import-objc-header c.h
$ ls
c.c  c.h  c.o  main  swift.h  swift.swift
$ ./main
Hello, I'm Swift.
Hello, I'm C.

did it. This is the end. It ’s easy, is n’t it?

by the way

You can also pass a function pointer, but that's another story [http://qiita.com/YOCKOW/items/9239e65995b8c4f8b14f).