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Learn how to use a Swift library in C


Learn how to construct a C suitable Swift library?


As a way to create a Swift library that is going to work with C, we now have to mess around with unsafe reminiscence pointers to create a C suitable interface. Thankfully I used to be capable of finding a pleasant instance, which served me as an excellent place to begin, on the Swift boards created by Cory Benfield, so that is what we’ll use on this case. Thanks you. 🙏


closing class MyType {
    var rely: Int = 69
}

@_cdecl("mytype_create")
public func mytype_create() -> OpaquePointer {
    let sort = MyType()
    let retained = Unmanaged.passRetained(sort).toOpaque()
    return OpaquePointer(retained)
}

@_cdecl("mytype_get_count")
public func mytype_get_count(_ sort: OpaquePointer) -> CInt {
    let sort = Unmanaged<MyType>.fromOpaque(UnsafeRawPointer(sort)).takeUnretainedValue()
    return CInt(sort.rely)
}

@_cdecl("mytype_destroy")
public func mytype_destroy(_ sort: OpaquePointer) {
    _ = Unmanaged<MyType>.fromOpaque(UnsafeRawPointer(sort)).takeRetainedValue()
}


The excellent news is that we do not essential must create a separate header file for our interfaces, however the Swift compiler can generate it for us if we offer the -emit-objc-header flag.


I’ve an article about the swiftc command for freshmen and I additionally wrote some issues about the Swift compiler, the place I speak concerning the out there flags. This time we’ll use the -module-name choice to specify our module identify, we’ll generate the required information utilizing the -emit-dependencies flag, parse the supply information as a library (-parse-as-library), since we might wish to generate a Swift library present the required goal and model info and emit a header file.



swiftc 
        -module-name mytype 
        -emit-dependencies 
        -parse-as-library 
        -c mytype.swift 
        -target arm64-apple-macosx12.0 
        -swift-version 5 
        -emit-objc-header 
        -emit-objc-header-path mytype.h


swiftc 
    -module-name mytype 
    -emit-dependencies 
    -parse-as-library 
    -c mytype.swift 
    -swift-version 5 
    -emit-objc-header 
    -emit-objc-header-path mytype.h


This could generate a mytype.h and a mytype.o file plus some extra Swift module associated output information. We will use these information to construct our closing executable, however there are a couple of extra extra issues I might like to say.


Beneath Linux the header file will not work. It accommodates a line #embody Basis/Basis.h and naturally there isn’t a such header file for Linux. It’s attainable to put in the GNUstep bundle (e.g. through yum: sudo yum set up gnustep-base gnustep-base-devel gcc-objc, however for me the clang command nonetheless complained concerning the location of the objc.h file. Anyway, I simply eliminated the iclude Basis assertion from the header file and I used to be good to go. 😅


The second factor I might like to say is that if you wish to export a category for Swift, that is going to be a bit tougher, as a result of courses will not be included within the generated header file. You may have two choices on this case. The primary one is to show them into Goal-C courses, however it will result in issues when utilizing Linux, anyway, that is how you are able to do it:


import Basis

@objc public closing class MyType: NSObject {
    public var rely: Int = 69
}


I choose the second choice, when you do not change the Swift file, however you create a separate header file and outline your object sort as a struct with a customized sort (mytype_struct.h).

typedef struct mytype mytype_t;


We will want this sort (with the corresponding header file), as a result of the mytype_create perform returns a pointer that we will use to name the opposite mytype_get_count methodology. 🤔


Compiling C sources utilizing Swift libraries


So how will we use these uncovered Swift objects in C? Within the C programming language you simply must import the headers after which voilá you should utilize the whole lot outlined in these headers.


#embody <stdio.h>
#embody "mytype.h"

int fundamental() {
    mytype_t *merchandise = mytype_create();

    int i = mytype_get_count(merchandise);
 
    printf("Howdy, World! %dn", i);

    return 0;
}


We are able to use clang to compile the fundamental.c file into an object file utilizing the required header information.



clang -x objective-c -include mytype.h -include mytype_struct.h -c fundamental.c


clang -include mytype.h -include mytype_struct.h -c fundamental.c


This command will construct a fundamental.o file, which we will use to create the ultimate executable. 💪


Linking the ultimate executable


This was the toughest half to determine, however I used to be in a position to hyperlink the 2 object information collectively after a couple of hours of battling the ld command and different framework instruments I made a decision to provide it up and let swiftc handle the job, since it may possibly construct and hyperlink each C and Swift-based executables.


We will want a listing of the thing information that we’ll hyperlink collectively.


ls *.o > LinkFileList


Then we will name swiftc to do the job for us. I suppose it’s going to invoke the ld command below the hood, however I am not a linker skilled, so if you realize extra about this, be at liberty to achieve out and present me extra information concerning the course of. I’ve to learn this e-book for positive. 📚



swiftc 
        -sdk /Functions/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX12.1.sdk 
        -F /Functions/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/Library/Frameworks 
        -I /Functions/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/usr/lib 
        -L /Functions/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/usr/lib 
        -L /Customers/tib/swiftfromc/ 
        -module-name Instance 
        -emit-executable 
        -Xlinker -rpath 
        -Xlinker @loader_path @/Customers/tib/swiftfromc/LinkFileList 
        -Xlinker -rpath 
        -Xlinker /Functions/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift/macosx 
        -Xlinker -rpath 
        -Xlinker /Functions/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/swift-5.5/macosx 
        -target arm64-apple-macosx12.1 
        -Xlinker -add_ast_path 
        -Xlinker /Customers/tib/swiftfromc/mytype.swiftmodule 
        -L /Functions/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib


swiftc 
    -L /residence/ec2-user/swiftfromc 
    -module-name Instance 
    -emit-executable 
    -Xlinker -rpath 
    -Xlinker @loader_path @/residence/ec2-user/swiftfromc/LinkFileList


The command above will produce the ultimate linked executable file that you would be able to run by utilizing the ./Instance snippet and hopefully you may see the “Howdy, World! 69” message. 🙈


If you wish to know extra concerning the rpath linker flag, I extremely advocate studying the article by Marcin Krzyzanowski. If you wish to learn extra about Swift / Goal-C interoperability and utilizing the swiftc command, it’s best to take a look at this text by RDerik. Lastly if you wish to name C code from Swift and go the opposite approach, it’s best to check out my different weblog put up.



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