Easy methods to use C libraries in Swift?

Constructing a customized C library utilizing SPM

You should utilize the Swift Package deal Supervisor to create C household primarily based supply information (C, C++, Goal-C and Goal-C++) and ship them as standalone elements. If you do not know a lot concerning the Swift Package deal Supervisor, you need to learn my complete tutorial about how SPM works. 📦

The one factor that you have to setup a library is a regular Package deal.swift manifest file with a barely altered listing construction to assist header information. Let’s make a MyPoint library.

import PackageDescription

let package deal = Package deal(
    identify: "MyPoint",
    merchandise: [
        .library(name: "MyPoint", targets: ["MyPoint"]),
    ],
    targets: [
        .target(name: "MyPoint"),
    ]
)

The whole lot that you just put into the header file might be publicly accessible for different builders to make use of, the implementation particulars are going to be situated instantly beneath the Sources/[target]/ listing, however you must create a further embody folder to your headers. Let’s make a MyPoint.h file beneath the Sources/MyPoint/embody path with the next contents.

struct MyPoint {
   int x;
   int y;
};

We have simply outlined the general public interface for our library. Now when you attempt to compile it by means of the swift construct command, it will complain that the mission is lacking some supply information. We are able to simply repair this by creating an empty MyPoint.c file beneath the Sources/MyPoint listing.

Whenever you import an area header file to make use of in your implementation code, you possibly can skip the “embody” path and easily write #embody “MyPoint.h”. You may additionally put all types of C household elements into this mission, this technique works with C++, Goal-C and even Goal-C++ information.

You may additionally place header information subsequent to the implementation supply code, however in that case the system will not have the ability to auto-locate your public (umbrella) header information, so that you additionally need to create a modulemap file and supply the proper location of your headers explicitly. In case you use the construction with the embody listing SPM will generate all the pieces for you robotically.

Congratulations, you simply shipped your first C code with Swift Package deal Supervisor. 🥳

Interacting with C libraries utilizing Swift

We’ll create a model new Swift package deal to construct an executable utility primarily based on the beforehand created C library. To be able to use an area package deal you possibly can merely specify it as with the trail argument beneath the dependencies in your Package deal.swift manifest file.

import PackageDescription

let package deal = Package deal(
    identify: "Pattern",
    merchandise: [
        .executable(name: "Sample", targets: ["Sample"]),
    ],
    dependencies: [
        .package(path: "../MyPoint")
    ],
    targets: [
        .target(name: "Sample", dependencies: [
            .product(name: "MyPoint", package: "MyPoint"),
        ]),
    ]
)

This time we’re going to use the MyPoint library as an area dependency, however in fact you possibly can handle and publish your personal libraries utilizing a git repository someplace within the cloud. Subsequent we must always create our Sources/Pattern/predominant.swift file, import the library and write some code.

import MyPoint

let p = MyPoint(x: 4, y: 20)
print("Howdy, world!", p.x, p.y)

If each packages can be found regionally, be sure to place them subsequent to one another, then all the pieces ought to work like a attraction. You may open the Pattern mission manifest file utilizing Xcode as properly, the IDE can resolve package deal dependencies robotically for you, however when you want the command line, you need to use the swift run command to compile & run the executable goal.

With this method you possibly can import the MyPoint module from every other Swift package deal and use the accessible public elements from it. You simply have so as to add this module as a dependency, by the way in which you possibly can even name this module from one other C (C++, ObjC, Objc++) mission made with SPM. 😎

Easy methods to use C system libraries from Swift?

There are millions of accessible instruments that you would be able to set up in your working system (Linux, macOS) with a package deal supervisor (apt, brew). For instance there may be the well-known curl command line instrument and library, that can be utilized for transferring information from or to a server. In different phrases, you may make HTTP requests with it, simply sort curl "https://www.apple.com/" right into a terminal window.

These system elements are often constructed round libraries. In our case curl comes with libcurl, the multiprotocol file switch library. Typically you may wish to use these low degree elements (often written in C) in your utility, however how can we add them as a dependency? 🤔

The reply is easy, we are able to outline a brand new systemLibrary goal in our package deal manifest file.

import PackageDescription

let package deal = Package deal(
    identify: "Pattern",
    merchandise: [
        .executable(name: "Sample", targets: ["Sample"]),
    ],
    dependencies: [
        .package(path: "../MyPoint")
    ],
    targets: [

        .systemLibrary(
            name: "libcurl",
            providers: [
                .apt(["libcurl4-openssl-dev"]),
                .brew(["curl"])
            ]
        ),

        .goal(identify: "Pattern", dependencies: [
            .product(name: "MyPoint", package: "MyPoint"),
            .target(name: "libcurl"),
        ]),
    ]
)

Contained in the Package deal.swift file you possibly can set the suppliers for the library (comparable to brew for macOS or aptitude for a lot of Linux distributions). Sadly you continue to need to manually set up these packages, as a result of SPM will not do that for you, consider it as “only a reminder” for now… 😅

This may permit us to create a customized modulemap file with further headers (common or umbrella) and linker flags inside our mission folder. First, we must always add the next modulemap definition to the Sources/libcurl/module.modulemap file. Please create the libcurl listing, if wanted.

module libcurl [system] {
    header "libcurl.h"
    hyperlink "curl"
    export *
}

The idea of modules are coming from (clang) LLVM, I extremely suggest checking the linked article if you wish to know extra about modulemaps. This manner we inform the compiler that we wish to construct a module primarily based on the curl library, therefore we hyperlink curl. We additionally wish to present our customized header file to make some further stuff accessible or extra handy. Individuals often name these header information shims, umbrella headers or bridging headers.

An umberlla header is the primary header file for a framework or library. A bridging header permits us to make use of two languages in the identical utility. The shim header works across the limitation that module maps should include absolute or native paths. All of them exposes APIs from a library or language to a different, they’re very related, however they aren’t the identical idea. 🙄

In our case we will create a libcurl.h header file contained in the Sources/libcurl folder. The module map merely refers to this header file. Here is what we will place within it.

#embody 
#embody 

typedef size_t (*curl_func)(void * ptr, size_t dimension, size_t num, void * ud);

CURLcode curl_easy_setopt_string(CURL *curl, CURLoption choice, const char *param) {
    return curl_easy_setopt(curl, choice, param);
}

CURLcode curl_easy_setopt_func(CURL *deal with, CURLoption choice, curl_func param) {
    return curl_easy_setopt(deal with, choice, param);
}

CURLcode curl_easy_setopt_pointer(CURL *deal with, CURLoption choice, void* param) {
    return curl_easy_setopt(deal with, choice, param);
}

This code comes from the archived SoTS/CCurl repository, however when you examine the shim file contained in the Kitura/CCurl package deal, you will discover a just about related method with much more handy helpers.

The principle motive why we’d like these capabilities is that variadic capabilities cannot be imported by Swift (but), so we now have to wrap the curl_easy_setopt calls, so we’ll have the ability to use it from Swift.

Okay, let me present you the right way to write a low-level curl name utilizing the libcurl & Swift.

import Basis
import MyPoint
import libcurl

class Response {
    var information = Information()

    var physique: String { String(information: information, encoding: .ascii)! }
}

var response = Response()

let deal with = curl_easy_init()
curl_easy_setopt_string(deal with, CURLOPT_URL, "http://www.google.com")

let pointerResult = curl_easy_setopt_pointer(deal with, CURLOPT_WRITEDATA, &response)
guard pointerResult == CURLE_OK else {
    fatalError("Couldn't set response pointer")
}
curl_easy_setopt_func(deal with, CURLOPT_WRITEFUNCTION) { buffer, dimension, n, reference in
    let size = dimension * n
    let information = buffer!.assumingMemoryBound(to: UInt8.self)
    let p = reference?.assumingMemoryBound(to: Response.self).pointee
    p?.information.append(information, rely: size)
    return size
}

let ret = curl_easy_perform(deal with)
guard ret == CURLE_OK else {

    fatalError("One thing went improper with the request")
}
curl_easy_cleanup(deal with)

print(response.physique)

I do know, I do know. This appears horrible for the primary sight, however sadly C interoperability is all about coping with pointers, unfamiliar varieties and reminiscence addresses. Anyway, this is what occurs within the code snippet. First we now have to outline a response object that may maintain the info coming from the server as a response. Subsequent we name the system capabilities from the curl library to create a deal with and set the choices on it. We merely present the request URL as a string, we go the end result pointer and a write operate that may append the incoming information to the storage when one thing arrives from the server. Lastly we carry out the request, examine for errors and cleanup the deal with.

It isn’t so dangerous, however nonetheless it appears nothing such as you’d count on from Swift. It is only a fundamental instance I hope it will enable you to grasp what is going on on beneath the hood and the way low degree C-like APIs can work in Swift. If you wish to apply you need to strive to try the Kanna library and parse the response utilizing a customized libxml2 wrapper (or you possibly can examine a SQLite3 wrapper). 🤓

The system library goal function is a pleasant approach of wrapping C [system] modules with SPM. You may learn extra about it on the official Swift boards. In case you are nonetheless utilizing the previous system library package deal sort format, please migrate, because it’s deprecated and it will be utterly eliminated in a while.