The way to use C libraries in Swift?

Constructing a customized C library utilizing SPM

You need to use the Swift Package deal Supervisor to create C household based mostly supply recordsdata (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, it is best to learn my complete tutorial about how SPM works. 📦

The one factor that it’s essential to setup a library is an ordinary Package deal.swift manifest file with a barely altered listing construction to help header recordsdata. Let’s make a MyPoint library.

import PackageDescription

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

All the things that you simply put into the header file will likely be publicly out there for different builders to make use of, the implementation particulars are going to be positioned instantly beneath the Sources/[target]/ listing, however you must create a further embody folder in 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’s going to complain that the mission is lacking some supply recordsdata. We will simply repair this by creating an empty MyPoint.c file beneath the Sources/MyPoint listing.

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

You can additionally place header recordsdata subsequent to the implementation supply code, however in that case the system will not have the ability to auto-locate your public (umbrella) header recordsdata, so that you additionally must create a modulemap file and supply the proper location of your headers explicitly. If you happen to use the construction with the embody listing SPM will generate all the things for you mechanically.

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 bundle to construct an executable software based mostly on the beforehand created C library. As a way to use an area bundle you’ll be able to merely specify it as with the trail argument beneath the dependencies in your Package deal.swift manifest file.

import PackageDescription

let bundle = 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’ll be able to handle and publish your personal libraries utilizing a git repository someplace within the cloud. Subsequent we should always create our Sources/Pattern/primary.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, ensure you place them subsequent to one another, then all the things ought to work like a allure. You possibly can open the Pattern mission manifest file utilizing Xcode as nicely, the IDE can resolve bundle dependencies mechanically for you, however when you desire the command line, you need to use the swift run command to compile & run the executable goal.

With this method you’ll be able to import the MyPoint module from some other Swift bundle and use the out there public elements from it. You simply have so as to add this module as a dependency, by the best way you’ll be able to even name this module from one other C (C++, ObjC, Objc++) mission made with SPM. 😎

The way to use C system libraries from Swift?

There are literally thousands of out there instruments which you can set up in your working system (Linux, macOS) with a bundle supervisor (apt, brew). For instance there’s the well-known curl command line device and library, that can be utilized for transferring knowledge from or to a server. In different phrases, you can also make HTTP requests with it, simply kind 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. Generally you would possibly need to use these low stage elements (often written in C) in your software, however how will we add them as a dependency? 🤔

The reply is easy, we will outline a brand new systemLibrary goal in our bundle manifest file.

import PackageDescription

let bundle = 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’ll be able to set the suppliers for the library (akin to brew for macOS or aptitude for a lot of Linux distributions). Sadly you continue to must 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 can enable us to create a customized modulemap file with extra headers (common or umbrella) and linker flags inside our mission folder. First, we should 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 advocate checking the linked article if you wish to know extra about modulemaps. This manner we inform the compiler that we need to construct a module based mostly on the curl library, therefore we hyperlink curl. We additionally need to present our customized header file to make some extra stuff out there or extra handy. Individuals often name these header recordsdata shims, umbrella headers or bridging headers.

An umberlla header is the principle header file for a framework or library. A bridging header permits us to make use of two languages in the identical software. 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 inside it.

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

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

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

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

This code comes from the archived SoTS/CCurl repository, however when you test the shim file contained in the Kitura/CCurl bundle, 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 tips on how to write a low-level curl name utilizing the libcurl & Swift.

import Basis
import MyPoint
import libcurl

class Response {
    var knowledge = Knowledge()

    var physique: String { String(knowledge: knowledge, 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, measurement, n, reference in
    let size = measurement * n
    let knowledge = buffer!.assumingMemoryBound(to: UInt8.self)
    let p = reference?.assumingMemoryBound(to: Response.self).pointee
    p?.knowledge.append(knowledge, depend: size)
    return size
}

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

    fatalError("One thing went unsuitable 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 sorts 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 funtions 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 move the end result pointer and a write perform that may append the incoming knowledge to the storage when one thing arrives from the server. Lastly we carry out the request, test for errors and cleanup the deal with.

It isn’t so unhealthy, however nonetheless it appears nothing such as you’d anticipate from Swift. It is only a primary instance I hope it’s going to assist you to to know what is going on on beneath the hood and the way low stage C-like APIs can work in Swift. If you wish to follow it is best to strive to check out the Kanna library and parse the response utilizing a customized libxml2 wrapper (or you’ll be able to examine a SQLite3 wrapper). 🤓

The system library goal characteristic is a pleasant approach of wrapping C [system] modules with SPM. You possibly can learn extra about it on the official Swift boards. In case you are nonetheless utilizing the outdated system library bundle kind format, please migrate, because it’s deprecated and it will be utterly eliminated afterward.