The best way to use C libraries in Swift?

Constructing a customized C library utilizing SPM

You need to use the Swift Bundle Supervisor to create C household based mostly supply information (C, C++, Goal-C and Goal-C++) and ship them as standalone elements. If you do not know a lot in regards to the Swift Bundle Supervisor, you need to learn my complete tutorial about how SPM works. 📦

The one factor that it’s worthwhile to setup a library is a typical Bundle.swift manifest file with a barely altered listing construction to assist header information. Let’s make a MyPoint library.

import PackageDescription

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

Every thing that you simply put into the header file will probably be publicly accessible for different builders to make use of, the implementation particulars are going to be situated immediately below the Sources/[target]/ listing, however it’s a must to create an extra embody folder to your headers. Let’s make a MyPoint.h file below 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 in the event you attempt to compile it by means of the swift construct command, it’s going to complain that the mission is lacking some supply information. We will simply repair this by creating an empty MyPoint.c file below the Sources/MyPoint listing.

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

You could possibly additionally place header information subsequent to the implementation supply code, however in that case the system will not be capable of auto-locate your public (umbrella) header information, so that you additionally should create a modulemap file and supply the proper location of your headers explicitly. When you use the construction with the embody listing SPM will generate all the things for you robotically.

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

Interacting with C libraries utilizing Swift

We’ll create a model new Swift package deal to construct an executable software based mostly on the beforehand created C library. In an effort to use a neighborhood package deal you may merely specify it as with the trail argument below the dependencies in your Bundle.swift manifest file.

import PackageDescription

let package deal = Bundle(
    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 a neighborhood dependency, however after all you may handle and publish your personal libraries utilizing a git repository someplace within the cloud. Subsequent we should always create our Sources/Pattern/most important.swift file, import the library and write some code.

import MyPoint

let p = MyPoint(x: 4, y: 20)
print("Hi there, 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 attraction. You may open the Pattern mission manifest file utilizing Xcode as nicely, the IDE can resolve package deal dependencies robotically for you, however in the event you favor the command line, you should use the swift run command to compile & run the executable goal.

With this method you may import the MyPoint module from another 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 may even name this module from one other C (C++, ObjC, Objc++) mission made with SPM. 😎

The best way to use C system libraries from Swift?

There are millions of accessible instruments that you could set up in your working system (Linux, macOS) with a package deal supervisor (apt, brew). For instance there’s the well-known curl command line instrument 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 may wish to use these low degree elements (often written in C) in your software, however how will 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 = Bundle(
    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 Bundle.swift file you may set the suppliers for the library (equivalent to brew for macOS or aptitude for a lot of Linux distributions). Sadly you continue to should 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 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 fashion we inform the compiler that we wish to construct a module based mostly 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. Folks 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 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 comparable, however they aren’t the identical idea. 🙄

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

#embody <stdbool.h>
#embody <curl/curl.h>

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 in the event you verify the shim file contained in the Kitura/CCurl package deal, you will discover a just about comparable strategy with much more handy helpers.

The principle cause why we want these features is that variadic features cannot be imported by Swift (but), so we’ve to wrap the curl_easy_setopt calls, so we’ll be capable of use it from Swift.

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

import Basis
import MyPoint
import libcurl

class Response {
    var knowledge = Information()

    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 to be like 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’ve to outline a response object that may maintain the info coming from the server as a response. Subsequent we name the system features 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 outcome 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, verify for errors and cleanup the deal with.

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

The system library goal characteristic 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 kind format, please migrate, because it’s deprecated and it will be fully eliminated in a while.