Constructing a customized C library utilizing SPM
You should utilize the Swift Package deal 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 concerning the Swift Package deal Supervisor, it is best to learn my complete tutorial about how SPM works. 📦
The one factor that it is advisable to setup a library is a regular Package deal.swift manifest file with a barely altered listing construction to help 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"),
]
)
Every thing that you simply put into the header file might be publicly out there for different builders to make use of, the implementation particulars are going to be positioned straight beneath the Sources/[target]/ listing, however it’s important to 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 in the event you attempt to compile it by the swift construct command, it’s going to 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’ll be able to skip the “embody” path and easily write #embody "MyPoint.h". You can additionally put every kind of C household elements into this mission, this methodology works with C++, Goal-C and even Goal-C++ information.
You can additionally place header information subsequent to the implementation supply code, however in that case the system will not be capable to 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 every little thing for you mechanically.
Congratulations, you simply shipped your first C code with Swift Package deal Supervisor. 🥳
Interacting with C libraries utilizing Swift
We will create a model new Swift package deal to construct an executable software based mostly on the beforehand created C library. With a purpose to use an area package deal 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 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’ll be able to handle and publish your personal libraries utilizing a git repository someplace within the cloud. Subsequent we must 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("Good day, world!", p.x, p.y)
If each packages can be found regionally, be sure to place them subsequent to one another, then every little thing ought to work like a appeal. You may open the Pattern mission manifest file utilizing Xcode as nicely, the IDE can resolve package deal dependencies mechanically for you, however in the event you choose the command line, you should utilize the swift run command to compile & run the executable goal.
With this system you’ll be able to import the MyPoint module from every other Swift package deal and use the out there public elements from it. You simply have so as to add this module as a dependency, by the way in which you’ll be able to even name this module from one other C (C++, ObjC, Objc++) mission made with SPM. 😎
How you can use C system libraries from Swift?
There are literally thousands of out there instruments you can 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 device and library, that can be utilized for transferring knowledge from or to a server. In different phrases, you may make HTTP requests with it, simply kind curl "https://www.apple.com/" right into a terminal window.
These system elements are normally constructed round libraries. In our case curl comes with libcurl, the multiprotocol file switch library. Typically you may need to use these low degree elements (normally written in C) in your software, however how will we add them as a dependency? 🤔
The reply is straightforward, we will 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’ll be able to set the suppliers for the library (similar 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… 😅
It will permit us to create a customized modulemap file with extra 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 advocate checking the linked article if you wish to know extra about modulemaps. This fashion 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 normally name these header information 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 comprise 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’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 inside it.
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 in the event you verify the shim file contained in the Kitura/CCurl package deal, you may discover a just about related method with much more handy helpers.
The primary cause why we want these features is that variadic features cannot be imported by Swift (but), so we’ve got to wrap the curl_easy_setopt calls, so we’ll be capable to 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, dimension, n, reference in
let size = dimension * n
let knowledge = buffer!.assumingMemoryBound(to: UInt8.self)
let p = reference?.assumingMemoryBound(to: Response.self).pointee
p?.knowledge.append(knowledge, rely: 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, here is what occurs within the code snippet. First we’ve got to outline a response object that may maintain the information 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 go the outcome pointer and a write operate 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 dangerous, however nonetheless it appears nothing such as you’d count on from Swift. It is only a fundamental instance I hope it’s going to aid 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 observe 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 manner of wrapping C [system] modules with SPM. You may learn extra about it on the official Swift boards. If you’re nonetheless utilizing the previous system library package deal kind format, please migrate, because it’s deprecated and it will be utterly eliminated in a while.
