Constructing static and dynamic Swift libraries utilizing the Swift compiler

What the heck is a library?

A library is a group of Swift parts that different purposes can use.

Think about that you’re making a easy software to pluralize a string. It really works nice, you end the app and also you begin working in your subsequent one. In your subsequent software, you face the very same subject, you need to print countable gadgets (e.g 2 bananas). What would you do? 🤔

The very first thing that may cross your thoughts is to repeat all of the supply code from the primary software into the second. Nicely, this might work in fact, however what occurs should you uncover a bug within the pluralization element? Now you need to repair the difficulty at two locations, since you have simply duplicated the complete stuff. There have to be a greater method… 🧠

Fortuitously laptop programmers confronted the very same subject, in order that they invented shared libraries. A shared library is a particular form of binary element that you should utilize in your fundamental software. This fashion you’ll be able to outsource Swift code right into a separate file (or bunch of information), throw in some entry management to permit different apps to make use of public strategies and name capabilities out of your library and right here we go, we simply shared our widespread code between our purposes.

Oh wait, there’s a bug within the lib, how can I repair it? Nicely, that is the place issues get a bit difficult, however don’t fret an excessive amount of, I will attempt to clarify the way it works. So, final time, you already know, after we talked concerning the Swift compiler and linker, I discussed, that they will resolve dependencies in your program. While you use a library you’ll be able to select between two approaches.

• static linking
• dynamic linking

Static linking implies that the supply code contained in the library will likely be actually copy-pasted into your software binary. Dynamic linking however implies that your library dependencies will likely be resolved at runtime. By the way in which, you need to resolve this upfront, since you need to construct both a static or a dynamic library. Huhh? Okay, let me do that once more… 🙃

The static library strategy is extra easy. You may simply construct a static library utilizing the compiler (you may see how you can make one in a while), then you’ll be able to import this library inside your software supply (import MyLibrary). Now while you compile the principle app, you need to inform the compiler the situation of your static (binary) library, and the publicly accessible objects (headers or module map) which are obtainable to make use of. This fashion when your app consists the symbols from the lib (lessons, strategies, and so forth) may be copied to the principle executable file). While you run the app, required objects will likely be there already contained in the binary file, so you’ll be able to run it as it’s.

The principle distinction between a static and a dynamic library is that you do not copy each required image to the executable software binary while you use a dylib file, however a number of the “undefined” symbols will likely be resolved at runtime. First you need to construct your library as a dynamic dependency utilizing the Swift compiler, this may produce a dynamic (binary) library file and a module map (header information). While you make the ultimate model of your app, the system will put references of the dynamic library to your executable as a substitute of copying the contents of the dylib file. If you wish to run your software you need to make it possible for the referenced dynamic library is accessible to make use of. The working system will attempt to load the generated dylib file so the appliance resolves the symbols based mostly on the reference pointers. 👈

Ought to I select dynamic or static linking?

Nicely, it depends upon the setting. For instance the Swift Package deal Supervisor prefers to make use of static linking, however Xcode will attempt to construct SPM packages as dynamic dependencies. You may also explicitly inform SPM to construct a static or dynamic library, however in many of the circumstances you must keep on with the automated worth, so the system can construct the appropriate module dependency for you.

import PackageDescription

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

By the way in which if you’re confused sufficient, I’ve an article for inexperienced persons about Swift packages, modules, frameworks and the instruments that makes this entire dependency administration attainable. It’s best to undoubtedly have a look, it is a some kind of a deep dive into FAT frameworks, however the first a part of the article is filled with helpful definitions and introductions to numerous instructions.

Again to the unique query: static vs dynamic? Do you bear in mind the bug within the library that we now have to repair? Should you use a static library you need to rebuild all of the apps which are relying on it (they have to be linked with the fastened library in fact) as a way to make the difficulty disappear. 🐛

Since a dynamic library is loaded at runtime and the symbols usually are not embedded into the appliance binary, you’ll be able to merely construct a brand new dylib file and substitute the outdated one to repair the bug. This fashion all of the apps which are referencing to this dependency could have the repair without cost. There is no such thing as a must recompile everyting, besides the defective code within the framework itself. 💪

Additionally it is value to say that the ultimate app dimension is smaller while you use a dylib.

Okay, however why ought to I ever use static linking if dylibz are so cool? The reality is that generally you need to encapsulate the whole lot right into a single binary, as a substitute of putting in a lot of different dylib information into the system. Additionally what occurs if one thing deletes a dylib that your app would require to work flawlessly? That’d suck for certain, particularly if it’s a mission-critical script on a server… 😳

Hopefully, I over-explained issues, so we will begin constructing our very first static library.

Compiling a static Swift library

Do you continue to have that little Level struct from the earlier tutorial? Let’s construct a static library from that file, however earlier than we achieve this, we now have to explicitly mark it as public, plus we want a public init methodology so as to have the ability to create a Level struct from our software. You recognize, in Swift, entry management permits us, programmers, to cover particular components of a library from different builders.

public struct Level {
    public let x: Int
    public let y: Int

    public init(x: Int, y: Int) {
        self.x = x
        self.y = y
    }
}

Now we’re able to construct our static library based mostly on this single level.swift supply file. As I discussed this earlier than, we want a binary file and a module map file that comprises the publicly accessible interface for the lib. You need to use the -emit-library flat to inform the Swift compiler that we want a binary library file plus utilizing the -emit-module parameter will produce a Swift module information file with all of the API and docs wanted for different modules. By default the compiler would emit a dylib (on macOS no less than), so we now have to make use of the -static flat to explicitly generate a static dependency. 🔨

swiftc level.swift -emit-module -emit-library -static

The command above ought to produce 4 new information:

• libpoint.a – The binary static library itself
• level.swiftdoc – Documentation for the module (binary format)
• level.swiftmodule – Information concerning the module, “Swift header file”
• level.swiftsourceinfo – Supply data file

Transfer these information inside a lib folder, so it will be simpler to work with them. That is actually it, we have simply created a working static library, however how can we use it to hyperlink them in opposition to our fundamental software? 🤔

To begin with, we now have to import our newly created module contained in the fundamental.swift file if we need to use the objects (in our case the Level struct) from it. By the way in which you’ll be able to add a customized module identify to your library should you use the -module-name [name] argument with the earlier swiftc command.

import level

let p = Level(x: 4, y: 20)

print("Howdy library!", p.x, p.y)

So, all of our library information are positioned in a lib folder, and our default module identify is level (based mostly on our single enter file). We are able to use the swiftc command once more, to compile the principle file, this time we use the -L flag so as to add a library search path, so the compiler can find our binary libpoint.a file. We additionally must set a search path for imports, the -I property will assist us, this manner the general public API (headers) of the module will likely be obtainable in our supply file. The very final thing that we now have to append to the top of the command is the -l[name] flag, this specifies the library identify we want to hyperlink in opposition to. Watch out, there isn’t a house in between the -l and the identify worth! ⚠️

swiftc fundamental.swift -L ./lib/ -I ./lib/ -lpoint


./fundamental

Voilá, we have simply separated a file from the principle software by utilizing a static dependency. 👏

Compiling a dynamic Swift library

In concept, we will use the identical code and construct a dynamic library from the level.swift file and compile our fundamental.swift file utilizing that shared framework. We simply drop the -static flag first.

swiftc level.swift -emit-module -emit-library

This time the output is barely totally different. We have a libpoint.dylib binary as a substitute of the libpoint.a, however all the opposite information look similar. Extension my range per working system:

• macOS – static: .a, dynamic: .dylib
• Linux – static: .so, dynamic: .dylib
• Home windows – static: .lib, dynamic: .dll

So we now have our dylib file, however the true query is: can we construct the principle.swift file with it?

swiftc fundamental.swift -L ./lib/ -I ./lib/ -lpoint


./fundamental

Now rename the libpoint.dylib file into libpoint.foo and run the principle app once more.

./fundamental

Whoops, looks like we now have an issue. Don’t fret, that is the anticipated output, since we renamed the dynamic library and the appliance cannot discover it. When the loader tries to get the referenced symbols from the file it seems to be up dynamic libraries at a number of totally different locations.

• The listing you specified by way of the -L flag (./lib/).
• The listing the place your executable file is (./)
• The /usr/lib/ or the /usr/native/lib/ directories

For the reason that /usr/lib/ listing is protected by the well-known SIP “guard”, you must ship your dylib information subsequent to your executable binary, or alternatively you’ll be able to set up them beneath the /usr/native/lib/ folder. Sadly, this lookup technique can result in all kind of points, I actually do not need to get into the small print this time, however it could possibly result in compatibility and safety points. 🤫

The excellent news is that now should you change one thing within the dylib, and also you merely rebuild & substitute the file you then run the ./fundamental once more (with out recompiling), the altered dynamic library will likely be used. Simply attempt to put a print assertion into the init methodology of the Level struct…

Abstract

Actually, I might fairly go along with a static library in many of the circumstances as a result of utilizing a static library will assure that your software has each vital dependency embedded into the binary file.

After all dynamic libraries are nice if you’re the creator of a generally used framework, such the Swift commonplace library, Basis or UIKit. These modules are shipped as shared libraries, as a result of they’re enormous and virtually each single app imports them. Simply give it some thought, if we might hyperlink these three frameworks statically that’d add quite a bit to the dimensions of our apps, plus it might be method more durable to repair system-wide bugs. That is the rationale why these packages are shipped as shared libz, plus Apple can offers us a promise that these parts will all the time be obtainable as a part of the working system. 😅

Anyhow, there are some instruments that you should utilize to change library loader paths, I will let you know extra about this subsequent time. It should be a extra superior subject together with totally different languages. I will present you how you can construct a library utilizing C and how you can name it utilizing Swift, with out SPM. 🤓