Constructing static and dynamic Swift libraries utilizing the Swift compiler

What the heck is a library?

A library) is a set 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 concern, it’s important to print countable objects (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. Properly, this might work after all, however what occurs in the event you uncover a bug within the pluralization part? Now it’s important to repair the problem at two locations, since you’ve got simply duplicated your entire stuff. There have to be a greater approach… 🧠

Luckily pc programmers confronted the very same concern, so that they invented shared libraries. A shared library is a particular type of binary part that you need to use in your essential software. This manner you may outsource Swift code right into a separate file (or bunch of recordsdata), throw in some entry management to permit different apps to make use of public strategies and name features out of your library and right here we go, we simply shared our frequent code between our purposes.

Oh wait, there’s a bug within the lib, how can I repair it? Properly, that is the place issues get a bit sophisticated, however don’t fret an excessive amount of, I am going to attempt to clarify the way it works. So, final time, you understand, after we talked in regards to the Swift compiler and linker, I discussed, that they will resolve dependencies in your program. Whenever you use a library you may 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 alternatively implies that your library dependencies will likely be resolved at runtime. By the best way, it’s important to determine this upfront, since it’s important to construct both a static or a dynamic library. Huhh? Okay, let me do this once more… 🙃

The static library method is extra easy. You’ll be able to simply construct a static library utilizing the compiler (you may see learn how to make one in a while), then you may import this library inside your software supply (import MyLibrary). Now while you compile the primary app, it’s important to inform the compiler the situation of your static (binary) library, and the publicly accessible objects (headers or module map) which can be accessible to make use of. This manner when your app consists the symbols from the lib (lessons, strategies, and many others) will be copied to the primary executable file). Whenever you run the app, required objects will likely be there already contained in the binary file, so you may 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 it’s important to construct your library as a dynamic dependency utilizing the Swift compiler, this can produce a dynamic (binary) library file and a module map (header recordsdata). Whenever you make the ultimate model of your app, the system will put references of the dynamic library to your executable as an alternative of copying the contents of the dylib file. If you wish to run your software it’s important to make it possible for the referenced dynamic library is out there to make use of. The working system will attempt to load the generated dylib file so the appliance resolves the symbols primarily based on the reference pointers. 👈

Ought to I select dynamic or static linking?

Properly, it will depend on the surroundings. 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 a lot of the circumstances it’s best to stick to the automated worth, so the system can construct the correct 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 best way in case you are confused sufficient, I’ve an article for learners about Swift packages, modules, frameworks and the instruments that makes this complete dependency administration doable. You must positively have a look, it is a some form of a deep dive into FAT frameworks, however the first a part of the article is stuffed with helpful definitions and introductions to varied instructions.

Again to the unique query: static vs dynamic? Do you keep in mind the bug within the library that we’ve got to repair? If you happen to use a static library it’s important to rebuild all of the apps which can be relying on it (they have to be linked with the fastened library after all) with a view to make the problem disappear. 🐛

Since a dynamic library is loaded at runtime and the symbols aren’t embedded into the appliance binary, you may merely construct a brand new dylib file and substitute the previous one to repair the bug. This manner all of the apps which can be referencing to this dependency could have the repair at no cost. There isn’t a must recompile everyting, besides the defective code within the framework itself. 💪

It’s also price 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 typically you wish to encapsulate all the things right into a single binary, as an alternative of putting in plenty of different dylib recordsdata 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 are able to 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’ve got 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 understand, 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 primarily based on this single level.swift supply file. As I discussed this earlier than, we want a binary file and a module map file that incorporates 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’ve got 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 recordsdata:

• libpoint.a – The binary static library itself
• level.swiftdoc – Documentation for the module (binary format)
• level.swiftmodule – Data in regards to the module, “Swift header file”
• level.swiftsourceinfo – Supply info file

Transfer these recordsdata inside a lib folder, so it will be easier 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 towards our essential software? 🤔

To begin with, we’ve got to import our newly created module contained in the essential.swift file if we wish to use the objects (in our case the Level struct) from it. By the best way you may add a customized module identify to your library in the event you use the -module-name [name] argument with the earlier swiftc command.

import level

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

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

So, all of our library recordsdata are situated in a lib folder, and our default module identify is level (primarily based on our single enter file). We will use the swiftc command once more, to compile the primary 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 accessible in our supply file. The very very last thing that we’ve got to append to the tip of the command is the -l[name] flag, this specifies the library identify we want to hyperlink towards. Watch out, there isn’t a area in between the -l and the identify worth! ⚠️

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

# run the app
./essential
# Hey library! 4 20

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

Compiling a dynamic Swift library

In concept, we are able to use the identical code and construct a dynamic library from the level.swift file and compile our essential.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 an alternative of the libpoint.a, however all the opposite recordsdata look equivalent. Extension my fluctuate per working system:

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

So we’ve got our dylib file, however the true query is: can we construct the primary.swift file with it?

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

# run the app
./essential
# Hey library! 4 20

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

./essential

# dyld: Library not loaded: libpoint.dylib
#   Referenced from: /Customers/tib/./essential
#   Purpose: picture not discovered
# zsh: abort      ./essential

Whoops, looks like we’ve got 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 appears to be like up dynamic libraries at a couple of totally different locations.

• The listing you specified by 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”, it’s best to ship your dylib recordsdata subsequent to your executable binary, or alternatively you may set up them beneath the /usr/native/lib/ folder. Sadly, this lookup technique can result in all form of points, I actually do not wish to get into the small print this time, however it could actually result in compatibility and safety points. 🤫

The excellent news is that now in the event you change one thing within the dylib, and also you merely rebuild & substitute the file you then run the ./essential 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

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

After all dynamic libraries are nice in case you are the creator of a generally used framework, such the Swift customary library, Basis or UIKit. These modules are shipped as shared libraries, as a result of they’re large and nearly each single app imports them. Simply give it some thought, if we might hyperlink these three frameworks statically that’d add lots to the dimensions of our apps, plus it might be approach tougher 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 accessible as a part of the working system. 😅

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