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 functions 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 situation, you need 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 in fact, however what occurs in the event you uncover a bug within the pluralization element? Now you need to repair the problem at two locations, since you’ve got simply duplicated your entire stuff. There have to be a greater approach… 🧠

Thankfully pc programmers confronted the very same situation, in order that they invented shared libraries. A shared library is a particular form of binary element that you should utilize in your primary 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 functions.

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 be concerned an excessive amount of, I am going to attempt to clarify the way it works. So, final time, you recognize, 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 might be actually copy-pasted into your software binary. Dynamic linking alternatively implies that your library dependencies might be resolved at runtime. By the best way, you need to determine this upfront, since you need to construct both a static or a dynamic library. Huhh? Okay, let me do this once more… 🙃

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

The primary 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 among the “undefined” symbols might 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 an alternative of copying the contents of the dylib file. If you wish to run your software you need to guarantee that 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 based mostly on the reference pointers. 👈

Ought to I select dynamic or static linking?

Properly, it relies on the setting. For instance the Swift Bundle Supervisor prefers to make use of static linking, however Xcode will attempt to construct SPM packages as dynamic dependencies. You can too explicitly inform SPM to construct a static or dynamic library, however in a lot of the instances you must keep on with the automated worth, so the system can construct the correct module dependency for you.

import PackageDescription

let package deal = Bundle(
    title: "MyLibrary",
    merchandise: [
        
        .library(name: "MyLibrary", targets: ["MyLibrary"]),
        
    ],
    targets: [
        .target(name: "MyLibrary", dependencies: []),
    ]
)

By the best way if you’re confused sufficient, I’ve an article for learners about Swift packages, modules, frameworks and the instruments that makes this entire dependency administration potential. You need to undoubtedly 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 numerous instructions.

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

Since a dynamic library is loaded at runtime and the symbols will not be embedded into the appliance binary, you’ll be able to merely construct a brand new dylib file and substitute the previous one to repair the bug. This fashion all of the apps which can be referencing to this dependency may have the repair totally free. There is no such thing as a must recompile everyting, besides the defective code within the framework itself. 💪

It’s also value to say that the ultimate app measurement 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 need to encapsulate all the things right into a single binary, as an alternative 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 accomplish that, we have now to explicitly mark it as public, plus we’d like a public init methodology so as to have the ability to create a Level struct from our software. You already know, 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’d like a binary file and a module map file that incorporates the publicly accessible interface for the lib. You should use the -emit-library flat to inform the Swift compiler that we’d like a binary library file plus utilizing the -emit-module parameter will produce a Swift module data 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 have now 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 – Data concerning the module, “Swift header file”
• level.swiftsourceinfo – Supply info file

Transfer these information inside a lib folder, so it’s going to be less difficult 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 primary software? 🤔

To begin with, we have now to import our newly created module contained in the primary.swift file if we need to use the objects (in our case the Level struct) from it. By the best way you’ll be able to add a customized module title 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 information are positioned in a lib folder, and our default module title is level (based mostly on our single enter file). We are able to 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 should set a search path for imports, the -I property will assist us, this manner the general public API (headers) of the module might be out there in our supply file. The very very last thing that we have now to append to the top of the command is the -l[name] flag, this specifies the library title we wish to hyperlink towards. Watch out, there isn’t a house in between the -l and the title worth! ⚠️

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


./primary

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

Compiling a dynamic Swift library

In principle, we will use the identical code and construct a dynamic library from the level.swift file and compile our primary.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 got a libpoint.dylib binary as an alternative of the libpoint.a, however all the opposite information look equivalent. Extension my range per working system:

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

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

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


./primary

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

./primary

Whoops, looks like we have now 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 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

Because the /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 form of points, I actually do not need to get into the small print this time, however it will possibly 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 then you definitely run the ./primary once more (with out recompiling), the altered dynamic library might be used. Simply attempt to put a print assertion into the init methodology of the Level struct…

Abstract

Truthfully, I would relatively go along with a static library in a lot of the instances as a result of utilizing a static library will assure that your software has each obligatory dependency embedded into the binary file.

After all dynamic libraries are nice if you’re the writer 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 nearly each single app imports them. Simply give it some thought, if we might hyperlink these three frameworks statically that’d add so much to the dimensions of our apps, plus it might be approach more durable to repair system-wide bugs. That is the rationale why these packages are shipped as shared libz, plus Apple can provides us a promise that these parts will at all times be out there as a part of the working system. 😅

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