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 functions can use.

Think about that you’re making a easy utility to pluralize a string. It really works nice, you end the app and also you begin working in your subsequent one. In your subsequent utility, you face the very same difficulty, 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 utility into the second. Effectively, this might work in fact, however what occurs for those who uncover a bug within the pluralization part? Now it’s important to repair the problem at two locations, since you have simply duplicated your complete stuff. There have to be a greater method… 🧠

Luckily pc programmers confronted the very same difficulty, so that they invented shared libraries. A shared library is a particular form of binary part that you need to use in your primary utility. This fashion you possibly can 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 features out of your library and right here we go, we simply shared our frequent code between our functions.

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

• static linking
• dynamic linking

Static linking signifies that the supply code contained in the library will probably be actually copy-pasted into your utility binary. Dynamic linking however signifies that your library dependencies will probably be resolved at runtime. By the best way, it’s important to resolve 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 strategy is extra easy. You’ll be able to simply construct a static library utilizing the compiler (you will see easy methods to make one afterward), then you possibly can import this library inside your utility 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 are accessible to make use of. This fashion when your app consists the symbols from the lib (lessons, strategies, and many others) may be copied to the primary executable file). Whenever you run the app, required objects will probably be there already contained in the binary file, so you possibly can 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 utility binary while you use a dylib file, however among the “undefined” symbols will probably 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 information). 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 utility it’s important to ensure that the referenced dynamic library is offered to make use of. The working system will attempt to load the generated dylib file so the applying resolves the symbols primarily based on the reference pointers. 👈

Ought to I select dynamic or static linking?

Effectively, it is determined by the atmosphere. 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 many of the instances it’s best to persist with the automated worth, so the system can construct the best 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 in case you are confused sufficient, I’ve an article for newcomers about Swift packages, modules, frameworks and the instruments that makes this complete dependency administration attainable. It’s best to undoubtedly have a look, it is a some type 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 keep in mind the bug within the library that we’ve to repair? Should you use a static library it’s important to rebuild all of the apps which are relying on it (they have to be linked with the mounted library in fact) with a view to make the problem disappear. 🐛

Since a dynamic library is loaded at runtime and the symbols are usually not embedded into the applying binary, you possibly can 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 may have the repair free of charge. There is no such thing as a must recompile everyting, besides the defective code within the framework itself. 💪

It is usually 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 wish to encapsulate every part right into a single binary, as an alternative of putting in plenty 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’ve to explicitly mark it as public, plus we want a public init technique so as to have the ability to create a Level struct from our utility. You realize, in Swift, entry management permits us, programmers, to cover particular elements 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 at the least), so we’ve 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 in regards to the module, “Swift header file”
• level.swiftsourceinfo – Supply info file

Transfer these information inside a lib folder, so it will 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 utility? 🤔

To begin with, we’ve to import our newly created module contained in the primary.swift file if we wish to use the objects (in our case the Level struct) from it. By the best way you possibly can add a customized module title to your library for those who use the -module-name [name] argument with the earlier swiftc command.

import level

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

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

So, all of our library information are situated in a lib folder, and our default module title 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 should set a search path for imports, the -I property will assist us, this fashion the general public API (headers) of the module will probably be accessible in our supply file. The very last item that we’ve to append to the tip of the command is the -l[name] flag, this specifies the library title we wish to hyperlink towards. Watch out, there is no such thing as a house in between the -l and the title worth! ⚠️

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

# run the app
./primary
# Hiya library! 4 20

Voilá, we have simply separated a file from the primary utility through the use of 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 completely different. We have got a libpoint.dylib binary as an alternative of the libpoint.a, however all the opposite information look similar. Extension my differ per working system:

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

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

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

# run the app
./primary
# Hiya library! 4 20

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

./primary

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

Whoops, looks as if we’ve an issue. Don’t be concerned, that is the anticipated output, since we renamed the dynamic library and the applying cannot discover it. When the loader tries to get the referenced symbols from the file it appears up dynamic libraries at just a few completely different locations.

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

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

Abstract

Truthfully, I might relatively go together with a static library in many of the instances as a result of utilizing a static library will assure that your utility has each obligatory 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 would hyperlink these three frameworks statically that’d add so much to the scale of our apps, plus it would be method more durable to repair system-wide bugs. That is the explanation why these packages are shipped as shared libz, plus Apple can provides us a promise that these parts will at all times be accessible as a part of the working system. 😅

Anyhow, there are some instruments that you need to use to change library loader paths, I will let you know extra about this subsequent time. It is going to be a extra superior matter together with completely different languages. I’ll present you easy methods to construct a library utilizing C and easy methods to name it utilizing Swift, with out SPM. 🤓