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 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 subject, you must 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. Properly, this might work in fact, however what occurs if you happen to uncover a bug within the pluralization element? Now you must repair the problem at two locations, since you’ve got simply duplicated your entire stuff. There should be a greater method… 🧠

Luckily pc programmers confronted the very same subject, in order that they invented shared libraries. A shared library is a particular sort of binary element that you should utilize in your fundamental 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 capabilities 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 difficult, however don’t fret an excessive amount of, I will 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’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 then again signifies that your library dependencies will probably be resolved at runtime. By the best way, you must resolve this upfront, since you must construct both a static or a dynamic library. Huhh? Okay, let me do that 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 the right way to make one in a while), then you possibly can import this library inside your utility supply (import MyLibrary). Now while you compile the principle app, you must inform the compiler the situation of your static (binary) library, and the publicly accessible objects (headers or module map) which are out there to make use of. This fashion when your app consists the symbols from the lib (lessons, strategies, and so on) could be copied to the principle 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 primary 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 a number of the “undefined” symbols will probably be resolved at runtime. First you must 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). Whenever 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 utility you must guarantee that the referenced dynamic library is accessible to make use of. The working system will attempt to load the generated dylib file so the applying resolves the symbols based mostly on the reference pointers. 👈

Ought to I select dynamic or static linking?

Properly, it will depend on the atmosphere. 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 can too explicitly inform SPM to construct a static or dynamic library, however in a lot of the instances it is best to follow the automated worth, so the system can construct the fitting 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 if you’re confused sufficient, I’ve an article for newbies 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 kind of a deep dive into FAT frameworks, however the first a part of the article is filled 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 have now to repair? In case you use a static library you must rebuild all of the apps which are relying on it (they should be linked with the mounted library in fact) with a purpose 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 previous one to repair the bug. This fashion all of the apps which are referencing to this dependency may have the repair totally free. There is no such thing as a have to recompile everyting, besides the defective code within the framework itself. 💪

It’s also 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 every thing right into a single binary, as a substitute of putting in numerous 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 technique so as to have the ability to create a Level struct from our utility. 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’d like 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’d like 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 not 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 data file

Transfer these information inside a lib folder, so it’s going to 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 utility? 🤔

To start with, we have now 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 best way you possibly can add a customized module identify to your library if you happen to 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 identify is level (based mostly on our single enter file). We will 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 fashion the general public API (headers) of the module will probably be out there in our supply file. The very last item that we have now to append to the top of the command is the -l[name] flag, this specifies the library identify we wish to hyperlink in opposition to. Watch out, there is no such thing as a area in between the -l and the identify worth! ⚠️

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

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

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

Compiling a dynamic Swift library

In idea, 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 completely different. We have got 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 have now our dylib file, however the actual query is: can we construct the principle.swift file with it?

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

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

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

./fundamental

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

Whoops, looks like we have now an issue. Don’t fret, 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 seems to be up dynamic libraries at a couple of completely 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”, it is best to ship your dylib information subsequent to your executable binary, or alternatively you possibly can set up them below 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 main points this time, however it will probably result in compatibility and safety points. 🤫

The excellent news is that now if you happen to 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 probably be used. Simply attempt to put a print assertion into the init technique of the Level struct…

Abstract

Truthfully, I would somewhat go together with a static library in a lot 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 if you’re the writer 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 enormous and nearly 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 explanation why these packages are shipped as shared libz, plus Apple can offers 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 will inform you extra about this subsequent time. It is going to be a extra superior matter together with completely different languages. I will present you the right way to construct a library utilizing C and the right way to name it utilizing Swift, with out SPM. 🤓