1 Introducing Zig

In this chapter, I want to introduce you to the world of Zig. Zig is a very young language that is being actively developed. As a consequence, its world is still very wild and to be explored. This book is my attempt to help you on your personal journey for understanding and exploring the exciting world of Zig.

I assume you have previous experience with some programming language in this book, not necessarily with a low-level one. So, if you have experience with Python, or Javascript, for example, it will be fine. But, if you do have experience with low-level languages, such as C, C++, or Rust, you will probably learn faster throughout this book.

1.1 What is Zig?

Zig is a modern, low-level, and general-purpose programming language. Some programmers think of Zig as a modern and better version of C.

In the author’s personal interpretation, Zig is tightly connected with “less is more”. Instead of trying to become a modern language by adding more and more features, many of the core improvements that Zig brings to the table are actually about removing annoying behaviours/features from C and C++. In other words, Zig tries to be better by simplifying the language, and by having more consistent and robust behaviour. As a result, analyzing, writing and debugging applications become much easier and simpler in Zig, than it is in C or C++.

This philosophy becomes clear with the following phrase from the official website of Zig:

“Focus on debugging your application rather than debugging your programming language knowledge”.

This phrase is specially true for C++ programmers. Because C++ is a gigantic language, with tons of features, and also, there are lots of different “flavors of C++”. These elements are what makes C++ so complex and hard to learn. Zig tries to go in the opposite direction. Zig is a very simple language, more closely related to other simple languages such as C and Go.

The phrase above is still important for C programmers too. Because, even C being a simple language, it is still hard sometimes to read and understand C code. For example, pre-processor macros in C are a frequent source of confusion. They really make it sometimes hard to debug C programs. Because macros are essentially a second language embedded in C that obscures your C code. With macros, you are no longer 100% sure about which pieces of the code are being sent to the compiler, i.e. they obscures the actual source code that you wrote.

You don’t have macros in Zig. In Zig, the code you write, is the actual code that get’s compiled by the compiler. You also don’t have a hidden control flow happening behind the scenes. And, you also don’t have functions or operators from the standard library that make hidden memory allocations behind your back.

By being a simpler language, Zig becomes much more clear and easier to read/write, but at the same time, it also achieves a much more robust state, with more consistent behaviour in edge situations. Once again, less is more.

1.2 Hello world in Zig

We begin our journey in Zig by creating a small “Hello World” program. To start a new Zig project in your computer, you simply call the init command from the zig compiler. Just create a new directory in your computer, then, init a new Zig project inside this directory, like this:

mkdir hello_world
cd hello_world
zig init

info: created build.zig
info: created build.zig.zon
info: created src/main.zig
info: created src/root.zig
info: see `zig build --help` for a menu of options

1.2.1 Understanding the project files

After you run the init command from the zig compiler, some new files are created inside of your current directory. First, a “source” (src) directory is created, containing two files, main.zig and root.zig. Each .zig file is a separate Zig module, which is simply a text file that contains some Zig code.

By convention, the main.zig module is where your main function lives. Thus, if you are building an executable program in Zig, you need to declare a main() function, which represents the entrypoint of your program, i.e. it is where the execution of your program begins.

However, if you are building a library (instead of an executable program), then, the normal procedure is to delete this main.zig file and start with the root.zig module. By convention, the root.zig module is the root source file of your library.

tree .

.
├── build.zig
├── build.zig.zon
└── src
    ├── main.zig
    └── root.zig

1 directory, 4 files

The ìnit command also creates two additional files in our working directory: build.zig and build.zig.zon. The first file (build.zig) represents a build script written in Zig. This script is executed when you call the build command from the zig compiler. In other words, this file contain Zig code that executes the necessary steps to build the entire project.

Low-level languages normally use a compiler to build your source code into binary executables or binary libraries. Nevertheless, this process of compiling your source code and building binary executables or binary libraries from it, became a real challenge in the programming world, once the projects became bigger and bigger. As a result, programmers created “build systems”, which are a second set of tools designed to make this process of compiling and building complex projects, easier.

Examples of build systems are CMake, GNU Make, GNU Autoconf and Ninja, which are used to build complex C and C++ projects. With these systems, you can write scripts, which are called “build scripts”. They simply are scripts that describes the necessary steps to compile/build your project.

However, these are separate tools, that do not belong to C/C++ compilers, like gcc or clang. As a result, in C/C++ projects, you have not only to install and manage your C/C++ compilers, but you also have to install and manage these build systems separately.

In Zig, we don’t need to use a separate set of tools to build our projects, because a build system is embedded inside the language itself. Therefore, Zig contains a native build system in it, and we can use this build system to write small scripts in Zig, which describes the necessary steps to build/compile our Zig project¹. So, everything you need to build a complex Zig project is the zig compiler, and nothing more.

The second generated file (build.zig.zon) is the Zig package manager configuration file, where you can list and manage the dependencies of your project. Yes, Zig has a package manager (like pip in Python, cargo in Rust, or npm in Javascript) called Zon, and this build.zig.zon file is similar to the package.json file in Javascript projects, or, the Pipfile file in Python projects, or the Cargo.toml file in Rust projects.

1.2.2 The file `root.zig`

Let’s take a look into the root.zig file. You might have noticed that every line of code with an expression ends with a semicolon (;). This follows the syntax of a C-family programming language².

Also, notice the @import() call at the first line. We use this built-in function to import functionality from other Zig modules into our current module. This @import() function works similarly to the #include pre-processor in C or C++, or, to the import statement in Python or Javascript code. In this example, we are importing the std module, which gives you access to the Zig Standard Library.

In this root.zig file, we can also see how assignments (i.e. creating new objects) are made in Zig. You can create a new object in Zig by using the following syntax (const|var) name = value;. In the example below, we are creating two constant objects (std and testing). At Section 1.4 we talk more about objects in general.

const std = @import("std");
const testing = std.testing;

export fn add(a: i32, b: i32) i32 {
    return a + b;
}

Functions in Zig are declared using the fn keyword. In this root.zig module, we are declaring a function called add(), which has two arguments named a and b. The function returns an integer of the type i32 as result.

Zig is a strongly-typed language. There are some specific situations where you can (if you want to) omit the type of an object in your code, if this type can be inferred by the zig compiler (we talk more about that at Section 2.4). But there are other situations where you do need to be explicit. For example, you do have to explicitly specify the type of each function argument, and also, the return type of every function that you create in Zig.

We specify the type of an object or a function argument in Zig by using a colon character (:) followed by the type after the name of this object/function argument. With the expressions a: i32 and b: i32, we know that both a and b arguments have type i32, which is a signed 32 bit integer. In this part, the syntax in Zig is identical to the syntax in Rust, which also specifies types by using the colon character.

Lastly, we have the return type of the function at the end of the line, before we open the curly braces to start writing the function’s body. In the example above, this type is also a signed 32 bit integer (i32) value.

Notice that we also have an export keyword before the function declaration. This keyword is similar to the extern keyword in C. It exposes the function to make it available in the library API. Therefore, if you are writing a library for other people to use, you have to expose the functions you write in the public API of this library by using this export keyword. If we removed the export keyword from the add() function declaration, then, this function would be no longer exposed in the library object built by the zig compiler.

1.2.3 The `main.zig` file

Now that we have learned a lot about Zig’s syntax from the root.zig file, let’s take a look at the main.zig file. A lot of the elements we saw in root.zig are also present in main.zig. But there are some other elements that we haven’t seen yet, so let’s dive in.

First, look at the return type of the main() function in this file. We can see a small change. The return type of the function (void) is accompanied by an exclamation mark (!). This exclamation mark tells us that this main() function might return an error.

In this example, the main() function can either return void or return an error. This is an interesting feature of Zig. If you write a function and something inside of the body of this function might return an error then you are forced to:

either add the exclamation mark to the return type of the function and make it clear that this function might return an error
explicitly handle this error inside the function

In most programming languages, we normally handle (or deal with) an error through a try catch pattern. Zig does have both try and catch keywords. But they work a little differently than what you’re probably used to in other languages.

If we look at the main() function below, you can see that we do have a try keyword on the 5th line. But we do not have a catch keyword in this code. In Zig, we use the try keyword to execute an expression that might return an error, which, in this example, is the stdout.print() expression.

In essence, the try keyword executes the expression stdout.print(). If this expression returns a valid value, then, the try keyword does nothing. It only passes the value forward. But if the expression does return an error, then, the try keyword just unwrap the error value, and return this error from the function and also prints the current stack trace to stderr.

This might sound weird to you if you come from a high-level language. Because in high-level languages, such as Python, if an error occurs somewhere, this error is automatically returned and the execution of your program will automatically stop even if you don’t want to stop the execution. You are obligated to face the error.

const std = @import("std");

pub fn main() !void {
    const stdout = std.io.getStdOut().writer();
    try stdout.print("Hello, {s}!\n", .{"world"});
}

Another thing that you might have noticed in this code example, is that the main() function is marked with the pub keyword. It marks the main() function as a public function from this module.

Every function in your Zig module is by default private to this Zig module and can only be called from within the module. Unless, you explicitly mark this function as a public function with the pub keyword. This means that the pub keyword in Zig does essentially the opposite of what the static keyword do in C/C++.

By making a function “public” you allow other Zig modules to access and call it. A calling Zig module imports the module with the @import() built-in. That makes all public functions from the imported module visible.

1.2.4 Compiling your source code

You can compile your Zig modules into a binary executable by running the build-exe command from the zig compiler. You simply list all the Zig modules that you want to build after the build-exe command, separated by spaces. In the example below, we are compiling the module main.zig.

zig build-exe src/main.zig

Since we are building an executable, the zig compiler will look for a main() function declared in any of the files that you list after the build-exe command. If the compiler does not find a main() function declared somewhere, a compilation error will be raised, warning about this mistake.

The zig compiler also offers a build-lib and build-obj commands, which work the exact same way as the build-exe command. The only difference is that, they compile your Zig modules into a portale C ABI library, or, into object files, respectively.

In the case of the build-exe command, a binary executable file is created by the zig compiler in the root directory of your project. If we take a look now at the contents of our current directory, with a simple ls command, we can see the binary file called main that was created by the compiler.

ls

build.zig  build.zig.zon  main  src

If I execute this binary executable, I get the “Hello World” message in the terminal , as we expected.

./main

Hello, world!

1.2.5 Compile and execute at the same time

On the previous section, I presented the zig build-exe command, which compiles Zig modules into an executable file. However, this means that, in order to execute the executable file, we have to run two different commands. First, the zig build-exe command, and then, we call the executable file created by the compiler.

But what if we wanted to perform these two steps, all at once, in a single command? We can do that by using the zig run command.

zig run src/main.zig

Hello, world!

1.2.6 Important note for Windows users

First of all, this is a Windows-specific thing, and, therefore, does not apply to other operational systems, such as Linux and MacOs. In summary, if you have a piece of Zig code that includes some global variables whose initialization rely on runtime resources, then, you might have some troubles while trying to compile this Zig code on Windows.

An example of that is accessing the stdout (i.e. the standard output of your system), which is usually done in Zig by using the expression std.io.getStdOut(). Thus, if you use this expression to instantiate a global variable in a Zig module, then, the compilation of your Zig code will very likely fail on Windows, with a “unable to evaluate comptime expression” error message.

This failure in the compilation process happens because all global variables in Zig are initialized at compile-time. However, operations like accessing the stdout (or opening a file) on Windows depends on resources that are available only at runtime (you will learn more about compile-time versus runtime at Section 3.1.1).

For example, if you try to compile this code example on Windows, you will likely get the error message exposed below:

const std = @import("std");
// ERROR! Compile-time error that emerge from
// this next line, on the `stdout` object
const stdout = std.io.getStdOut().writer();

pub fn main() !void {
    _ = try stdout.write("Hello\n");
}

t.zig:2107:28: error: unable to evaluate comptime expression
    break :blk asm {
               ^~~

To avoid this problem on Windows, we need to force the zig compiler to instantiate this stdout object only at runtime, instead of instantiating it at compile-time. We can achieve that by simply moving the expression to a function body.

This solves the problem because all expressions that are inside a function body in Zig are evaluated only at runtime, unless you use the comptime keyword explicitly to change this behaviour. You will learn more about this comptime keyword at Section 12.1.

const std = @import("std");
pub fn main() !void {
    // SUCCESS: Stdout initialized at runtime.
    const stdout = std.io.getStdOut().writer();
    _ = try stdout.write("Hello\n");
}

Hello

You can read more details about this Windows-specific limitation at a couple of GitHub issues opened at the official Zig repository. More specifically, the issues 17186 ³ and 19864 ⁴.

1.2.7 Compiling the entire project

Just as I described at Section 1.2.1, as our project grows in size and complexity, we usually prefer to organize the compilation and build process of the project into a build script, using some sort of “build system”.

In other words, as our project grows in size and complexity, the build-exe, build-lib and build-obj commands become harder to use directly. Because then, we start to list multiple and multiple modules at the same time. We also start to add built-in compilation flags to customize the build process for our needs, etc. It becomes a lot of work to write the necessary commands by hand.

In C/C++ projects, programmers normally opt to use CMake, Ninja, Makefile or configure scripts to organize this process. However, in Zig, we have a native build system in the language itself. So, we can write build scripts in Zig to compile and build Zig projects. Then, all we need to do, is to call the zig build command to build our project.

So, when you execute the zig build command, the zig compiler will search for a Zig module named build.zig inside your current directory, which should be your build script, containing the necessary code to compile and build your project. If the compiler does find this build.zig file in your directory, then, the compiler will essentially execute a zig run command over this build.zig file, to compile and execute this build script, which in turn, will compile and build your entire project.

zig build

After you execute this “build project” command, a zig-out directory is created in the root of your project directory, where you can find the binary executables and libraries created from your Zig modules accordingly to the build commands that you specified at build.zig. We will talk more about the build system in Zig latter in this book.

In the example below, I’m executing the binary executable named hello_world that was generated by the compiler after the zig build command.

./zig-out/bin/hello_world

Hello, world!

1.3 How to learn Zig?

What are the best strategies to learn Zig? First of all, of course this book will help you a lot on your journey through Zig. But you will also need some extra resources if you want to be really good at Zig.

As a first tip, you can join a community with Zig programmers to get some help , when you need it:

Reddit forum: https://www.reddit.com/r/Zig/;
Ziggit community: https://ziggit.dev/;
Discord, Slack, Telegram, and others: https://github.com/ziglang/zig/wiki/Community;

Now, one of the best ways to learn Zig is to simply read Zig code. Try to read Zig code often, and things will become more clear. A C/C++ programmer would also probably give you this same tip. Because this strategy really works!

Now, where can you find Zig code to read? I personally think that, the best way of reading Zig code is to read the source code of the Zig Standard Library. The Zig Standard Library is available at the lib/std folder ⁵ on the official GitHub repository of Zig. Access this folder, and start exploring the Zig modules.

Also, a great alternative is to read code from other large Zig codebases, such as:

the Javascript runtime Bun ⁶.
the game engine Mach ⁷.
a LLama 2 LLM model implementation in Zig ⁸.
the financial transactions database tigerbeetle⁹.
the command-line arguments parser zig-clap¹⁰.
the UI framework capy¹¹.
the Language Protocol implementation for Zig, zls¹².
the event-loop library libxev¹³.

All these assets are available on GitHub, and this is great, because we can use the GitHub search bar in our advantage, to find Zig code that fits our description. For example, you can always include lang:Zig in the GitHub search bar when you are searching for a particular pattern. This will limit the search to only Zig modules.

Also, a great alternative is to consult online resources and documentations. Here is a quick list of resources that I personally use from time to time to learn more about the language each day:

Zig Language Reference: https://ziglang.org/documentation/master/;
Zig Standard Library Reference: https://ziglang.org/documentation/master/std/;
Zig Guide: https://zig.guide/;
Karl Seguin Blog: https://www.openmymind.net/;
Zig News: https://zig.news/;
Read the code written by one of the Zig core team members: https://github.com/kubkon;
Some livecoding sessions are transmitted in the Zig Showtime Youtube Channel: https://www.youtube.com/@ZigSHOWTIME/videos;

Another great strategy to learn Zig, or honestly, to learn any language you want, is to practice it by solving exercises. For example, there is a famous repository in the Zig community called Ziglings ¹⁴ , which contains more than 100 small exercises that you can solve. It is a repository of tiny programs written in Zig that are currently broken, and your responsibility is to fix these programs, and make them work again.

A famous tech YouTuber known as The Primeagen also posted some videos (at YouTube) where he solves these exercises from Ziglings. The first video is named “Trying Zig Part 1”¹⁵.

Another great alternative, is to solve the Advent of Code exercises ¹⁶. There are people that already took the time to learn and solve the exercises, and they posted their solutions on GitHub as well, so, in case you need some resource to compare while solving the exercises, you can look at these two repositories:

1.4 Creating new objects in Zig (i.e. identifiers)

Let’s talk more about objects in Zig. Readers that have past experience with other programming languages might know this concept through a different name, such as: “variable” or “identifier”. In this book, I choose to use the term “object” to refer to this concept.

To create a new object (or a new “identifier”) in Zig, we use the keywords const or var. These keywords specify if the object that you are creating is mutable or not. If you use const, then the object you are creating is a constant (or immutable) object, which means that once you declare this object, you can no longer change the value stored inside this object.

On the other side, if you use var, then, you are creating a variable (or mutable) object. You can change the value of this object as many times you want. Using the keyword var in Zig is similar to using the keywords let mut in Rust.

1.4.1 Constant objects vs variable objects

In the code example below, we are creating a new constant object called age. This object stores a number representing the age of someone. However, this code example does not compiles successfully. Because on the next line of code, we are trying to change the value of the object age to 25.

The zig compiler detects that we are trying to change the value of an object/identifier that is constant, and because of that, the compiler will raise a compilation error, warning us about the mistake.

const age = 24;
// The line below is not valid!
age = 25;

t.zig:10:5: error: cannot assign to constant
    age = 25;
      ~~^~~

In contrast, if you use var, then, the object created is a variable object. With var you can declare this object in your source code, and then, change the value of this object how many times you want over future points in your source code.

So, using the same code example exposed above, if I change the declaration of the age object to use the var keyword, then, the program gets compiled successfully. Because now, the zig compiler detects that we are changing the value of an object that allows this behaviour, because it is an “variable object”.

var age: u8 = 24;
age = 25;

1.4.2 Declaring without an initial value

By default, when you declare a new object in Zig, you must give it an initial value. In other words, this means that we have to declare, and, at the same time, initialize every object we create in our source code.

On the other hand, you can, in fact, declare a new object in your source code, and not give it an explicit value. But we need to use a special keyword for that, which is the undefined keyword.

Is important to emphasize that, you should avoid using undefined as much as possible. Because when you use this keyword, you leave your object uninitialized, and, as a consequence, if for some reason, your code use this object while it is uninitialized, then, you will definitely have undefined behaviour and major bugs in your program.

In the example below, I’m declaring the age object again. But this time, I do not give it an initial value. The variable is only initialized at the second line of code, where I store the number 25 in this object.

var age: u8 = undefined;
age = 25;

Having these points in mind, just remember that you should avoid as much as possible to use undefined in your code. Always declare and initialize your objects. Because this gives you much more safety in your program. But in case you really need to declare an object without initializing it… the undefined keyword is the way to do it in Zig.

1.4.3 There is no such thing as unused objects

Every object (being constant or variable) that you declare in Zig must be used in some way. You can give this object to a function call, as a function argument, or, you can use it in another expression to calculate the value of another object, or, you can call a method that belongs to this particular object.

It doesn’t matter in which way you use it. As long as you use it. If you try to break this rule, i.e. if your try to declare a object, but not use it, the zig compiler will not compile your Zig source code, and it will issue a error message warning that you have unused objects in your code.

Let’s demonstrate this with an example. In the source code below, we declare a constant object called age. If you try to compile a simple Zig program with this line of code below, the compiler will return an error as demonstrated below:

const age = 15;

t.zig:4:11: error: unused local constant
    const age = 15;
          ^~~

Everytime you declare a new object in Zig, you have two choices:

you either use the value of this object;
or you explicitly discard the value of the object;

To explicitly discard the value of any object (constant or variable), all you need to do is to assign this object to an special character in Zig, which is the underscore (_). When you assign an object to a underscore, like in the example below, the zig compiler will automatically discard the value of this particular object.

You can see in the example below that, this time, the compiler did not complain about any “unused constant”, and successfully compiled our source code.

// It compiles!
const age = 15;
_ = age;

Now, remember, everytime you assign a particular object to the underscore, this object is essentially destroyed. It is discarded by the compiler. This means that you can no longer use this object further in your code. It doesn’t exist anymore.

So if you try to use the constant age in the example below, after we discarded it, you will get a loud error message from the compiler (talking about a “pointless discard”) warning you about this mistake.

// It does not compile.
const age = 15;
_ = age;
// Using a discarded value!
std.debug.print("{d}\n", .{age + 2});

t.zig:7:5: error: pointless discard
    of local constant

This same rule applies to variable objects. Every variable object must also be used in some way. And if you assign a variable object to the underscore, this object also get’s discarded, and you can no longer use this object.

1.4.4 You must mutate every variable objects

Every variable object that you create in your source code must be mutated at some point. In other words, if you declare an object as a variable object, with the keyword var, and you do not change the value of this object at some point in the future, the zig compiler will detect this, and it will raise an error warning you about this mistake.

The concept behind this is that every object you create in Zig should be preferably a constant object, unless you really need an object whose value will change during the execution of your program.

So, if I try to declare a variable object such as where_i_live below, and I do not change the value of this object in some way, the zig compiler raises an error message with the phrase “variable is never mutated”.

var where_i_live = "Belo Horizonte";
_ = where_i_live;

t.zig:7:5: error: local variable is never mutated
t.zig:7:5: note: consider using 'const'

1.5 Primitive Data Types

Zig have many different primitive data types available for you to use. You can see the full list of available data types at the official Language Reference page ¹⁷.

But here is a quick list:

Unsigned integers: u8, 8-bit integer; u16, 16-bit integer; u32, 32-bit integer; u64, 64-bit integer; u128, 128-bit integer.
Signed integers: i8, 8-bit integer; i16, 16-bit integer; i32, 32-bit integer; i64, 64-bit integer; i128, 128-bit integer.
Float number: f16, 16-bit floating point; f32, 32-bit floating point; f64, 64-bit floating point; f128, 128-bit floating point;
Boolean: bool, represents true or false values.
C ABI compatible types: c_long, c_char, c_short, c_ushort, c_int, c_uint, and many others.
Pointer sized integers: isize and usize.

1.6 Arrays

You create arrays in Zig by using a syntax that resembles the C syntax. First, you specify the size of the array (i.e. the number of elements that will be stored in the array) you want to create inside a pair of brackets.

Then, you specify the data type of the elements that will be stored inside this array. All elements present in an array in Zig must have the same data type. For example, you cannot mix elements of type f32 with elements of type i32 in the same array.

After that, you simply list the values that you want to store in this array inside a pair of curly braces. In the example below, I am creating two constant objects that contain different arrays. The first object contains an array of 4 integer values, while the second object, an array of 3 floating point values.

Now, you should notice that in the object ls, I am not explicitly specifying the size of the array inside of the brackets. Instead of using a literal value (like the value 4 that I used in the ns object), I am using the special character underscore (_). This syntax tells the zig compiler to fill this field with the number of elements listed inside of the curly braces. So, this syntax [_] is for lazy (or smart) programmers who leave the job of counting how many elements there are in the curly braces for the compiler.

const ns = [4]u8{48, 24, 12, 6};
const ls = [_]f64{432.1, 87.2, 900.05};
_ = ns; _ = ls;

Is worth noting that these are static arrays, meaning that they cannot grow in size. Once you declare your array, you cannot change the size of it. This is very common in low level languages. Because low level languages normally wants to give you (the programmer) full control over memory, and the way in which arrays are expanded is tightly related to memory management.

1.6.1 Selecting elements of the array

One very common activity is to select specific portions of an array you have in your source code. In Zig, you can select a specific element from your array, by simply providing the index of this particular element inside brackets after the object name. In the example below, I am selecting the third element from the ns array. Notice that Zig is a “zero-index” based language, like C, C++, Rust, Python, and many other languages.

const ns = [4]u8{48, 24, 12, 6};
try stdout.print("{d}\n", .{ ns[2] });

In contrast, you can also select specific slices (or sections) of your array, by using a range selector. Some programmers also call these selectors of “slice selectors”, and they also exist in Rust, and have the exact same syntax as in Zig. Anyway, a range selector is a special expression in Zig that defines a range of indexes, and it have the syntax start..end.

In the example below, at the second line of code, the sl object stores a slice (or a portion) of the ns array. More precisely, the elements at index 1 and 2 in the ns array.

const ns = [4]u8{48, 24, 12, 6};
const sl = ns[1..3];
_ = sl;

When you use the start..end syntax, the “end tail” of the range selector is non-inclusive, meaning that, the index at the end is not included in the range that is selected from the array. Therefore, the syntax start..end actually means start..end - 1 in practice.

You can for example, create a slice that goes from the first to the last elements of the array, by using ar[0..ar.len] syntax In other words, it is a slice that access all elements in the array.

const ar = [4]u8{48, 24, 12, 6};
const sl = ar[0..ar.len];
_ = sl;

You can also use the syntax start.. in your range selector. Which tells the zig compiler to select the portion of the array that begins at the start index until the last element of the array. In the example below, we are selecting the range from index 1 until the end of the array.

const ns = [4]u8{48, 24, 12, 6};
const sl = ns[1..];
_ = sl;

1.6.2 More on slices

As we discussed before, in Zig, you can select specific portions of an existing array. This is called slicing in Zig (Sobeston 2024), because when you select a portion of an array, you are creating a slice object from that array.

A slice object is essentially a pointer object accompanied by a length number. The pointer object points to the first element in the slice, and the length number tells the zig compiler how many elements there are in this slice.

Slices can be thought of as a pair of [*]T (the pointer to the data) and a usize (the element count) (Sobeston 2024).

Through the pointer contained inside the slice you can access the elements (or values) that are inside this range (or portion) that you selected from the original array. But the length number (which you can access through the len property of your slice object) is the really big improvement (over C arrays for example) that Zig brings to the table here.

Because with this length number the zig compiler can easily check if you are trying to access an index that is out of the bounds of this particular slice, or, if you are causing any buffer overflow problems. In the example below, we access the len property of the slice sl, which tells us that this slice have 2 elements in it.

const ns = [4]u8{48, 24, 12, 6};
const sl = ns[1..3];
try stdout.print("{d}\n", .{sl.len});

1.6.3 Array operators

There are two array operators available in Zig that are very useful. The array concatenation operator (++), and the array multiplication operator (**). As the name suggests, these are array operators.

One important detail about these two operators is that they work only when both operands have a size (or “length”) that is compile-time known. We are going to talk more about the differences between “compile-time known” and “runtime known” at Section 3.1.1. But for now, keep this information in mind, that you cannot use these operators in every situation.

In summary, the ++ operator creates a new array that is the concatenation, of both arrays provided as operands. So, the expression a ++ b produces a new array which contains all the elements from arrays a and b.

const a = [_]u8{1,2,3};
const b = [_]u8{4,5};
const c = a ++ b;
try stdout.print("{any}\n", .{c});

{ 1, 2, 3, 4, 5 }

This ++ operator is particularly useful to concatenate strings together. Strings in Zig are described in depth at Section 1.8. In summary, a string object in Zig is essentially an arrays of bytes. So, you can use this array concatenation operator to effectively concatenate strings together.

In contrast, the ** operator is used to replicate an array multiple times. In other words, the expression a ** 3 creates a new array which contains the elements of the array a repeated 3 times.

const a = [_]u8{1,2,3};
const c = a ** 2;
try stdout.print("{any}\n", .{c});

{ 1, 2, 3, 1, 2, 3 }

1.6.4 Runtime versus compile-time known length in slices

We are going to talk a lot about the differences between compile-time known and runtime known across this book, especially at Section 3.1.1. But the basic idea is that a thing is compile-time known, when we know everything (the value, the attributes and the characteristics) about this thing at compile-time. In contrast, a runtime known thing is when the exact value of a thing is calculated only at runtime. Therefore, we don’t know the value of this thing at compile-time, only at runtime.

We have learned at Section 1.6.1 that slices are created by using a range selector, which represents a range of indexes. When this “range of indexes” (i.e. the start and the end of this range) is known at compile-time, the slice object that get’s created is actually, under the hood, just a single-item pointer to an array.

You don’t need to precisely understand what that means now. We are going to talk a lot about pointers at Chapter 6. For now, just understand that, when the range of indexes is known at compile-time, the slice that get’s created is just a pointer to an array, accompanied by a length value that tells the size of the slice.

If you have a slice object like this, i.e. a slice that has a compile-time known range, you can use common pointer operations over this slice object. For example, you can dereference the pointer of this slice, by using the .* method, like you would do on a normal pointer object.

const arr1 = [10]u64 {
    1, 2, 3, 4, 5,
    6, 7, 8, 9, 10
};
// This slice have a compile-time known range.
// Because we know both the start and end of the range.
const slice = arr1[1..4];

On the other hand, if the range of indexes is not known at compile time, then, the slice object that get’s created is not a pointer anymore, and, thus, it does not support pointer operations. For example, maybe the start index is known at compile time, but the end index is not. In such case, the range of the slice becomes runtime known only.

In the example below, the slice object have a runtime known range, because the end index of the range is not known at compile time. In other words, the size of the array at buffer is not known at compile time. When we execute this program, the size of the array might be 10, or, it might be 12 depending on where we execute it. Therefore, we don’t know at compile time if the slice object have a range of size 10, or, a range of size 12.

const std = @import("std");
const builtin = @import("builtin");

pub fn main() !void {
    var gpa = std.heap.GeneralPurposeAllocator(.{}){};
    const allocator = gpa.allocator();
    var n: usize = 0;
    if (builtin.target.os.tag == .windows) {
        n = 10;
    } else {
        n = 12;
    }
    const buffer = try allocator.alloc(u64, n);
    const slice = buffer[0..];
    _ = slice;
}

1.7 Blocks and scopes

Blocks are created in Zig by a pair of curly braces. A block is just a group of expressions (or statements) contained inside of a pair of curly braces. All of these expressions that are contained inside of this pair of curly braces belongs to the same scope.

In other words, a block just delimits a scope in your code. The objects that you define inside the same block belongs to the same scope, and, therefore, are accessible from within this scope. At the same time, these objects are not accessible outside of this scope. So, you could also say that blocks are used to limit the scope of the objects that you create in your source code. In less technical terms, blocks are used to specify where in your source code you can access whatever object you have in your source code.

So, a block is just a group of expressions contained inside a pair of curly braces. And every block have its own scope separated from the others. The body of a function is a classic example of a block. If statements, for and while loops (and any other structure in the language that uses the pair of curly braces) are also examples of blocks.

This means that, every if statement, or for loop, etc., that you create in your source code have its own separate scope. That is why you can’t access the objects that you defined inside of your for loop (or if statement) in an outer scope, i.e. a scope outside of the for loop. Because you are trying to access an object that belongs to a scope that is different than your current scope.

You can create blocks within blocks, with multiple levels of nesting. You can also (if you want to) give a label to a particular block, with the colon character (:). Just write label: before you open the pair of curly braces that delimits your block. When you label a block in Zig, you can use the break keyword to return a value from this block, like as if it was a function’s body. You just write the break keyword, followed by the block label in the format :label, and the expression that defines the value that you want to return.

Like in the example below, where we are returning the value from the y object from the block add_one, and saving the result inside the x object.

var y: i32 = 123;
const x = add_one: {
    y += 1;
    break :add_one y;
};
if (x == 124 and y == 124) {
    try stdout.print("Hey!", .{});
}

Hey!

1.8 How strings work in Zig?

The first project that we are going to build and discuss in this book is a base64 encoder/decoder (Chapter 4). But in order for us to build such a thing, we need to get a better understanding on how strings work in Zig. So let’s discuss this specific aspect of Zig.

In summary, there are two types of string values that you care about in Zig, which are:

String literal values.
String objects.

A string literal value is just a pointer to a null-terminated array of bytes (i.e. similar to a C string). But in Zig, a string literal value also embeds the length of the string into the data type of the value itself. Therefore, a string literal value have a data type in the format *const [n:0]u8. The n in the data type indicates the size of the string.

On the other hand, a string object in Zig is basically a slice to an arbitrary sequence of bytes, or, in other words, a slice of u8 values (slices were presented at Section 1.6). Thus, a string object have a data type of []u8 or []const u8, depending if the string object is marked as constant with const, or as variable with var.

Because a string object is essentially a slice, it means that a string object always contains two things: a pointer to an array of bytes (i.e. u8 values) that represents the string value; and also, a length value, which specifies the size of the slice, or, how many elements there is in the slice. Is worth to emphasize that the array of bytes in a string object is not null-terminated, like in a string literal value.

// This is a string literal value:
"A literal value";
// This is a string object:
const object: []const u8 = "A string object";

Zig always assumes that the sequence of bytes in your string is UTF-8 encoded. This might not be true for every sequence of bytes you’re working with, but is not really Zig’s job to fix the encoding of your strings (you can use iconv¹⁸ for that). Today, most of the text in our modern world, especially on the web, should be UTF-8 encoded. So if your string literal is not UTF-8 encoded, then, you will likely have problems in Zig.

Let’s take for example the word “Hello”. In UTF-8, this sequence of characters (H, e, l, l, o) is represented by the sequence of decimal numbers 72, 101, 108, 108, 111. In hexadecimal, this sequence is 0x48, 0x65, 0x6C, 0x6C, 0x6F. So if I take this sequence of hexadecimal values, and ask Zig to print this sequence of bytes as a sequence of characters (i.e. a string), then, the text “Hello” will be printed into the terminal:

const std = @import("std");
const stdout = std.io.getStdOut().writer();

pub fn main() !void {
    const bytes = [_]u8{0x48, 0x65, 0x6C, 0x6C, 0x6F};
    try stdout.print("{s}\n", .{bytes});
}

Hello

If you want to see the actual bytes that represents a string in Zig, you can use a for loop to iterate through each byte in the string, and ask Zig to print each byte as an hexadecimal value to the terminal. You do that by using a print() statement with the X formatting specifier, like you would normally do with the printf() function ¹⁹ in C.

const std = @import("std");
const stdout = std.io.getStdOut().writer();
pub fn main() !void {
    const string_object = "This is an example";
    try stdout.print("Bytes that represents the string object: ", .{});
    for (string_object) |byte| {
        try stdout.print("{X} ", .{byte});
    }
    try stdout.print("\n", .{});
}

Bytes that represents the string object: 54 68 69 
   73 20 69 73 20 61 6E 20 65 78 61 6D 70 6C 65

1.8.1 Strings in C

At first glance, a string literal value in Zig looks very similar to how C treats strings as well. In more details, string values in C are treated internally as an array of arbitrary bytes, and this array is also null-terminated.

But one key difference between a Zig string literal and a C string, is that Zig also stores the length of the string inside the object itself. In the case of a string literal value, this length is stored in the data type of the value (i.e. the n variable in [n:0]u8). While, in a string object, the length is stored in the len attribute of the slice that represents the string object. This small detail makes your code safer, because is much easier for the Zig compiler to check if you are trying to access an element that is “out of bounds”, i.e. if your trying to access memory that does not belong to you.

To achieve this same kind of safety in C, you have to do a lot of work that kind of seems pointless. So getting this kind of safety is not automatic and much harder to do in C. For example, if you want to track the length of your string throughout your program in C, then, you first need to loop through the array of bytes that represents this string, and find the null element ('\0') position to discover where exactly the array ends, or, in other words, to find how much elements the array of bytes contain.

To do that, you would need something like this in C. In this example, the C string stored in the object array is 25 bytes long:

#include <stdio.h>
int main() {
    char* array = "An example of string in C";
    int index = 0;
    while (1) {
        if (array[index] == '\0') {
            break;
        }
        index++;
    }
    printf("Number of elements in the array: %d\n", index);
}

Number of elements in the array: 25

You don’t have this kind of work in Zig. Because the length of the string is always present and accessible. In a string object for example, you can easily access the length of the string through the len attribute. As an example, the string_object object below is 43 bytes long:

const std = @import("std");
const stdout = std.io.getStdOut().writer();
pub fn main() !void {
    const string_object = "This is an example of string literal in Zig";
    try stdout.print("{d}\n", .{string_object.len});
}

1.8.2 A better look at the object type

Now, we can inspect better the type of objects that Zig create. To check the type of any object in Zig, you can use the @TypeOf() function. If we look at the type of the simple_array object below, you will find that this object is an array of 4 elements. Each element is a signed integer of 32 bits which corresponds to the data type i32 in Zig. That is what an object of type [4]i32 is.

But if we look closely at the type of the string literal value exposed below, you will find that this object is a constant pointer (hence the *const annotation) to an array of 16 elements (or 16 bytes). Each element is a single byte (more precisely, an unsigned 8 bit integer - u8), that is why we have the [16:0]u8 portion of the type below. In other words, the string literal value exposed below is 16 bytes long.

Now, if we create an pointer to the simple_array object, then, we get a constant pointer to an array of 4 elements (*const [4]i32), which is very similar to the type of the string literal value. This demonstrates that a string literal value in Zig is already a pointer to a null-terminated array of bytes.

Furthermore, if we take a look at the type of the string_obj object, you will see that it is a slice object (hence the [] portion of the type) to a sequence of constant u8 values (hence the const u8 portion of the type).

const std = @import("std");
pub fn main() !void {
    const simple_array = [_]i32{1, 2, 3, 4};
    const string_obj: []const u8 = "A string object";
    std.debug.print(
        "Type 1: {}\n", .{@TypeOf(simple_array)}
    );
    std.debug.print(
        "Type 2: {}\n", .{@TypeOf("A string literal")}
    );
    std.debug.print(
        "Type 3: {}\n", .{@TypeOf(&simple_array)}
    );
    std.debug.print(
        "Type 4: {}\n", .{@TypeOf(string_obj)}
    );
}

Type 1: [4]i32Type 2: *const [16:0]u8Type 3: *cons
  st [4]i32Type 4: []const u8

Type 1: [4]i32
Type 2: *const [16:0]u8
Type 3: *const [4]i32
Type 4: []const u8

1.8.3 Byte vs unicode points

Is important to point out that each byte in the array is not necessarily a single character. This fact arises from the difference between a single byte and a single unicode point.

The encoding UTF-8 works by assigning a number (which is called a unicode point) to each character in the string. For example, the character “H” is stored in UTF-8 as the decimal number 72. This means that the number 72 is the unicode point for the character “H”. Each possible character that can appear in a UTF-8 encoded string have its own unicode point.

For example, the Latin Capital Letter A With Stroke (Ⱥ) is represented by the number (or the unicode point) 570. However, this decimal number (570) is higher than the maximum number stored inside a single byte, which is 255. In other words, the maximum decimal number that can be represented with a single byte is 255. That is why, the unicode point 570 is actually stored inside the computer’s memory as the bytes C8 BA.

const std = @import("std");
const stdout = std.io.getStdOut().writer();
pub fn main() !void {
    const string_object = "Ⱥ";
    _ = try stdout.write(
        "Bytes that represents the string object: "
    );
    for (string_object) |char| {
        try stdout.print("{X} ", .{char});
    }
}

Bytes that represents the string object: C8 BA

This means that to store the character Ⱥ in an UTF-8 encoded string, we need to use two bytes together to represent the number 570. That is why the relationship between bytes and unicode points is not always 1 to 1. Each unicode point is a single character in the string, but not always a single byte corresponds to a single unicode point.

All of this means that if you loop trough the elements of a string in Zig, you will be looping through the bytes that represents that string, and not through the characters of that string. In the Ⱥ example above, the for loop needed two iterations (instead of a single iteration) to print the two bytes that represents this Ⱥ letter.

Now, all english letters (or ASCII letters if you prefer) can be represented by a single byte in UTF-8. As a consequence, if your UTF-8 string contains only english letters (or ASCII letters), then, you are lucky. Because the number of bytes will be equal to the number of characters in that string. In other words, in this specific situation, the relationship between bytes and unicode points is 1 to 1.

But on the other side, if your string contains other types of letters… for example, you might be working with text data that contains, chinese, japanese or latin letters, then, the number of bytes necessary to represent your UTF-8 string will likely be much higher than the number of characters in that string.

If you need to iterate through the characters of a string, instead of its bytes, then, you can use the std.unicode.Utf8View struct to create an iterator that iterates through the unicode points of your string.

In the example below, we loop through the japanese characters “アメリカ”. Each of the four characters in this string is represented by three bytes. But the for loop iterates four times, one iteration for each character/unicode point in this string:

const std = @import("std");
const stdout = std.io.getStdOut().writer();

pub fn main() !void {
    var utf8 = try std.unicode.Utf8View.init("アメリカ");
    var iterator = utf8.iterator();
    while (iterator.nextCodepointSlice()) |codepoint| {
        try stdout.print(
            "got codepoint {}\n",
            .{std.fmt.fmtSliceHexUpper(codepoint)},
        );
    }
}

got codepoint E382A2
got codepoint E383A1
got codepoint E383AA
got codepoint E382AB

1.8.4 Some useful functions for strings

In this section, I just want to quickly describe some functions from the Zig Standard Library that are very useful to use when working with strings. Most notably:

std.mem.eql(): to compare if two strings are equal.
std.mem.splitScalar(): to split a string into an array of substrings given a delimiter value.
std.mem.splitSequence(): to split a string into an array of substrings given a substring delimiter.
std.mem.startsWith(): to check if string starts with substring.
std.mem.endsWith(): to check if string ends with substring.
std.mem.trim(): to remove specific values from both start and end of the string.
std.mem.concat(): to concatenate strings together.
std.mem.count(): to count the occurrences of substring in the string.
std.mem.replace(): to replace the occurrences of substring in the string.

Notice that all of these functions come from the mem module of the Zig Standard Library. This module contains multiple functions and methods that are useful to work with memory and sequences of bytes in general.

The eql() function is used to check if two arrays of data are equal or not. Since strings are just arbitrary arrays of bytes, we can use this function to compare two strings together. This function returns a boolean value indicating if the two strings are equal or not. The first argument of this function is the data type of the elements of the arrays that are being compared.

const name: []const u8 = "Pedro";
try stdout.print(
    "{any}\n", .{std.mem.eql(u8, name, "Pedro")}
);

true

The splitScalar() and splitSequence() functions are useful to split a string into multiple fragments, like the split() method from Python strings. The difference between these two methods is that the splitScalar() uses a single character as the separator to split the string, while splitSequence() uses a sequence of characters (a.k.a. a substring) as the separator. There is a practical example of these functions later in the book.

The startsWith() and endsWith() functions are pretty straightforward. They return a boolean value indicating if the string (or, more precisely, if the array of data) begins (startsWith) or ends (endsWith) with the sequence provided.

const name: []const u8 = "Pedro";
try stdout.print(
    "{any}\n", .{std.mem.startsWith(u8, name, "Pe")}
);

true

The concat() function, as the name suggests, concatenate two or more strings together. Because the process of concatenating the strings involves allocating enough space to accomodate all the strings together, this concat() function receives an allocator object as input.

const str1 = "Hello";
const str2 = " you!";
const str3 = try std.mem.concat(
    allocator, u8, &[_][]const u8{ str1, str2 }
);
try stdout.print("{s}\n", .{str3});

As you can imagine, the replace() function is used to replace substrings in a string by another substring. This function works very similarly to the replace() method from Python strings. Therefore, you provide a substring to search, and every time that the replace() function finds this substring within the input string, it replaces this substring with the “replacement substring” that you provided as input.

In the example below, we are taking the input string “Hello”, and replacing all occurrences of the substring “el” inside this input string with “34”, and saving the results inside the buffer object. As result, the replace() function returns an usize value that indicates how many replacements were performed.

const str1 = "Hello";
var buffer: [5]u8 = undefined;
const nrep = std.mem.replace(
    u8, str1, "el", "34", buffer[0..]
);
try stdout.print("New string: {s}\n", .{buffer});
try stdout.print("N of replacements: {d}\n", .{nrep});

New string: H34lo
N of replacements: 1

1.9 Safety in Zig

A general trend in modern low-level programming languages is safety. As our modern world becomes more interconnected with technology and computers, the data produced by all of this technology becomes one of the most important (and also, one of the most dangerous) assets that we have.

This is probably the main reason why modern low-level programming languages have been giving great attention to safety, especially memory safety, because memory corruption is still the main target for hackers to exploit. The reality is that we don’t have an easy solution for this problem. For now, we only have techniques and strategies that mitigates these problems.

As Richard Feldman explains on his most recent GOTO conference talk ²⁰ , we haven’t figured it out yet a way to achieve true safety in technology. In other words, we haven’t found a way to build software that won’t be exploited with 100% certainty. We can greatly reduce the risks of our software being exploited, by ensuring memory safety for example. But this is not enough to achieve “true safety” territory.

Because even if you write your program in a “safe language”, hackers can still exploit failures in the operational system where your program is running (e.g. maybe the system where your code is running has a “backdoor exploit” that can still affect your code in unexpected ways), or also, they can exploit the features from the architecture of your computer. A recently found exploit that involves memory invalidation through a feature of “memory tags” present in ARM chips is an example of that (Kim et al. 2024).

The question is: what have Zig and other languages been doing to mitigate this problem? If we take Rust as an example, Rust is, for the most part²¹, a memory safe language by enforcing specific rules to the developer. In other words, the key feature of Rust, the borrow checker, forces you to follow a specific logic when you are writing your Rust code, and the Rust compiler will always complain everytime you try to go out of this pattern.

In contrast, the Zig language is not a memory safe language by default. There are some memory safety features that you get for free in Zig, especially in arrays and pointer objects. But there are other tools offered by the language, that are not used by default. In other words, the zig compiler does not obligates you to use such tools.

The tools listed below are related to memory safety. That is, they help you to achieve memory safety in your Zig code:

defer allows you to keep free operations physically close to allocations. This helps you to avoid memory leaks, “use after free”, and also “double-free” problems. Furthermore, it also keeps free operations logically tied to the end of the current scope, which greatly reduces the mental overhead about object lifetime.
errdefer helps you to guarantee that your program frees the allocated memory, even if a runtime error occurs.
pointers and objects are non-nullable by default. This helps you to avoid memory problems that might arise from de-referencing null pointers.
Zig offers some native types of allocators (called “testing allocators”) that can detect memory leaks and double-frees. These types of allocators are widely used on unit tests, so they transform your unit tests into a weapon that you can use to detect memory problems in your code.
arrays and slices in Zig have their lengths embedded in the object itself, which makes the zig compiler very effective on detecting “index out-of-range” type of errors, and avoiding buffer overflows.

Despite these features that Zig offers that are related to memory safety issues, the language also has some rules that help you to achieve another type of safety, which is more related to program logic safety. These rules are:

pointers and objects are non-nullable by default. Which eliminates an edge case that might break the logic of your program.
switch statements must exaust all possible options.
the zig compiler forces you to handle every possible error in your program.

1.10 Other parts of Zig

We already learned a lot about Zig’s syntax, and also, some pretty technical details about it. Just as a quick recap:

We talked about how functions are written in Zig at Section 1.2.2 and Section 1.2.3.
How to create new objects/identifiers at Section 1.2.2 and especially at Section 1.4.
How strings work in Zig at Section 1.8.
How to use arrays and slices at Section 1.6.
How to import functionality from other Zig modules at Section 1.2.2.

But, for now, this amount of knowledge is enough for us to continue with this book. Later, over the next chapters we will still talk more about other parts of Zig’s syntax that are also equally important. Such as:

How Object-Oriented programming can be done in Zig through struct declarations at Section 2.3.
Basic control flow syntax at Section 2.1.
Enums at Section 7.6;
Pointers and Optionals at Chapter 6;
Error handling with try and catch at Chapter 10;
Unit tests at Chapter 8;
Vectors at Chapter 17;
Build System at Chapter 9;

https://ziglang.org/learn/overview/#zig-build-system.↩︎
https://en.wikipedia.org/wiki/List_of_C-family_programming_languages ↩︎
https://github.com/ziglang/zig/issues/17186 ↩︎
https://github.com/ziglang/zig/issues/19864 ↩︎
https://github.com/ziglang/zig/tree/master/lib/std ↩︎
https://github.com/oven-sh/bun.↩︎
https://github.com/hexops/mach ↩︎
https://github.com/cgbur/llama2.zig/tree/main ↩︎
https://github.com/tigerbeetle/tigerbeetle ↩︎
https://github.com/Hejsil/zig-clap ↩︎
https://github.com/capy-ui/capy ↩︎
https://github.com/zigtools/zls ↩︎
https://github.com/mitchellh/libxev ↩︎
https://ziglings.org.↩︎
https://www.youtube.com/watch?v=OPuztQfM3Fg&t=2524s&ab_channel=TheVimeagen.↩︎
https://adventofcode.com/↩︎
https://ziglang.org/documentation/master/#Primitive-Types.↩︎
https://www.gnu.org/software/libiconv/↩︎
https://cplusplus.com/reference/cstdio/printf/↩︎
https://www.youtube.com/watch?v=jIZpKpLCOiU&ab_channel=GOTOConferences ↩︎
Actually, a lot of existing Rust code is still memory unsafe, because they communicate with external libraries through FFI (foreign function interface), which disables the borrow-checker features through the unsafe keyword.↩︎