FFI (Foreign Function Interface) for native code in Deno - Mini Project: Build & Apply

Using FFI in Deno to Call Native C Functions

📖 Scenario: You want to use a simple native C function from your Deno script. This helps when you need fast or system-level code that JavaScript alone can't do easily.

🎯 Goal: Build a Deno script that calls a native C function using FFI (Foreign Function Interface). You will load the native library, define the function signature, and call the function.

📋 What You'll Learn

Create a Deno ForeignLibrary object to load the native library

Define the native function signature with correct parameter and result types

Call the native function from Deno and store the result

Print the result using Deno's console.log

💡 Why This Matters

🌍 Real World

Using FFI lets you run fast native code or access system features not available in JavaScript, useful for performance-critical apps or hardware access.

💼 Career

Many jobs require integrating JavaScript with native code for performance or legacy reasons. Knowing FFI in Deno is valuable for backend and systems programming.

1

Load the native library with Deno's FFI

Create a constant called lib that loads the native library ./libsimple.so using Deno.dlopen with an empty symbols object {}.

Deno

const lib = Deno.dlopen("./libsimple.so", {})
// Your code here

Hint

Use Deno.dlopen with the path and an empty object for symbols first.

2

Define the native function signature

Update the lib constant to load ./libsimple.so with a symbol called add. Define add as a function with parameters i32 and i32 and result i32.

Deno

const lib = Deno.dlopen("./libsimple.so", {
  // Define the 'add' function symbol here
})

Hint

Inside the second argument object, add a key add with parameters array and result type.

3

Call the native function and store the result

Create a constant called sum and assign it the result of calling lib.symbols.add(10, 20).

Deno

const lib = Deno.dlopen("./libsimple.so", {
  add: { parameters: ["i32", "i32"], result: "i32" }
})

// Call the native 'add' function with 10 and 20
// Your code here

Hint

Use lib.symbols.add(10, 20) to call the native function.

4

Print the result and close the library

Use console.log to print the value of sum. Then call lib.close() to release the native library resources.

Deno

const lib = Deno.dlopen("./libsimple.so", {
  add: { parameters: ["i32", "i32"], result: "i32" }
})

const sum = lib.symbols.add(10, 20)

// Print the sum and close the library
// Your code here

Hint

Print sum with console.log and then call lib.close().

Practice

1. What is the main purpose of using Deno.dlopen in Deno's FFI?

easy

A. To load a native shared library and access its functions

B. To compile TypeScript code into JavaScript

C. To start a web server in Deno

D. To read files from the filesystem

Solution

1. Step 1: Understand the role of Deno.dlopen

   Deno.dlopen is used to load native shared libraries (like .dll, .so, .dylib) so Deno can call native functions.

2. Step 2: Compare with other options

   Options B, C, and D relate to other Deno features but not FFI or native code loading.

3. Final Answer:

   To load a native shared library and access its functions -> Option A

4. Quick Check:

   Deno.dlopen loads native libraries = A [OK]

Hint: Remember: dlopen means 'dynamic library open' [OK]

Common Mistakes:

• Confusing dlopen with file reading functions
• Thinking dlopen compiles code
• Mixing up server start with FFI loading

2. Which of the following is the correct way to define a function signature for a native function returning an integer in Deno.dlopen?

easy

A. { myFunc: { parameters: ["i32"], result: "void" } }

B. { myFunc: { parameters: ["i32"], result: "i32" } }

C. { myFunc: { parameters: ["void"], result: "i32" } }

D. { myFunc: { parameters: [], result: "string" } }

Solution

1. Step 1: Understand function signature format

   Function signatures in Deno.dlopen require specifying parameter types and the return type. For a function taking an integer and returning an integer, both parameters and result should be "i32".

2. Step 2: Analyze each option

   { myFunc: { parameters: ["i32"], result: "void" } } returns void, not integer. { myFunc: { parameters: ["void"], result: "i32" } } has parameters as void which is invalid. { myFunc: { parameters: [], result: "string" } } returns string but parameters are empty. Only { myFunc: { parameters: ["i32"], result: "i32" } } correctly matches integer parameter and integer return.

3. Final Answer:

   { myFunc: { parameters: ["i32"], result: "i32" } } -> Option B

4. Quick Check:

   Integer parameter and return = C [OK]

Hint: Match parameter and result types exactly as strings [OK]

Common Mistakes:

• Using 'void' as a parameter type incorrectly
• Mixing return and parameter types
• Using unsupported types like 'string' without proper handling

3. Given the following code snippet, what will be the output?

const lib = Deno.dlopen("./libmath.so", {
  add: { parameters: ["i32", "i32"], result: "i32" }
});

const result = lib.symbols.add(10, 5);
console.log(result);
lib.close();

medium

A. undefined

B. 105

C. 15

D. Runtime error

Solution

1. Step 1: Understand the function signature and call

   The native function 'add' takes two 32-bit integers and returns a 32-bit integer. Calling add(10, 5) should return 15.

2. Step 2: Analyze the output

   The console.log prints the returned value, which is 15. No errors occur because the signature matches and the library is closed properly.

3. Final Answer:

   15 -> Option C

4. Quick Check:

   Adding 10 + 5 = 15 [OK]

Hint: Check parameter types and expected return for output [OK]

Common Mistakes:

• Concatenating numbers as strings (getting 105)
• Forgetting to close the library
• Assuming undefined return without checking signature

4. Identify the error in this Deno FFI code snippet:

const lib = Deno.dlopen("./libcalc.so", {
  multiply: { parameters: ["i32", "i32"], result: "i64" }
});

const output = lib.symbols.multiply(3, 4);
console.log(output);
lib.close();

medium

A. The result type should be "i32" instead of "i64"

B. The parameters array should be empty

C. The library path is incorrect

D. The function call syntax is invalid

Solution

1. Step 1: Check function signature consistency

   The function 'multiply' takes two 32-bit integers and likely returns a 32-bit integer. Using "i64" as result type can cause mismatch errors if the native function returns 32-bit.

2. Step 2: Validate other parts

   Parameters are correctly specified as two "i32". The library path and function call syntax are valid assuming the file exists and function is exported.

3. Final Answer:

   The result type should be "i32" instead of "i64" -> Option A

4. Quick Check:

   Result type must match native function = D [OK]

Hint: Match return type exactly with native function signature [OK]

Common Mistakes:

• Changing parameter types incorrectly
• Assuming library path error without checking
• Misusing function call syntax

5. You want to call a native function that returns a pointer to a UTF-8 string. Which approach correctly handles this in Deno FFI?

hard

A. Define the function with result type "string" and use it directly

B. Define the function with result type "i32" and convert to string

C. Define the function with result type "void" and cast the result to string

D. Define the function with result type "pointer", then use Deno.UnsafePointerView to read the string

Solution

1. Step 1: Understand pointer return handling

   Native functions returning strings usually return pointers. Deno FFI requires defining the result as "pointer" to get the address.

2. Step 2: Use Deno.UnsafePointerView to read UTF-8

   After getting the pointer, Deno.UnsafePointerView can read the memory as a UTF-8 string safely.

3. Final Answer:

   Define the function with result type "pointer", then use Deno.UnsafePointerView to read the string -> Option D

4. Quick Check:

   Pointer + UnsafePointerView for strings = B [OK]

Hint: Use pointer type and UnsafePointerView for native strings [OK]

Common Mistakes:

• Using 'string' as result type directly
• Casting void or integer to string incorrectly
• Ignoring memory safety when reading pointers