FFI (Foreign Function Interface) for native code in Deno - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to load a native library using Deno FFI.

Deno

const dylib = Deno.dlopen("./libexample.so", [1]);

Drag options to blanks, or click blank then click option'

A"./libexample.so"

B["add", "sub"]

Cnew Deno.DynamicLibrary()

D{ "add": { parameters: ["i32", "i32"], result: "i32" } }

Attempts:

3 left

💡 Hint

Common Mistakes

Passing an array instead of an object for symbols.

Omitting the parameter or result types.

Trying to pass the library path as the symbols argument.

You must provide a symbols object describing the native functions you want to use. This object maps function names to their parameter and result types.

2fill in blank

medium

Complete the code to call the native 'add' function with arguments 5 and 3.

Deno

const result = dylib.symbols.add([1], [2]);

Drag options to blanks, or click blank then click option'

A5, 3

B5

C3, 5

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Passing only one argument.

Swapping the argument order.

Passing arguments as strings.

The 'add' function expects two 32-bit integers. You must pass both arguments in the correct order.

3fill in blank

hard

Fix the error in the native function declaration to correctly specify a function returning void.

Deno

const dylib = Deno.dlopen("./libexample.so", { "log": { parameters: ["pointer"], result: [1] } });

Drag options to blanks, or click blank then click option'

A"null"

B"void"

C"undefined"

D"voidptr"

Attempts:

3 left

💡 Hint

Common Mistakes

Using "null" or "undefined" instead of "void".

Using a pointer type for a void return.

Omitting the result property.

In Deno FFI, a native function returning nothing uses the result type "void".

4fill in blank

hard

Fill both blanks to define a native function 'multiply' that takes two 64-bit floats and returns a 64-bit float.

Deno

const dylib = Deno.dlopen("./libexample.so", { "multiply": { parameters: [[1], [2]], result: "f64" } });

Drag options to blanks, or click blank then click option'

A"f64"

B"i32"

C"pointer"

D"bool"

Attempts:

3 left

💡 Hint

Common Mistakes

Using integer types instead of float types.

Mixing parameter types.

Using pointer or boolean types incorrectly.

The 'multiply' function takes two 64-bit floats as parameters, so both parameters must be "f64".

5fill in blank

hard

Fill all three blanks to create a safe wrapper function that calls the native 'subtract' function and returns its result.

Deno

function safeSubtract(a: number, b: number): number {
  return dylib.symbols.[1]([2], [3]);
}

Drag options to blanks, or click blank then click option'

Asubtract

Ba

Cb

Dadd

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong function name like 'add'.

Swapping the argument order.

Passing incorrect variable names.

The wrapper calls the native 'subtract' function with arguments 'a' and 'b' and returns the result.

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