FFI (Foreign Function Interface) for native code in Deno - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What does this Deno FFI code output?

Consider this Deno FFI snippet that calls a native C function to add two numbers. What will be printed?

Deno

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

const result = dylib.symbols.add(5, 7);
console.log(result);
dylib.close();

A 57

B 12

C undefined

D Throws a runtime error

💡 Hint

Think about what the native function 'add' is expected to do with two integers.

The native function 'add' takes two 32-bit integers and returns their sum. Calling add(5,7) returns 12, which is printed.

📝 Syntax

intermediate

Which option correctly defines a Deno FFI function signature for a C function returning a double?

You want to call a native C function that returns a double and takes a single float parameter. Which Deno FFI signature is correct?

A { parameters: ["f32"], result: "f64" }

B { parameters: ["float"], result: "double" }

C { parameters: ["double"], result: "float" }

D { parameters: ["f64"], result: "f32" }

💡 Hint

Deno FFI uses specific type strings like 'f32' and 'f64' for floats and doubles.

In Deno FFI, 'f32' represents a 32-bit float parameter and 'f64' represents a 64-bit double return type. Option A matches this.

🔧 Debug

advanced

Why does this Deno FFI call throw a TypeError?

Given this code snippet, why does calling dylib.symbols.multiply(3, '4') throw a TypeError?

Deno

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

const result = dylib.symbols.multiply(3, '4');
console.log(result);
dylib.close();

A Because the second argument is a string, not an integer as required

B Because the native function multiply is not exported correctly

C Because the dylib.close() is called before the function call

D Because the parameters array is missing a third argument type

💡 Hint

Check the types of arguments passed versus the expected parameter types.

The FFI expects two 32-bit integers. Passing a string '4' causes a TypeError because the type does not match.

🧠 Conceptual

advanced

What is the purpose of the 'unsafe' keyword in Deno FFI function definitions?

In Deno FFI, some function definitions include the 'unsafe' flag. What does this flag indicate?

A The function runs asynchronously and returns a Promise

B The function is deprecated and should not be used

C The function may cause undefined behavior if called incorrectly and bypasses some safety checks

D The function is only available on Windows platforms

💡 Hint

Consider what 'unsafe' usually means in programming contexts related to memory and calls.

The 'unsafe' flag marks functions that can cause crashes or undefined behavior if used incorrectly, disabling some runtime safety checks.

❓ state_output

expert

What is the value of 'result' after this Deno FFI call sequence?

Given this code, what is the final value of 'result'?

Deno

const dylib = Deno.dlopen("./libcounter.so", {
  increment: { parameters: ["pointer"], result: "void" },
  getValue: { parameters: ["pointer"], result: "i32" }
});

const counter = new Uint32Array(1);
counter[0] = 0;
const ptr = Deno.UnsafePointer.of(counter.buffer);

dylib.symbols.increment(ptr);
dylib.symbols.increment(ptr);
const result = dylib.symbols.getValue(ptr);
dylib.close();

A Throws a TypeError

B 0

C undefined

D 2

💡 Hint

The native 'increment' function increases the value pointed to by the pointer by 1 each call.

The counter starts at 0. Two increments increase it to 2. Then getValue reads 2.

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