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WebAssembly (WASM) for High Performance Apps · Lesson

Passing Complex Data Structures

Learn strategies for efficiently transferring strings, arrays, and custom objects between WASM and JavaScript.

Passing Complex Data Structures is a free WebAssembly (WASM) for High Performance Apps lesson on CoddyKit — lesson 1 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the WebAssembly (WASM) for High Performance Apps learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Complex Data in WASM

When building high-performance applications with WebAssembly (WASM), you often need to exchange more than just simple numbers between your WASM module and JavaScript.

  • How do you pass a string?
  • What about an array of numbers?
  • Or even a custom object structure?

This lesson explores strategies for efficiently handling these complex data types.

WASM's Linear Memory

The key to passing complex data lies in understanding WASM's linear memory. This is a contiguous block of memory (like a large array of bytes) that is shared between your WASM module and the JavaScript environment.

WASM functions can only return numbers. So, instead of passing the entire data, we pass a pointer (a memory address, which is just a number!) to where the data resides in this shared memory.

Strings: Pointer & Length

Strings are sequences of characters. To pass a string from WASM to JavaScript, the WASM module will:

  1. Write the string's bytes into its linear memory.
  2. Return two pieces of information to JavaScript:
    • The starting memory address (pointer) of the string.
    • The length of the string (how many bytes it occupies).

JavaScript then uses these to read the bytes from the shared memory and decode them into a JavaScript string.

C String to JS Example

Here's a C function that returns a pointer to a string. When compiled with Emscripten, JavaScript can call this function and then read the string from the WASM memory.

EMSCRIPTEN_KEEPALIVE makes the function callable from JavaScript.

#include <emscripten/emscripten.h>
#include <string.h>

// A static string for demonstration
const char* myString = "Hello WASM!";

EMSCRIPTEN_KEEPALIVE
const char* getGreetingPtr() {
  return myString;
}

EMSCRIPTEN_KEEPALIVE
int getGreetingLength() {
  return strlen(myString);
}

int main() {
  // Required for a complete C program
  return 0;
}

Arrays: Contiguous Data

Numeric arrays (like int[] or float[]) are similar to strings. They are stored as a contiguous block of values in WASM's linear memory.

To pass an array, WASM returns:

  • A pointer to the first element of the array.
  • The number of elements in the array.

JavaScript then reads the raw bytes from the shared memory, knowing the type and size of each element (e.g., 4 bytes for an integer).

C Array to JS Example

This C code creates an array, populates it, and provides functions to get its pointer and length. JavaScript would then use Module.HEAP32 (for 32-bit integers) to access the elements.

#include <emscripten/emscripten.h>
#include <stdlib.h> // For malloc

int* dynamicArray = NULL;
int currentArrayLength = 0;

EMSCRIPTEN_KEEPALIVE
int* createAndGetArray(int size) {
  if (dynamicArray) {
    free(dynamicArray);
  }
  dynamicArray = (int*)malloc(size * sizeof(int));
  currentArrayLength = size;
  for (int i = 0; i < size; ++i) {
    dynamicArray[i] = i * 10 + 5;
  }
  return dynamicArray;
}

EMSCRIPTEN_KEEPALIVE
int getArrayLength() {
  return currentArrayLength;
}

int main() {
  // Required for a complete C program
  return 0;
}

Custom Objects (Structs)

Passing custom objects (like structs in C/C++) is more involved. These objects are collections of different data types (e.g., an integer, a float, and a string).

The strategy is to manually lay out the object's fields in WASM's linear memory. WASM then returns a pointer to the start of this memory block. JavaScript must then know the exact memory layout (offsets and types of each field) to correctly read the data.

C Struct to JS Example

This C example defines a Person struct and a function to create one. JavaScript would receive a pointer and then read values from specific offsets in the WASM memory buffer, knowing the struct's layout.

  • id (int) at offset 0
  • age (int) at offset 4
  • height (float) at offset 8
#include <emscripten/emscripten.h>
#include <stdlib.h> // For malloc
#include <string.h> // For strcpy

typedef struct {
  int id;
  int age;
  float height; // assuming 4 bytes after age
  char name[16]; // Fixed-size string buffer
} Person;

EMSCRIPTEN_KEEPALIVE
Person* createPerson(int id, int age, float height, const char* name) {
  Person* p = (Person*)malloc(sizeof(Person));
  p->id = id;
  p->age = age;
  p->height = height;
  strncpy(p->name, name, sizeof(p->name) - 1);
  p->name[sizeof(p->name) - 1] = '\0'; // Ensure null termination
  return p;
}

int main() {
  // Required for a complete C program
  return 0;
}

JS Allocates WASM Memory

What if JavaScript needs to pass complex data *into* WASM? For example, a large image array for WASM to process.

JavaScript can call WASM's exposed memory allocation functions (like Emscripten's _malloc) to reserve space within WASM's linear memory. Then, JavaScript writes the data into that allocated space and passes the pointer to a WASM function for processing.

Quick Check

You've learned how WASM handles complex data. Let's test your understanding!

Recap: Complex Data

Congratulations! You've learned the fundamental techniques for passing complex data between WebAssembly and JavaScript.

  • WASM and JavaScript communicate via a shared linear memory.
  • Complex data (strings, arrays, structs) are passed by their pointer (memory address) and often their length or size.
  • JavaScript then reads the raw bytes from the shared memory, interpreting them based on the known data structure.
  • For custom objects, understanding the exact memory layout is crucial.

These techniques allow for efficient, high-performance data exchange!

Frequently asked questions

Is the “Passing Complex Data Structures” lesson free?

Yes — the full text of “Passing Complex Data Structures” is free to read here on the web, and the WebAssembly (WASM) for High Performance Apps course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the WebAssembly (WASM) for High Performance Apps course, upgrade to CoddyKit PRO.

What will I learn in “Passing Complex Data Structures”?

Learn strategies for efficiently transferring strings, arrays, and custom objects between WASM and JavaScript. You practise WebAssembly (WASM) for High Performance Apps with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start WebAssembly (WASM) for High Performance Apps?

No prior experience is required. WebAssembly (WASM) for High Performance Apps on CoddyKit is structured for beginners through advanced learners; this is — lesson 1 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Passing Complex Data Structures” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this WebAssembly (WASM) for High Performance Apps lesson?

Yes. Every WebAssembly (WASM) for High Performance Apps lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

  1. Passing Complex Data Structures
  2. WASM Memory Model & Management
  3. Shared Memory & Atomics
  4. Growing & Managing Linear Memory
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