Emscripten Compiler

# Compile C/C++ files
emcc [options] file...
# Display help and version
emcc --help
emcc --version

# Docker example:
docker run --rm -v $(pwd):/src -u $(id -u):$(id -g) emscripten/emsdk
# Inside container:
emcc helloworld.cpp -o helloworld.js

emcc -v

emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 3.1.45-git
clang version 18.0.0 (...)
Target: wasm32-unknown-emscripten
...

#include <stdio.h>
int main() {
    printf("Hello, World!\n");
    return 0;
}

emcc hello_world.c

node a.out.js
# → Hello, World!

emcc hello_world.c -s WASM=0

emcc hello_world.c -o hello.html

// hello.cpp
#include <iostream>
#include <stdlib.h>
int main() {
    char *buffer = (char*)malloc(10); // Deprecated in C++
    if (!buffer) return 1;
    std::cout << "Memory allocated." << std::endl;
    free(buffer);
    return 0;
}

emcc hello.cpp -o hello.html -s STRICT=1

Warning: ‘malloc’ is not recommended in modern C++. Use ‘new’ instead
Warning: inclusion of the C header file <stdlib.h> is deprecated in STRICT mode

#include <emscripten.h>
EMSCRIPTEN_KEEPALIVE
int multiplyNumbers(int a, int b) {
    return a * b;
}

emcc multiplyNumbers.c -o multiplyNumbers.js \
    -s EXPORTED_FUNCTIONS="['_multiplyNumbers']"

Module._multiplyNumbers(3, 4); // 12

#include <stdio.h>
int main() {
    FILE *file = fopen("hello_world_file.txt", "rb");
    if (!file) {
        printf("cannot open file\n");
        return 1;
    }
    int c;
    while ((c = fgetc(file)) != EOF) putchar(c);
    fclose(file);
    return 0;
}

emcc test/hello_world_file.cpp -o hello.html \
    --preload-file test/hello_world_file.txt

emcc -O1 hello_world.cpp  # safe transformations
emcc -O2 hello_world.cpp  # balanced speed and size
emcc -O3 hello_world.cpp  # aggressive inlining & loop unrolling