Embed C in Java Using GraalWasm

The example below demonstrates how to compile a C function to WebAssembly and run it embedded in a Java application.

Prerequisites

To run the demo, you need the following:

Demo Part

Put the following C program in a file named floyd.c:

 #include <stdio.h>

 void floyd() {
     int number = 1;
     int rows = 10;
     for (int i = 1; i <= rows; i++) {
         for (int j = 1; j <= i; j++) {
             printf("%d ", number);
             ++number;
         }
         printf(".\n");
     }
 }

 int main() {
     floyd();
     return 0;
 }

Note that floyd is defined as a separate function and can be exported.

Compile the C code using the most recent version of the Emscripten compiler frontend:
```
 emcc --no-entry -s EXPORTED_FUNCTIONS=_floyd -o floyd.wasm floyd.c
```
The exported functions must be prefixed by _. If you reference that function in, for example, the Java code, the exported name should not contain the underscore.

It produces a standalone file floyd.wasm in the current working directory.

Add dependencies. The GraalVM SDK Polyglot API can be easily added as a Maven dependency to your Java project. The GraalWasm artifact should be on the Java module or class path too. Add the following set of dependencies to the project configuration file (pom.xml in the case of Maven).

To add the Polyglot API:

  <dependency>
      <groupId>org.graalvm.polyglot</groupId>
      <artifactId>polyglot</artifactId> 
      <version>${graalwasm.version}</version>
  </dependency>

To add GraalWasm:

  <dependency>
      <groupId>org.graalvm.polyglot</groupId>
      <artifactId>wasm</artifactId> 
      <version>${graalwasm.version}</version>
      <type>pom</type>
  </dependency>

Now you can embed this WebAssembly function in a Java application, for example:

 import org.graalvm.polyglot.*;
 import org.graalvm.polyglot.io.ByteSequence;

 // Load the WebAssembly contents into a byte array
 byte[] binary = Files.readAllBytes(Path.of("path", "to", "wasm", "file", "floyd.wasm"));

 // Setup context
 Context.Builder contextBuilder = Context.newBuilder("wasm").option("wasm.Builtins", "wasi_snapshot_preview1");
 Source.Builder sourceBuilder = Source.newBuilder("wasm", ByteSequence.create(binary), "example");
 Source source = sourceBuilder.build();
 Context context = contextBuilder.build();

 // Evaluate the WebAssembly module
 context.eval(source);

 // Execute the floyd function
 context.getBindings("wasm").getMember("example").getMember("_initialize").executeVoid();
 Value mainFunction =context.getBindings("wasm").getMember("example").getMember("floyd");
 mainFunction.execute();
 context.close();

Compile and run this Java application with Maven as usual.

Embed C in Java Using GraalWasm

Prerequisites

Demo Part

Related Documentation