cesp32webassembly

Compiling Wasm Programs for esp32 wamr

I want to be able to compile a C program to Wasm in a way that the Espressif Wasm micro runtime(espressif) will accept it.

I have tried following the guide in the esp-wdf github project, which supposedly gives insight into how it works, however to no avail. I tried compiling with various compilers such as emscripten or zig but no binary produced is accepted by the Wasm runtime. Sadly the examples from the Wasm micro runtime repository do not explain the compilation step, but provide a Wasm application as a hardcoded binary array (which works as expected). It is weird, since when I use it with some Node.js glue code, it works fine.

Solution

  • I have found a solution which works for the following "hello world" program provided from the espressif Wasm micro runtime repository:

    #include <stdio.h>
#include <stdlib.h>

int
main(int argc, char **argv)
{
    char *buf;

    printf("Hello world!\n");

    buf = malloc(1024);
    if (!buf) {
        printf("malloc buf failed\n");
        return -1;
    }

    printf("buf ptr: %p\n", buf);

    snprintf(buf, 1024, "%s", "1234\n");
    printf("buf: %s", buf);

    free(buf);
    return 0;
}

    In the directory with path wasm-micro-runtime/product-mini/app-samples/hello-world is the program, as well as the build.sh file:

    # Copyright (C) 2019 Intel Corporation.  All rights reserved.
# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception

WAMR_DIR=${PWD}/../../..

echo "Build wasm app .."
/opt/wasi-sdk/bin/clang -O3 \
        -z stack-size=4096 -Wl,--initial-memory=65536 \
        -o test.wasm main.c \
        -Wl,--export=main -Wl,--export=__main_argc_argv \
        -Wl,--export=__data_end -Wl,--export=__heap_base \
        -Wl,--strip-all,--no-entry \
        -Wl,--allow-undefined \
        -nostdlib \

echo "Build binarydump tool .."
rm -fr build && mkdir build && cd build
cmake ../../../../test-tools/binarydump-tool
make
cd ..

echo "Generate test_wasm.h .."
./build/binarydump -o test_wasm.h -n wasm_test_file test.wasm

echo "Done"

    where we can clearly extract the necessary compiler (it is necessary to use the wasi-sdk clang compiler) as well as the compiler flags. What the purpose of each flag is can be found here, but for the sake of completeness I list what each flag does:

    Those exports are defined by wasm-ld (the WebAssembly linker backend in lld, and used by clang). The linker is adding those symbols because they are required - they aren't extra symbols exported on accident. The __heap_base symbol is used to indicate the start of the heap in linear memory, and end of the stack. While __data_end marks the end of the data/bss sections and the start of the stack.