WebAssembly

Using libc with WebAssembly enabled DataFrames

Using libc with WebAssembly enabled DataFrames

This post describes how to use the C standard library with WebAssembly enabled
DataFrames. You can find the source code
here.

Introduction

In my previous
post
I avoided solving the problem of linking to standard
library to provide memory allocation for my WebAssembly project by writing my
own memory allocator. In order to use external C packages I will need a
standard library.

wsi-libc

The wasi-libc project does what it
says on the tin by providing a standard library for wasm.

I have changed my setup as my laptop required reinstallation. I’m now running
Ubuntu 20.04 LTS.

I installed the latest (version 10) LLVM tool chain as a Debian package which
I got from this page. I did the following.

1
2
wget https://apt.llvm.org/llvm.sh
sudo ./llvm.sh

That installed clang et al with the suffix “-10”, such that clang could be
found as /usr/bin/clang-10.

I installed wabt in /opt/wabt.

I cloned the wasi-libc library and built it as follows.

1
2
3
git clone git@github.com:WebAssembly/wasi-libc.git
cd wasi-libc
sudo WASM_CC=clang-10 WASM_NM=llvm-nm-10 WASM_AR=llvm-ar-10 INSTALL_DIR=/opt/wasi-libc make install

The C Source Code

In my previous post I had two source files: memory-allocation.c and array-methods.c.
We can delete memory-allocation.c as we’ll use malloc and free from the
standard library. I split he array methods into two files: array-methods-int.c
and array-methods-double.c, included stdlib.h and changed the memory allocation
to use the standard library.

Here is the start of array-methods-int.c.

1
2
3
4
5
6
7
8
9
10
11
12
13
14
#include <stdlib.h>

__attribute__((used)) int* addInt32Arrays (int *array1, int* array2, int length)
{
  int* result = (int*) malloc(length * sizeof(int));
  if (result == 0)
    return 0;

  for (int i = 0; i < length; ++i) {
    result[i] = array1[i] + array2[i];
  }

  return result;
}

Then I changed the makefile as follows.

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
SYSROOT=/opt/wasi-libc

LLVM_VERSION=10
CC=clang-$(LLVM_VERSION)
LD=wasm-ld-$(LLVM_VERSION)
CFLAGS=--target=wasm32-unknown-unknown-wasm --optimize=3 --sysroot=$(SYSROOT)
LDFLAGS=--export-all --no-entry --allow-undefined -L$(SYSROOT)/lib/wasm32-wasi -lc -lm

WASM2WAT=/opt/wabt/bin/wasm2wat

sources = array-methods-int.c array-methods-double.c
objects = $(sources:%.c=%.bc)
target = data-frame

.PHONY: all

all: $(target).wat

$(target).wat: $(target).wasm
	$(WASM2WAT) $(target).wasm -o $(target).wat

$(target).wasm: $(objects)
	$(LD) $(objects) $(LDFLAGS) -o $(target).wasm
	
%.bc: %.c
	$(CC) -c -emit-llvm $(CFLAGS) $< -o $@

clean:
	rm -f $(target).wasm
	rm -f $(target).wat
	rm -f $(objects)

The compilation stage node sets --sysroot. This appears to set up the include
paths correctly.

The link stage adds the path to the libraries -L$(SYSROOT)/lib/wasm32-wasi and
and includes libc and libm -lc -lm.

The JavaScript Code

All I needed to do was to change the memory allocation imports to use malloc and free.

Everything just worked!

As a test I added a unary function logFloat64Array. This required sme minor
changes to the javascript to allow ‘int’ type series to fall through to the double
methods. Again it all just worked.

Things To Do

I need to understand what the crt1.o file is all about. I tried to link, but
that gives me other errors.