Compiling c++ project with 3rd party lib to WebAssembly - c++

Question

I have a simple c++ project which includes eigen. I'm able to compile the project on my own machine but having trouble to compile it to webassembly with emscripten.
Project structure:
.
├── CMakeLists.txt
├── include
│   └── HelloWasm
│   └── my_lib.h
└── src
├── main.cpp
└── my_lib.cpp
File contents:
CMakeLists.txt
cmake_minimum_required( VERSION 3.0 )
project( HelloWasm )
# flags
# include files
include_directories( ./include .include/HelloWasm ./src )
# target
add_executable( HelloWasm ./src/main.cpp ./src/my_lib.cpp )
# 3rd party libs
find_package(Eigen3 REQUIRED NO_MODULE)
include_directories(${EIGEN3_INCLUDE_DIR})
include/HelloWasm/my_lib.h
#include <iostream>
#include <Eigen/Dense>
using namespace std;
using Eigen::MatrixXd;
class MyLib
{
private:
protected:
public:
MyLib()
{
}
~MyLib()
{
}
void eigen_test();
};
src/main.cpp
#include <iostream>
#include "HelloWasm/my_lib.h"
using namespace std;
int main()
{
MyLib my_lib;
my_lib.eigen_test();
}
src/my_lib.cpp
#include "HelloWasm/my_lib.h"
void MyLib::eigen_test()
{
MatrixXd m(2, 2);
m(0, 0) = 3;
m(1, 0) = 2.5;
m(0, 1) = -1;
m(1, 1) = m(1, 0) + m(0, 1);
cout << '\n'
<< m << endl;
}
Compiling the project successfully locally:
mkdir build && cd build
cmake ..
make
➜ ./HelloWasm
3 -1
2.5 1.5
Errors when trying to compile to webassemply
(I tried following the steps provided in the emscripten docs)
mkdir build && cd build
cmake ..
emcmake cmake ..
output:
configure: cmake .. -DCMAKE_TOOLCHAIN_FILE=/Users/me/programming/emsdk/upstream/emscripten/cmake/Modules/Platform/Emscripten.cmake -DCMAKE_CROSSCOMPILING_EMULATOR="/Users/me/programming/emsdk/node/12.9.1_64bit/bin/node"
-- Configuring done
-- Generating done
CMake Warning:
Manually-specified variables were not used by the project:
CMAKE_TOOLCHAIN_FILE
-- Build files have been written to: /Users/me/programming/sandbox/cpp_sandbox/so_question_project/build
Now running make:
➜ emmake make
make: make
Scanning dependencies of target HelloWasm
[ 33%] Building CXX object CMakeFiles/HelloWasm.dir/src/main.cpp.o
[ 66%] Building CXX object CMakeFiles/HelloWasm.dir/src/my_lib.cpp.o
[100%] Linking CXX executable HelloWasm
[100%] Built target HelloWasm
Obviously that did not create a .wasm file...
Doing the following:
em++ CMakeFiles/HelloWasm.dir/src/main.cpp.o CMakeFiles/HelloWasm.dir/src/my_lib.cpp.o -o helloWasm.js
output:
em++: warning: CMakeFiles/HelloWasm.dir/src/main.cpp.o is not a valid input file [-Winvalid-input]
em++: warning: CMakeFiles/HelloWasm.dir/src/my_lib.cpp.o is not a valid input file [-Winvalid-input]
em++: error: no input files
note that input files without a known suffix are ignored, make sure your input files end with one of: ('.c', '.i', '.cpp', '.cxx', '.cc', '.c++', '.CPP', '.CXX', '.C', '.CC', '.C++', '.ii', '.m', '.mi', '.mm', '.mii', '/dev/null', '.bc', '.o', '.obj', '.lo', '.dylib', '.so', '.a', '.ll', '.h', '.hxx', '.hpp', '.hh', '.H', '.HXX', '.HPP', '.HH')
What am I missing here...?

Answer 1

Change:
em++ CMakeFiles/HelloWasm.dir/src/main.cpp.o CMakeFiles/HelloWasm.dir/src/my_lib.cpp.o -o helloWasm.js
To:
em++ CMakeFiles/HelloWasm.dir/src/main.cpp.o CMakeFiles/HelloWasm.dir/src/my_lib.cpp -o helloWasm.js
Emscripten can't compile my_lib.cpp.o because it's already compiled to machine code (it's an object file). You have to use a .cpp file, not .cpp.o.

Answer 2

I have a suggestion for you.
First, you can put your lib files (my_lib.h and my_lib.cpp) on same directory.
Second, you can create a folder for your applications (HelloWasm) and put your executable codes (main.cpp) in this folder.
Finally, you can create CMakeLists.txt file for each folder.
This is your new directory tree:
.
├── CMakeLists.txt
├── Applications
│ └── HelloWasm
│ ├── CMakeLists.txt
│ └── main.cpp
└── Libraries
├── CMakeLists.txt
├── my_lib.h
└── my_lib.cpp
CMakeLists.txt (the first one) :
cmake_minimum_required(VERSION 3.0)
project(LIBRARYANDAPPS)
#This function is starting build and looking all folders.
function( start_build )
file( GLOB_RECURSE components "${CMAKE_SOURCE_DIR}/*/CMakeLists.txt" )
foreach( component ${components} )
get_filename_component( path ${component} PATH )
add_subdirectory( ${path} )
endforeach( )
endfunction( )
start_build()
CMakeLists.txt (for HelloWasm):
#You can add Emscripten flags like this.
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} \
-std=c++11 --bind \
-s USE_WEBGL2=1 -s FULL_ES3=1 --memory-init-file 0")
find_package(Eigen3 REQUIRED NO_MODULE)
include_directories(${EIGEN3_INCLUDE_DIR})
include_directories("${CMAKE_SOURCE_DIR}/Libraries")
ADD_EXECUTABLE(HelloWasm main.cpp)
TARGET_LINK_LIBRARIES(HelloWasm MyLib Eigen)
CMakeLists.txt (for Library):
find_package(Eigen3 REQUIRED NO_MODULE)
include_directories(${EIGEN3_INCLUDE_DIR})
ADD_LIBRARY(MyLib STATIC my_lib.cpp my_lib.h)
TARGET_LINK_LIBRARIES(MyLib Eigen)
With these edits, you can create bigger projects, libraries. And I think more useful like this.