Background info
In this thread I will explain how to compile the latest Mono from source for the Onion Omega2 and Omega2+, including some things to customize Mono for your needs.
Mono compilation consists of two things
- Native code (C Code). The main program, virtual execution system, garbage collector etc.
- Managed code (C# Code). The Mono Library.
The first thing you will need to compile is the main program written in C. After you have done this, you can compile the Library (C#). Under normal circumstances you can do both in one make command. That only works when you are compiling on the device itself. In this tutorial I will use a cross toolchain to compile the main program for the Omega2. Mono does not support compiling managed code when cross compiling. For that you will need the Linux host on which you are cross compiling or a different Linux host to compile the managed code. After compilation is done you can just copy the C# library from the Linux host.
Cross compile distribution
Ubuntu Server 18.04 LTS
Step 0. Compiling the cross toolchain
I used the following cross toolchain for cross compiling. Build the cross toolchain explained there.
https://github.com/OnionIoT/source
Note: Make sure the toolchain's usr/lib has libraries like libz. Otherwise the mono build will fail while cross compiling.
Step 1. Compile Mono on the Linux host
I used the following working directory
The first step is to compile Mono on the Linux host. Go to the working directory
cd /home/project/
Download and install all the necessary packages
sudo apt-get install subversion g++ zlib1g-dev build-essential git python libncurses5-dev gawk gettext unzip file libssl-dev wget libtool-bin autoconf cmake
Clone the mono source
git clone –recursive https://github.com/mono/mono
Change to mono directory
cd mono
Configure and make mono
./autogen.sh --prefix=/ && make
Create directory for exporting mono installation
mkdir ../BuildHost
Make install Mono
make install DESTDIR=/home/project/BuildHost
Now you have a full working Mono installation for your Linux host. We only need the library /home/project/BuildHost/lib/mono
Clean up source directory
make clean
Step 2. Cross Compile mono for Omega2
Then we will build the main program for the Omega2.
Create the cross toolchain environment. For that you will need the cross toolchain binaries from the Openwrt SDK directory. In my case /home/project/openwrt-sdk-ramips-mt76x8_gcc-7.3.0_musl.Linux-x86_64/staging_dir/toolchain-mipsel_24kc_gcc-7.3.0_musl/bin/
export CROSS=/home/project/openwrt-sdk-ramips-mt76x8_gcc-7.3.0_musl.Linux-x86_64/staging_dir/toolchain-mipsel_24kc_gcc-7.3.0_musl/bin/mipsel-openwrt-linux
export CC=$CROSS-gcc
export CXX=$CROSS-g++
export CPP=$CROSS-cpp
export AR=$CROSS-ar
export AS=$CROSS-as
Ok so the cross toolchain environment is set up. At this point you will need to tweak the configure script so that it forces to use __thread as Thread Local Storage for Mono. The configure script automatically selects pthread instead of __thread. The reason for doing this is because the standard garbage collector for mono, sgen does not support pthread. Normally you can select boehm as the alternative garbage collector, but since boehm doesn’t support the musl library used for the Omega2 there is no other choice then to tweak the configure script. Also using the option --with-tls=__thread with the configure script doesn’t work. So tweaking it is. Fortunately __thread works just fine on the Omega2.
Tweak the configure script
sed -i -- 's/with_tls=pthread/with_tls=__thread/g' configure configure.ac
What I could not figure out was this error when I was cross compiling Mono
In file included from mono-context.c:10:0:
mono-context.c: In function 'mono_sigctx_to_monoctx':
../../mono/utils/mono-sigcontext.h:480:73: error: request for member 'fp_r' in something not a structure or union
# define UCONTEXT_FPREGS(ctx) (((ucontext_t *)(ctx))->uc_mcontext.fpregs.fp_r.fp_dregs)
^
mono-context.c:448:24: note: in expansion of macro 'UCONTEXT_FPREGS'
mctx->sc_fpregs[i] = UCONTEXT_FPREGS (sigctx) [i];
^
mono-context.c: In function 'mono_monoctx_to_sigctx':
../../mono/utils/mono-sigcontext.h:480:73: error: request for member 'fp_r' in something not a structure or union
# define UCONTEXT_FPREGS(ctx) (((ucontext_t *)(ctx))->uc_mcontext.fpregs.fp_r.fp_dregs)
^
mono-context.c:460:3: note: in expansion of macro 'UCONTEXT_FPREGS'
UCONTEXT_FPREGS (sigctx) [i] = mctx->sc_fpregs[i];
^
If you have suggestions on how to fix this, comment it please. I have been working on this error for quite some time. It has to do with the musl library used with the Mips architecture. It compiles normally when glibc library is used.
For now just remove the lines which gives the make error.
sed -i '/# define UCONTEXT_FPREGS(ctx)/c\' mono/utils/mono-sigcontext.h
sed -i '/UCONTEXT_FPREGS (sigctx)/c\' mono/utils/mono-context.c
Then you can configure Mono. Because boehm doesn’t work we can disable it. At this point you can select which features you want to use with Mono. I highly recommend to create a small footprint since the Omega2 does not have much space. You can of course expand the root partition with a USB flash drive or a MicroSD card and use a completer version of Mono. In this tutorial I will explain how to build a small footprint http://www.mono-project.com/docs/compiling-mono/small-footprint/
Configure mono with small footprint
./autogen.sh --prefix=/ --enable-minimal=aot,profiler,debug,logging --host=mipsel-openwrt-linux --enable-small-config=yes --with-shared_mono=no --with-static_mono=yes --disable-mcs-build --without-mcs-docs --with-moonlight=no --disable-boehm
Then make mono with optimization for size
make CFLAGS=-Os
Create directory for exporting Omega2 Mono installation
mkdir ../BuildOmega
Export Omega2 Mono installation
make install DESTDIR=/home/project/BuildOmega
Change to BuildOmega directory
cd ../BuildOmega
Remove include and share. We don’t need those
rm -r include share
Change to bin directory
cd bin
Remove all binaries except mono and mono-sgen
mv mono mono-sgen ../
rm *
mv ../mono ../mono-sgen .
Then strip the mono binary
$CROSS-strip mono-sgen
Change to lib directory
cd ../lib
Strip some libraries
$CROSS-strip lib*
Remove unnecessary files and directories
rm *.la *.a
rm -r pkgconfig mono
Now we can copy the .NET library to the Mono Omega build directory
If size doesn’t matter (chuckle), just merge the whole directory /home/project/BuildHost/lib/mono with /home/project/BuildOmega/lib/mono so that it has the complete mono library.
Or just choose which .NET features you want to use and copy those to /home/project/BuildOmega/lib/mono
I will explain how to create a very small library, almost equal to Debian’s mono-runtime
cd /home/project/BuildHost/lib/mono
mv 4.5 gac ../
rm -r *
mv ../4.5 ../gac .
cd 4.5
mv cert-sync.exe gacutil.exe I18N.dll I18N.West.dll Mono.Security.dll mscorlib.dll System.Configuration.dll System.dll System.Security.dll System.Xml.dll ../
rm -r *
mv ../cert-sync.exe ../gacutil.exe ../I18N.dll ../I18N.West.dll ../Mono.Security.dll ../mscorlib.dll ../System.Configuration.dll ../System.dll ../System.Security.dll ../System.Xml.dll .
cd ../gac
mv I18N I18N.West Mono.Security System System.Configuration System.Security System.Xml ../
rm -r *
mv ../I18N ../I18N.West ../Mono.Security ../System ../System.Configuration ../System.Security ../System.Xml .
rm I18N/4.0.0.0*/*.pdb I18N.West/4.0.0.0*/*.pdb Mono.Security/4.0.0.0*/*.pdb System/4.0.0.0*/*.pdb System.Configuration/4.0.0.0*/*.pdb System.Security/4.0.0.0*/*.pdb System.Xml/4.0.0.0*/*.pdb
Then copy the library to the BuildOmega directory
cp -R /home/project/BuildHost/lib/mono /home/project/BuildOmega/lib/mono
You can then copy the contents of /home/project/BuildOmega to the root of your Omega2(+).
This is how you compile Mono for the Omega2. If there are any questions, please ask them in the comments.
Timezone error
Some .NET apps requires /etc/localtime which is not available by standard. To fix this download and install zoneinfo. Before you do this you need to select your contintent. In this example zoneinfo-europe
opkg install http://downloads.lede-project.org/snapshots/packages/mipsel_24kc/packages/zoneinfo-europe_2017b-1_mipsel_24kc.ipk
Copy timezone to /etc/localtime (In this example London)
cp /usr/share/zoneinfo/Europe/London /etc/localtime
