Makers Blog Archive

Crosscompile Guide: Libwebsockets API

Oliver PLCnext Team 09 August 2018 min. read
570 views 0 comments

The Libwebsockets API (“LWS”) covers a lot of interesting features for people making embedded servers or clients:

  • HTTP(S) serving and client operation
  • HTTP/2 support for serving
  • WS(S) serving and client operation
  • HTTP(S) apis for file transfer and upload
  • HTTP 1 + 2 POST form handling (including multipart / file upload)
  • cookie-based sessions
  • account management (including registration, email verification, lost pw etc)
  • strong SSL / TLS PFS support (A+ on SSLlabs test)
  • ssh server integration
  • serving gzipped files directly from inside zip files, without conversion
  • support for linux, bsd, windows etc… and very small non-linux targets like ESP32Here’s how to crosscompile it – first the easy way, and then for advanced users:

Crosscompiling made easy

  1. git clone
  2. cd libwebsockets
  3. mkdir build
  4. cd build
  5. source /home/ow/SDK/FW1.1/environment-setup-cortexa9t2hf-neon-pxc-linux-gnueabi
  6. cmake $CFLAGS -DCMAKE_INSTALL_PREFIX:PATH=../../Install ..
  7. make # -> if this fails with CMake Error: cmake_symlink_library: System Error: Operation not supported make sure to use a drive that supports symlinks
  8. make install

Check results

  1. execute file libwebsocket.* to check if it is an Arm 32 / ELF Binary ->ELF 32-bit LSB shared object, ARM,
  2. arm-pxc-linux-gnueabi-objdump -p ~/CrossCompiling/Install/lib/ | grep NEEDED

CrossCompiling with all dependencies

There is a good guide at /libwebsockets/READMEs/

The following description simply has been adapted to the Phoenix Contact (PxC) toolchain and the AXC F 2152 controller. FW 1.1 already has libz that is used by libwebsockets. We could include MBEDTLS but won´t because of memory usage and speed. We still need to crosscompile LIBUV.

Compiling LIBUV


libuv is a multi-platform support library with a focus on asynchronous I/O. It was primarily developed for use by Node.js, but it’s also used by LuvitJuliapyuv, and others.

  • MIT License

Feature highlights

  • Full-featured event loop backed by epoll, kqueue, IOCP, event ports
  • Asynchronous TCP and UDP sockets
  • Asynchronous DNS resolution
  • Asynchronous file and file system operations
  • File system events
  • ANSI escape code controlled TTY
  • IPC with socket sharing, using Unix domain sockets or named pipes (Windows)
  • Child processes
  • Thread pool
  • Signal handling
  • High resolution clock
  • Threading and synchronization primitives

1/3: Building libuv cross:

  1. git clone get libuv
  2. cd libuv
  3. source /home/ow/SDK/FW1.1/environment-setup-cortexa9t2hf-neon-pxc-linux-gnueabi
  4. ./ libtoolize --copy libtoolize: putting auxiliary files in '.'. libtoolize: copying file './' libtoolize: putting macros in AC_CONFIG_MACRO_DIRS, 'm4'. libtoolize: copying file 'm4/libtool.m4' libtoolize: copying file 'm4/ltoptions.m4' libtoolize: copying file 'm4/ltsugar.m4' libtoolize: copying file 'm4/ltversion.m4' libtoolize: copying file 'm4/lt~obsolete.m4' + aclocal -I m4 + autoconf + automake --add-missing --copy installing './ar-lib' installing './compile' installing './config.guess' installing './config.sub' installing './install-sh' installing './missing' installing './depcomp'If it has problems, you will need to install automakelibtool etc.
  5. ./configure --host=arm-linux-gnueabihf --prefix=/home/ow/CrossCompiling/Install
  6. make && make install
  7. file /home/ow/CrossCompiling/Install/lib/libuv.* Check if it’s really built for ARM

2/3: Building zlib cross

FW 1.1 already has libz that is used by libwebsocket. You can find it in the SDK /sysroots/cortexa9t2hf-neon-pxc-linux-gnueabi/lib and the headers at /sysroots/cortexa9t2hf-neon-pxc-linux-gnueabi/usr/include

Or compile it again with:

  1. git clone
  2. source /home/ow/SDK/FW1.1/environment-setup-cortexa9t2hf-neon-pxc-linux-gnueabi
  3. ./configure --prefix=~/CrossCompiling/Install
  4. file ~/CrossCompiling/Install/libz.*

3/3: Building libwebsockets with everything

Prepare the CMake toolchain file

For crosscompiling we need a CMake toolchain file. Here we use the file supplied by Bjoern Sauer, ready for download.

{jd_file onlinelayout==Simple File List}{jd_file file==57}

Here’s an instruction how to make it yourself.

  1. cd /CrossCompiling
  2. git clone ssh://
  3. cd libwebsockets ; mkdir build ; cd build
  4. cmake .. -DCMAKE_TOOLCHAIN_FILE=../../axcf2152.cmake \ -DTOOLCHAIN_ROOT=/home/ow/SDK/FW1.1 \ -DTOOLCHAIN_PREFIX=~/CrossCompiling/Install \ -DCMAKE_BUILD_TYPE=Release \ -DCMAKE_INSTALL_PREFIX:PATH=../../Install \ -DLWS_WITH_LWSWS=ON \ -DLWS_LIBUV_LIBRARIES=~/CrossCompiling/Install/lib/ \ -DLWS_LIBUV_INCLUDE_DIRS=~/CrossCompiling/Install/include \ -DLWS_ZLIB_LIBRARIES=~/CrossCompiling/Install/lib/ \ -DLWS_ZLIB_INCLUDE_DIRS=~/CrossCompiling/Install/include (this all is one line on the commandline)
  5. make && make install
  6. file ~/CrossCompiling/Install/lib/libwebsockets.*
  7. rm-pxc-linux-gnueabi-objdump -p ~/CrossCompiling/Install/lib/ | grep NEEDEDConfirm that the LWS library was linked against everything we expect (libm / libc are provided by your toolchain) NEEDED NEEDED NEEDED NEEDED NEEDED NEEDED
  • you will also find the LWS test apps in ~/CrossCompiling/Install/bin
  • to run LWS on the target, copy the related things from ~/CrossCompiling/Install…
  • copy them to the ‘/usr/local/” directory in the PLC

4/3: Building mbedtls cross (optional)

  1. cd /tmp
  2. git clone
  3. cd mbedtls ; mkdir build ; cd build
  4. cmake .. \ -DCMAKE_TOOLCHAIN_FILE=../../axcf2152.cmake \ -DTOOLCHAIN_ROOT=/home/ow/SDK/FW1.1 \ -DTOOLCHAIN_PREFIX=~/CrossCompiling/Install \ -DCMAKE_BUILD_TYPE=Release \ -DCMAKE_INSTALL_PREFIX:PATH=../../Install \ -DUSE_SHARED_MBEDTLS_LIBRARY=1
  5. make && make install
  6. file ../../Install/lib/libmbed*


The Makers Blog shows applications and user stories of community members that are not tested or reviewed by Phoenix Contact. Use them at your own risk.


Please login/register to comment


Leave a Reply

Never miss a new article
Sign up for the newsletter
Never miss news about PLCnext Technology
Get interesting content via newsletter four times a year
Receive exclusive information before all other users