Compiling on OpenWRT

These are some notes, we'll see if it works and warrants a full article...

• Entware has better resources documentation for compiling on a router: https://github.com/Entware/Entware/wiki/Using-GCC-for-native-compilation
• A key resource are headers;
  • To keep OpenWRT and Entware stuff separate, create the /opt directory: mkdir /opt
  • Then it is safe to install / download this: wget -qO- http://bin.entware.net/armv7sf-k3.2/include/include.tar.gz | tar xvz -C /opt/include
• And from this source, it is easy to add the path when compiling: https://gcc.gnu.org/onlinedocs/cpp/Search-Path.html
  • By default, gcc searches these directories first: /usr/include and /usr/lib/gcc/arm-openwrt-linux-muslgnueabi/7.4.0/include
  • Downloading the above Entware Header File, this command can be used to search a different directory, like /opt/include, first: -IWhatEverDirectory
• Items to install;
  • Make: https://man7.org/linux/man-pages/man1/make.1.html
    • opkg install gcc make
  • Entware recommends installing the following: opkg install busybox ldd make gawk sed (a version of ldd and sed are included within the OpenWRT version of BusyBox)
  • Additional utilities are recommended
    • Install the full version of BASH: opkg install bash
• Environment variables to add;
  • export LDFLAGS="-Wl,-rpath=/opt/lib -Wl,--dynamic-linker=/opt/lib/ld-linux.so.3 -L/opt/lib", but... (rpath is for run time (not compile time) libraries, https://en.wikipedia.org/wiki/Rpath)
    • needs to be adjusted for OpenWRT (/usr/lib) instead of DD-WRT (/opt/lib)
  • This is needed too, but...: export CFLAGS="-O2 -pipe -march=armv7-a -mtune=cortex-a9 -fno-caller-saves -mfloat-abi=soft "
    • ...but add this to it: export CFLAGS="-O2 -pipe -march=armv7-a -mtune=cortex-a9 -fno-caller-saves -mfloat-abi=soft -I/opt/include"
  • CFLAGS = "-I/opt/include" is the same as: export CPATH=/opt/include (https://gcc.gnu.org/onlinedocs/cpp/Environment-Variables.html)
  • LDLFLAGS = "-L/opt/include" is the same as: export LIBRARY_PATH=/opt/include (https://gcc.gnu.org/onlinedocs/gcc/Environment-Variables.html)
    • LIBRARY_PATH is used by gcc before compilation to search directories containing static and shared libraries that need to be linked to your program
  • LDLFLAGS = "-rpath=/opt/lib" is the same as: export LD_LIBRARY_PATH = "/opt/lib LIBRARY_PATH, but not the same, below is a good explanation (from https://stackoverflow.com/questions/4250624/ld-library-path-vs-library-path);
    • LD_LIBRARY_PATH is used by your program to search directories containing shared libraries after it has been successfully compiled and linked.