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;
- GCC:
  - opkg install gcc
- Make: https://man7.org/linux/man-pages/man1/make.1.html
  - opkg install make
- Entware recommends installing the following;
  - opkg install busybox ldd make gawk sed (ldd and sed are not available as separate packages in OpenWRT, but both are included within the OpenWRT full version of BusyBox)
- Additional utilities are recommended
  - Install the full version of BASH: opkg install bash
Environment Variables to add (these can be added in a myriad of ways, see the end of this section;
- export LDFLAGS="-Wl,-rpath=/usr/lib:/opt/lib -Wl,--dynamic-linker=/usr/lib/gcc/arm-openwrt-linux-muslgnueabi/7.4.0/ld-musl-armhf.so.1 -L/usr/lib:/opt/lib:/opt/include
- NOTE: The original DD-WRT / Entware version is this: export LDFLAGS="-Wl,-rpath=/opt/lib -Wl,--dynamic-linker=/opt/lib/ld-linux.so.3 -L/opt/lib", and needs to be modified for OpenWRT as shown above.
  - -WI is a switch that allows for additional items to be added via the GNU LD (https://stackoverflow.com/questions/6562403/i-dont-understand-wl-rpath-wl) command
  - -rpath in LDFLAGS = "-rpath=/usr/lib:/opt/lib" is the same as executing this on the command line: export LD_LIBRARY_PATH = "/opt/lib
    - rpath AND/OR LD_LIBRARY_PATH is a path used by an executable program to search directories containing shared libraries after the program has been compiled (from https://en.wikipedia.org/wiki/Rpath and https://stackoverflow.com/questions/4250624/ld-library-path-vs-library-path)
    - OpenWRT has its own equivalent path, which is /usr/lib, however there are some variations in files and sizes. Solution? Include both paths (from https://www.linuxtopia.org/online_books/an_introduction_to_gcc/gccintro_24.html);
      - -rpath=/usr/lib:/opt/lib (this will search OpenWRT's /usr/lib directory first, then DD-WRT / Entware's /opt/lib directory second)
  - --dynamic-linker
    - For DD-WRT / Entware, the GNU LD file is located here: /opt/lib/ld-linux.so.3, which is a symbolic link to ld-2.27.so (and here too: /lib/gcc/arm-openwrt-linux-gnueabi/7.4.0)
    - OpenWRT seems to have an equivalent GNU LD file located here: /usr/lib/gcc/arm-openwrt-linux-muslgnueabi/7.4.0/ld-musl-armhf.so.1, so use that path instead (from https://community.arm.com/developer/ip-products/processors/b/processors-ip-blog/posts/file-not-found-when-executing-assembled-program)
      - --dynamic-linker /usr/lib/gcc/arm-openwrt-linux-muslgnueabi/7.4.0/ld-musl-armhf.so.1
    - LD is not the same as GNU LD (https://en.wikipedia.org/wiki/Linker_(computing)#GNU_linker)
  - -L in LDFLAGS = "-L/usr/lib:/opt/lib:/opt/include" is the same as executing this on the command line: export LIBRARY_PATH=/usr/lib:/opt/lib:/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 (from https://stackoverflow.com/questions/4250624/ld-library-path-vs-library-path)
- export CFLAGS="-O2 -pipe -march=armv7-a -mtune=cortex-a9 -fno-caller-saves -mfloat-abi=soft -I/opt/include "
- NOTE: The original DD-WRT / Entware version is this: export CFLAGS="-O2 -pipe -march=armv7-a -mtune=cortex-a9 -fno-caller-saves -mfloat-abi=soft ", and needs to be modified for OpenWRT as shown above.
  - -I in CFLAGS = "-I/opt/include" is the same as: export CPATH=/opt/include (from https://gcc.gnu.org/onlinedocs/cpp/Environment-Variables.html)
  - NOTE: that's an "I" (eye) in the above command, as in the word "Inside, Inner, Implode, etc., not a "l" (el), as in the word Little, Lime, etc.
- Adding the above Environment Variables;
  - Run each of the below command lines
    - export LDFLAGS="-Wl,-rpath=/usr/lib:/opt/lib -Wl,--dynamic-linker=/usr/lib/gcc/arm-openwrt-linux-muslgnueabi/7.4.0/ld-musl-armhf.so.1 -L/usr/lib:/opt/lib:/opt/include
    - export CFLAGS="-O2 -pipe -march=armv7-a -mtune=cortex-a9 -fno-caller-saves -mfloat-abi=soft -I/opt/include "
  - Put the above two EXPORT commands in a single file, and run the following command;
    - source WhatEverFileName
  - Add the above two EXPORT commands to a user's profile
    - nano ~/.profile
    - NOTE: For the above to work, the full BASH shell must be installed (opkg install bash) AND the ~/.profile file must include the directive: SHELL=/bin/bash
...as far as the Entware documentation for Compiling: https://github.com/Entware/Entware/wiki/Using-GCC-for-native-compilation
- Additional information for the ./configure --prefix /opt command, and what --prefix means: https://askubuntu.com/questions/891835/what-does-prefix-do-exactly-when-used-in-configure
- Although they offer a link and a command for downloading additional libraries to /opt/include, there doesn't appear to be any notation of where that is utilized. Just having it as a bunch of files in a directory is useless unless a compiler like GCC can make use of the files. It should be
ELF Definition: https://en.wikipedia.org/wiki/Executable_and_Linkable_Format

Example

Create a file named ex.sh: nano ex.sh
Add the following directives (from the above section) and save it;

export LDFLAGS="-Wl,-rpath=/usr/lib:/opt/lib -Wl,--dynamic-linker=/usr/lib/gcc/arm-openwrt-linux-muslgnueabi/7.4.0/ld-musl-armhf.so.1 -L/usr/lib:/opt/lib:/opt/include

export CFLAGS="-O2 -pipe -march=armv7-a -mtune=cortex-a9 -fno-caller-saves -mfloat-abi=soft -I/opt/include "

Create a "Hello World" source code file: nano hw.c
Add the following text and save it;

#include <stdio.h>    

int main(void)    
{
printf("\nHello, world!\n\n");        
return 0;        
}

The below two commands are from an OpenWRT example here: https://openwrt.org/docs/guide-developer/helloworld/chapter2

Type this command: gcc -c -o hw.o hw.c -Wall
Type this command: gcc -o hw hw.o

...and then run it: ./hw, which should produce the below output;

Hello, world!

Compiling on OpenWRT

Example

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