Tinkering OpenWRT (12) : Cross-Compile the Helloworld Program

Step 1: Download the SDK 

Download
OpenWrt-SDK-ar71xx-for-linux-x86_64-gcc-4.8-linaro_uClibc-0.9.33.2.tar.bz2

from

http://archive.openwrt.org/barrier_breaker/14.07/ar71xx/generic/

Two things you need to know. The openWRT version running on your board, in my case, barrier_breaker. The chipset of your board, in my case, ar9331.

Step 2: Create the working structure

cd package
mkdir helloworld
cd helloworld
mkdir src
cd src
touch helloworld.c
touch Makefile

tree:

OpenWrt-SDK-ar71xx-.../package/helloworld
├── Makefile
└── src
    ├── helloworld.c
    └── Makefile

The Makefile in /src/ is just local compiling for checking if there is any coding errors.
The Makefile ourside /src/ is the real cross-compiling makefile.

Step 3: coding

~/helloworld/src/helloworld.c:

#include <stdio.h> int main(void) { printf("Hell! O' world, why won't my code compile?\n\n"); return 0; }

~/helloworld/src/Makefile:

# build helloworld executable when user executes "make" helloworld: helloworld.o $(CC) $(LDFLAGS) helloworld.o -o helloworld helloworld.o: helloworld.c $(CC) $(CFLAGS) -c helloworld.c # remove object files and executable when user executes "make clean" clean: rm *.o helloworld

Step 3: Cross-compiling Makefile

##############################################
# OpenWrt Makefile for helloworld program
#
#
# Most of the variables used here are defined in
# the include directives below. We just need to
# specify a basic description of the package,
# where to build our program, where to find
# the source files, and where to install the
# compiled program on the router.
#
# Be very careful of spacing in this file.
# Indents should be tabs, not spaces, and
# there should be no trailing whitespace in
# lines that are not commented.
#
##############################################
 
include $(TOPDIR)/rules.mk
 
# Name and release number of this package
PKG_NAME:=helloworld
PKG_RELEASE:=1
 
 
# This specifies the directory where we're going to build the program. 
# The root build directory, $(BUILD_DIR), is by default the build_mipsel
# directory in your OpenWrt SDK directory
PKG_BUILD_DIR := $(BUILD_DIR)/$(PKG_NAME)
 
 
include $(INCLUDE_DIR)/package.mk
 
 
 
# Specify package information for this program.
# The variables defined here should be self explanatory.
# If you are running Kamikaze, delete the DESCRIPTION
# variable below and uncomment the Kamikaze define
# directive for the description below
define Package/helloworld
 SECTION:=utils
 CATEGORY:=Utilities
 TITLE:=Helloworld -- prints a snarky message
endef
 
 
# Uncomment portion below for Kamikaze and delete DESCRIPTION variable above
define Package/helloworld/description
        If you can't figure out what this program does, you're probably
        brain-dead and need immediate medical attention.
endef
 
 
 
# Specify what needs to be done to prepare for building the package.
# In our case, we need to copy the source files to the build directory.
# This is NOT the default.  The default uses the PKG_SOURCE_URL and the
# PKG_SOURCE which is not defined here to download the source from the web.
# In order to just build a simple program that we have just written, it is
# much easier to do it this way.
define Build/Prepare
 mkdir -p $(PKG_BUILD_DIR)
 $(CP) ./src/* $(PKG_BUILD_DIR)/
endef
# We do not need to define Build/Configure or Build/Compile directives
# The defaults are appropriate for compiling a simple program such as this one
# Specify where and how to install the program. Since we only have one file,
# the helloworld executable, install it by copying it to the /bin directory on
# the router. The $(1) variable represents the root directory on the router running
# OpenWrt. The $(INSTALL_DIR) variable contains a command to prepare the install
# directory if it does not already exist.  Likewise $(INSTALL_BIN) contains the
# command to copy the binary file from its current location (in our case the build
# directory) to the install directory.
define Package/helloworld/install
 $(INSTALL_DIR) $(1)/bin
 $(INSTALL_BIN) $(PKG_BUILD_DIR)/helloworld $(1)/bin/
endef
# This line executes the necessary commands to compile our program.
# The above define directives specify all the information needed, but this
# line calls BuildPackage which in turn actually uses this information to
# build a package.
$(eval $(call BuildPackage,helloworld))

Step 4: Compile

Go to the root directory of the compiler and make:

make V=s

Step 5: Find the .ipk

If the compiling passes, you can find helloworld_1_ar71xx.ipk in

OpenWrt-SDK-ar71xx-for-linux-i686-gcc-4.6-linaro_uClibc-0.9.33.2/bin/ar71xx/packages/base

Step 6: upload the .ipk to the board

scp helloworld_1_ar71xx.ipk root@192.168.0.24:/root/

Step 7: Install the package

opkg install helloworld_1_ar71xx.ipk

Step 8: run the program
just run

$ helloworld

or

$ /bin/helloworld

or

check it with
$ opkg list

ref:
http://archive.openwrt.org/barrier_breaker/14.07/ar71xx/generic/
https://www.gargoyle-router.com/old-openwrt-coding.html
https://blog.csdn.net/luck_good/article/details/23517135
http://www.uubook.net/article/detail/116354.html