这次讲讲openwrt的结构.
1.代码上来看有几个重要目录package, target, build_root, bin, dl....
---build_dir/host目录是建立工具链时的临时目录
---build_dir/toolchain-是对应硬件的工具链的目录
---staging_dir/toolchain- 则是工具链的安装位置
---target/linux/目录里面是各个平台(arch)的相关代码
---target/linux//config-3.10文件就是配置文件了
---dl目录是'download'的缩写, 在编译前期，需要从网络下载的数据包都会放在这个目录下，这些软件包的一个特点就是，会自动安装在所编译的固件中，也就是我们make menuconfig的时候，为固件配置的一些软件包。如果我们需要更改这些源码包，只需要将更改好的源码包打包成相同的名字放在这个目录下，然后开始编译即可。编译时，会将软件包解压到build_dir目录下。
---而在build_dir/目录下进行解压，编译和打补丁等。
---package目录里面包含了我们在配置文件里设定的所有编译好的软件包。默认情况下，会有默认选择的软件包。在openwrt中ipk就是一切, 我们可以使用
$ ./scripts/feeds update来对软件包进行更新.
$ ./scripts/feeds search nmap 查找软件包'nmap'
Search results in feed ’packages’:
nmap Network exploration and/or security auditing utility
$ ./scripts/feeds install nmap 安装'nmap'这个软件
$ make package/symlinks //估计意思是更新软件源之类的
---bin目录下生成了很多bin文件，根据不同的平台来区分。另外bin//package目录，里面有很多ipk后缀的文件，都是package目录下的源码在build_dir目录下编译后的生成的结果。
2.新建自己的packages
对于自己新建的package，而这个package又不需要随固件一起安装，换句话说，就是可以当做一个可选软件包的话。我们可以利用我们的SDK环境来单独编译，编译后会生成一个ipk的文件包。然后利用 opkg install xxx.ipk 来安装这个软件。
下面具体说下，如何编译一个helloword的软件包。
（1）首先，编写helloworld程序
编写helloworld.c
/****************
* Helloworld.c
* The most simplistic C program ever written.
* An epileptic monkey on crack could write this code.
*****************/
#include <stdio.h>
#include <unistd.h>
int main(void)
{
printf("Hell! O' world, why won't my code compile?\n\n");
return 0;
}
编写Makefile文件
# 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
在这两个文件的目录下，执行make 应该可以生成helloworld的可执行文件。执行helloworld后，能够打印出“Hell!O' world, why won't my code compile?”。 这一步，主要保证我们的源程序是可以正常编译的。
下面我们将其移植到OpenWRT上。
（2）将OpenWrt-SDK-brcm47xx-for-Linux-x86_64-gcc-4.3.3+cs_uClibc-0.9.30.1.tar.bz2解压
tar –xvf OpenWrt-SDK-brcm47xx-for-Linux-x86_64-gcc-4.3.3+cs_uClibc-0.9.30.1.tar.bz2
（3）进入SDK
cd OpenWrt-SDK-brcm47xx-for-Linux-x86_64-gcc-4.3.3+cs_uClibc-0.9.30.1
可以看到里面的目录结构跟我们之前source的目录结构基本相同，所需要编译的软件包，需要放置在package目录下
（4）在package目录下创建helloworld目录
cd package
mkdir helloworld
cd helloworld
（5）创建src目录，拷贝 helloworld文件
mkdir src
cp /home/wrt/test/helloworld.c src
cp /home/wrt/test/Makefile src
（6）在helloworld目录下创建Makefile文件
这个Makefile文件是给OpenWRT读的，而之前写的那个Makefile文件是针对helloworld给编译其读的。两个Makefile不在同一层目录下。
touch Makefile
vim Makefile
Makefile文件模板内容如下：
##############################################
# 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))
（7）返回到SDK的根目录
执行make进行编译
编译过程会在build_dir目录下完成
编译结果会放在 bin/[yourtarget]/package目录下helloworld_1_bcm47xx.ipk
（8）上传helloworld_1_bcm47xx.ipk
上传helloworld_1_bcm47xx.ipk至路由器
执行# opkg install helloworld_1_bcm47xx.ipk
然后输入hello然后按Tab键，发现openwrt中已经有helloworld可执行命令。
执行 helloworld命令来查看程序的效果。
Hell! O' world, why won't my code compile?
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