OpenWRT之HelloWorld

    前些日子又搞起了OpenWRT，之前一直只是使用shell或者Lua实现自己的想法，随着C渐渐使用的顺畅了，打算学习下OpenWRT上程序的编写以及交叉编译。
首先列下目录结构

lee@ubuntu:~/OpenWrt-SDK-ar71xx-for-linux-i486-gcc-4.6-linaro_uClibc-0.9.33.2/package$ tree
 .
 |-- hiOpenWRT
 |   |-- Makefile
 |   `-- src
 |       |-- hiOpenwrt.c
 |       `-- Makefile
 |-- Makefile
 `-- rules.mk

    这是在OpenWRT-SDK/package下的文件结构，需要自己建的目录有hiOpenwrt/ 和hiOpenwrt/src/ ；需要自己建的文件有:
src下的hiOpenwrt.c

#include <stdlib.h>
 #include <stdio.h>
int main(void){
     printf("Hi,OpenWRT!\n");
     return 0;
}

src下Makefile：src下Makefile：

# build hiOpenwrt executable when user executes "make"
 hiOpenwrt: hiOpenwrt.o
 $(CC) $(LDFLAGS) hiOpenwrt.o -o hiOpenwrt

 hiOpenwrt.o: hiOpenwrt.c
 $(CC) $(CFLAGS) -c hiOpenwrt.c

 #remove object files and executable when user executes "make clean"
 clean:
 rm *.o hiOpenwrt

当然，在hiOpenwrt/Makefile也需要写，只不过这个要求比较多，格式如下：

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#-----官方文档如下
This is the OpenWrt SDK. It contains a stripped-down version of the buildroot. You can use it to test/develop packages without having to compile your own toolchain or any of the libraries included with OpenWrt.
 To use it, just put your buildroot-compatible package directory in the subdir 'package/' and run 'make' from this directory.


 #------ OPENWRT集成非官方包之Makefile规则
include $(TOPDIR)/rules.mk


 PKG_NAME:=[软件包名字 和文件夹名称一样]
PKG_VERSION:=[软件包版本 自己写个]
PKG_RELEASE:=1
 PKG_BUILD_DIR := $(BUILD_DIR)/$(PKG_NAME)
 include $(INCLUDE_DIR)/package.mk
 define Package/$(PKG_NAME)
 SECTION:=utils
 CATEGORY:=[软件包在menuconfig里的位置 比如Base system]
 DEPENDS:=[依赖包 两个之间通过空格分隔 前面加+为默认显示 选中该软件包自动选中依赖包 不加+为默认不显示 选中依赖包才显示]
TITLE:=[标题]
PKGARCH:=[平台 比如ar71xx 全部写all]
 MAINTAINER:=[作者]
endef
 define Package/$(PKG_NAME)/description
 [软件包简介]
endef

 #非本目录下的源码文件, 拷贝到此相应目录下.

 # 如../../xucommon/xucommon.c, 则将 xucommon.c 拷贝到此目录下的源码的 ../../
define Build/Prepare
 mkdir -p $(PKG_BUILD_DIR)
 $(CP) ./src/* $(PKG_BUILD_DIR)/
 endef

 define Build/Configure
 endef

 define Build/Compile
 endef

 define Package/$(PKG_NAME)/conffiles
 [升级时保留文件/备份时备份文件 一个文件一行]
endef

 define Package/$(PKG_NAME)/install
 $(CP) ./files/* $(1)/
 endef

 define Package/$(PKG_NAME)/preinst
 [安装前执行的脚本 记得加上#!/bin/sh 没有就空着]
 #!/bin/sh
 uci -q batch <<-EOF >/dev/null
 delete ucitrack.@aria2[-1]
 add ucitrack aria2
 set ucitrack.@aria2[-1].init=aria2
 commit ucitrack
 EOF
 exit 0
 endef

 define Package/$(PKG_NAME)/postinst
 [安装后执行的脚本 记得加上#!/bin/sh 没有就空着]
 #!/bin/sh
 rm -f /tmp/luci-indexcache
 exit 0
 endef

 Package/$(PKG_NAME)/prerm
 [删除前执行的脚本 记得加上#!/bin/sh 没有就空着]
endef
 Package/$(PKG_NAME)/postrm
 [删除后执行的脚本 记得加上#!/bin/sh 没有就空着]
endef
 $(eval $(call BuildPackage,$(PKG_NAME)))

自己的hiOpenwrt的Makefile自己的hiOpenwrt的Makefile

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##############################################
 # OpenWrtMakefile for hiOpenwrt program
 #
 #
 # Most ofthe variables used here are defined in
 # theinclude directives below. We just need to
 # specifya basic description of the package,
 # whereto build our program, where to find
 # thesource files, and where to install the
 # compiled program on the router.
 #
 # Be verycareful of spacing in this file.
 # Indentsshould be tabs, not spaces, and
 # thereshould be no trailing whitespace in
 # linesthat are not commented.
 #
 ##############################################
 include $(TOPDIR)/rules.mk
 # Nameand release number of this package
 PKG_NAME:=hiOpenWRT
 PKG_RELEASE:=1

 # Thisspecifies the directory where we're going to build the program.
 # Theroot 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

 # Specifypackage information for this program.
 # Thevariables defined here should be self explanatory.
 # If youare running Kamikaze, delete the DESCRIPTION
 #variable below and uncomment the Kamikaze define
 # directivefor the description below
 define Package/$(PKG_NAME)
 SECTION:=utils
 CATEGORY:=Utilities
 TITLE:=hiOpenwrt-- prints a snarky message
 endef

 # Specifywhat needs to be done to prepare for building the package.
 # In ourcase, we need to copy the source files to the build directory.
 # This isNOT the default.  The default uses thePKG_SOURCE_URL and the
 #PKG_SOURCE which is not defined here to download the source from the web.
 # Inorder to just build a simple program that we have just written, it is
 # mucheasier to do it this way.
 define Build/Prepare
 mkdir -p $(PKG_BUILD_DIR)
 $(CP) ./src/* $(PKG_BUILD_DIR)/
 endef


 # We donot need to define Build/Configure or Build/Compile directives
 # Thedefaults are appropriate for compiling a simple program such as this one


 # Specifywhere and how to install the program. Since we only have one file,
 # thehelloworld executable, install it by copying it to the /bin directory on
 # therouter. 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 $(INSTALL_BIN) contains the
 # commandto copy the binary file from its current location (in our case the build
 #directory) to the install directory.
 define Package/$(PKG_NAME)/install
 $(INSTALL_DIR) $(1)/bin
 $(INSTALL_BIN) $(PKG_BUILD_DIR)/hiOpenwrt $(1)/bin/
 endef


 # Thisline executes the necessary commands to compile our program.
 # Theabove define directives specify all the information needed, but this
 # linecalls BuildPackage which in turn actually uses this information to
 # build apackage.
 $(eval $(call BuildPackage,$(PKG_NAME)))

生成ipk的命令：

make package/hiOpenWRT/compile V=s

加上V=s的目的是为了详细打印。

生成出来的ipk文件将在bin/ar71xx/packages/下，同时，也可以在build_dir/ build_dir/下找到可执行文件以及源码。

lee@ubuntu:~/OpenWrt-SDK-ar71xx-for-linux-i486-gcc-4.6-linaro_uClibc-0.9.33.2$ tree | head -n 20
 .
 |-- bin
 |   |-- ar71xx
 |   |   `-- packages
 |   |       `-- hiOpenWRT_1_ar71xx.ipk
 |   `-- packages
 |-- build_dir
 |   `-- build_dir
 |       `-- hiOpenWRT
 |           |-- hiOpenwrt
 |           |-- hiOpenwrt.c
 |           |-- hiOpenwrt.o
 |           |-- ipkg-ar71xx
 |           |   `-- hiOpenWRT
 |           |       |-- bin
 |           |       |   `-- hiOpenwrt
 |           |       `-- CONTROL
 |           |           `-- control
 |           `-- Makefile
 |-- Config.in

一点总结：

    目录结构很重要，如果目录结构不正确，那么Openwrt是不可能正确编译出ipk的。
    在学习编译过程中出现了很多处错误，最让我头疼的是Makefile的格式，这部分很容易错误而编译失败。
Makefile编写规则：

target ... : prerequisites ...
 <tab>command
 ...
 ...

target也就是一个目标文件，可以是Object File，也可以是执行文件。还可以是一个标签
（Label），对于标签这种特性，在后续的“伪目标”章节中会有叙述。
prerequisites就是，要生成那个target所需要的文件或是目标。
command也就是make需要执行的命令。（任意的Shell命令）

这是一个文件的依赖关系，也就是说，target这一个或多个的目标文件依赖于prerequisi
tes中的文件，其生成规则定义在command中。说白一点就是说，prerequisites中如果有一个以上的文件比target文件要新的话，command所定义的命令就会被执行。这就是 Makefile的规则。也就是Makefile中最核心的内容。