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：

#-----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

##############################################
# 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 ...
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。



参考资料
http://www.cnblogs.com/laojie4321/archive/2012/03/27/2420241.html
http://blog.chinaunix.net/uid-27194309-id-3432651.html