$ mkdir helloworld
$ mkdir helloworld/src

helloworld/src目录下创建源码文件helloworld.c：

#include <stdio.h>

int main(int argc, char** argv)
{
    printf("hello world!\n");
    
    return 0;
}

helloworld/src目录下创建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

在helloworld目录下创建Makefile文件：

##############################################
# 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))

Makefile语法可参考官网：http://wiki.openwrt.org/doc/devel/packages

编译：

$ make package/helloworld/compile -j1 V=s
$ ls bin/ramips/packages/base/helloworld_1_ramips_24kec.ipk 
bin/ramips/packages/base/helloworld_1_ramips_24kec.ipk

安装：

root@OpenWrt:/# opkg install helloworld_1_ramips_24kec.ipk
Installing helloworld (1) to root...
Configuring helloworld.

运行：

root@OpenWrt:/# helloworld
hello world!

参考文章

Openwrt之helloworld
openWRT自学---如何开发新的用户态模块-helloworld
用CMakeLists.txt替代makefile创建openwrt软件包