OpenWrt: Add Hello World Application

This document describes how to create, compile, package, and install a minimal “Hello World” application inside OpenWrt. The workflow matches the OpenWrt buildroot package model, including directory layout, build phases, Makefile definitions, menuconfig selection, package compilation, installation, and runtime verification.

In this section, you are going to learn

How to create a new application package inside OpenWrt buildroot?

How OpenWrt prepares, compiles, and installs application binaries using its cross‑toolchain?

How to select the application in menuconfig and build only the package?

How to install the generated package inside a QEMU-based OpenWrt environment?

Topics in this section,

Create a new package directory inside the OpenWrt buildroot:

~/openwrt-build/openwrt/

Run:

mkdir -p package/helloworld

OpenWrt will detect this folder automatically during menuconfig and package builds.

  • Every OpenWrt package lives under package/<name>/.

  • The package name becomes the IPK identifier.

  • OpenWrt buildroot is Makefile-driven and automatically discovers packages placed under the package/ hierarchy.

Create the Makefile:

package/helloworld/Makefile

Contents:

include $(TOPDIR)/rules.mk

PKG_NAME:=helloworld
PKG_RELEASE:=1

PKG_BUILD_DIR := $(BUILD_DIR)/$(PKG_NAME)

include $(INCLUDE_DIR)/package.mk

define Package/helloworld
  SECTION:=utils
  CATEGORY:=Utilities
  TITLE:=Hello World Program
endef

define Package/helloworld/description
 A simple hello world application for OpenWrt testing.
endef

define Build/Prepare
    mkdir -p $(PKG_BUILD_DIR)
    $(CP) ./src/* $(PKG_BUILD_DIR)/
endef

define Package/helloworld/install
    $(INSTALL_DIR) $(1)/usr/bin
    $(INSTALL_BIN) $(PKG_BUILD_DIR)/helloworld $(1)/usr/bin/
endef

$(eval $(call BuildPackage,helloworld))

  • PKG_BUILD_DIR is where OpenWrt compiles the program using its cross‑compiler.

  • Build/Prepare copies source files to the isolated build directory.

  • Package/helloworld/install describes installation into the OpenWrt rootfs.

  • BuildPackage registers and integrates the package with OpenWrt’s package system.

Create the source directory:

mkdir -p package/helloworld/src

Create the file:

package/helloworld/src/helloworld.c

Contents:

#include <stdio.h>

int main() {
    printf("Hello from OpenWrt!\n");
    return 0;
}

  • OpenWrt compiles using TARGET_CC targeting your architecture (e.g., x86_64).

  • Compiled output becomes $(PKG_BUILD_DIR)/helloworld.

  • Applications typically install into /usr/bin or /usr/sbin.

OpenWrt requires explicitly selecting the package:

make menuconfig

Navigate:

Utilities  --->
    <M> helloworld

Save and exit.

  • <M> marks the package as a module, generating a standalone IPK.

  • Avoids rebuilding a full firmware image.

  • Regenerates .config, integrating the package into the build pipeline.

Build the package without rebuilding the entire firmware:

make package/helloworld/{clean,compile} V=s

Output package appears under:

bin/packages/x86_64/base/helloworld*.apk

  • clean wipes previous build artifacts.

  • compile runs Prepare, Compile, Install, and package creation.

  • Output directories vary per architecture.

Copy the built package into a running OpenWrt VM:

scp -P 2223 bin/packages/x86_64/base/helloworld*.apk root@127.0.0.1:/tmp/

Install it inside OpenWrt:

apk add --allow-untrusted /tmp/helloworld*.apk

  • --allow-untrusted bypasses signature verification (normal for test packages).

  • apk extracts files exactly per the Makefile install section.

  • Binary is installed under /usr/bin/helloworld.

Execute the installed program:

helloworld

Expected Output:

Hello from OpenWrt!

  • Confirms successful compile + install.

  • Verifies correct OpenWrt toolchain and rootfs integration.