Cross compile applications for OpenWrt

In the previous article Build your own OpenWrt firmware for armsr target, We have seen how to build OpenWrt firmware for armsr target. OpenWrt is emulated using qemu for armsr target with the custom generated image.

In this article, we can explore how to pack and release OpenWrt toolchain as a tarball using which OpenWrt application can be developed without the need for complete OpenWrt build system. We can also explore about how a simple hello_world.c program can be cross compiled using the released OpenWrt toolchain and how to execute the pre-compiled binary in the armsr target.

Cross-compilation - A brief intro

Cross-compilation is the process of building software on your development environment (like PC) to run on different architecture like embedded systems. OpenWrt runs on embedded routers with different hardware architectures like ARM, MIPS, x86 and so on.

OpenWrt helps in providing the cross-compilation toolchain as a separate package since Application developers doesn’t need the complete build system to develop and test their applications in their Target.

Packing OpenWrt Toolchain

OpenWrt toolchain can be packed as a tarball by enabling the below package in the menuconfig command.

make menuconfig -> Package the OpenWrt-based Toolchain (CONFIG_MAKE_TOOLCHAIN)

Enabling the config will help to generate the toolchain tarball in the target’s bin directory

• armsr Toolchain artifacts : bin/targets/armsr/armv8/openwrt-toolchain-armsr-armv8_gcc-13.3.0_musl.Linux-x86_64.tar.zst

Unpacking the OpenWrt Toolchain

• Download the OpenWrt toolchain tarball into the development environment

• Unpack the OpenWrt toolchain using the command

tar --zstd -xvf openwrt-toolchain-armsr-armv8_gcc-13.3.0_musl.Linux-x86_64.tar.zst

• After extracting the files, Cross-compilation Toolchain relate files are available in below path

openwrt-toolchain-armsr-armv8_gcc-13.3.0_musl.Linux-x86_64/toolchain-aarch64_generic_gcc-13.3.0_musl/
|-- aarch64-openwrt-linux -> aarch64-openwrt-linux-musl
|-- aarch64-openwrt-linux-musl
|-- bin
|-- include
|-- info.mk
|-- lib
|-- lib32 -> lib
|-- lib64 -> lib
|-- libexec
|-- share
`-- usr

Cross Compile hello world for armsr-armv8 target

• Create a Simple C program - hello_world.c

#include <stdio.h>

int main()
{
	printf("Hello World - Welcome to OpenWrt\n");

	return 0;
}

• Export Toolchain’s bin directory path

$ export PATH=~/Downloads/openwrt-toolchain-armsr-armv8_gcc-13.3.0_musl.Linux-x86_64/toolchain-aarch64_generic_gcc-13.3.0_musl/bin/:$PATH

• cross compile the hello_world.c program using the OpenWrt toolchain for armsr target

$ aarch64-openwrt-linux-gcc ~/Downloads/hello_world.c -o ~/Downloads/armsr_hello_world

• Verify the generated binary format using file command

$ file ~/Downloads/armsr_hello_world
/home/user/Downloads/armsr_hello_world: ELF 64-bit LSB executable, ARM aarch64, version 1 (SYSV), dynamically linked, interpreter /lib/ld-musl-aarch64.so.1, with debug_info, not stripped

Validate hello world binary

• In the development environment/Host machine (like PC), Emulate OpenWrt for armsr target using qemu

• Find the Gateway (or) host machine’s IP address using the route -n command

root@OpenWrt:~# route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         10.0.2.2        0.0.0.0         UG    0      0        0 eth1
.... skipped ....

• Verify whether the host machine is reachable from qemu using the ping command

root@OpenWrt:~# ping 10.0.2.2
PING 10.0.2.2 (10.0.2.2): 56 data bytes
64 bytes from 10.0.2.2: seq=0 ttl=255 time=0.745 ms

• Copy the generated binary from the host machine into the qemu using the scp command

root@OpenWrt:~#  scp user@10.0.2.2:~/Downloads/armsr_hello_world ~/

• Execute the generated binary in the armsr target

root@OpenWrt:~# ~/armsr_hello_world
Hello World - Welcome to OpenWrt

Future Explorations in OpenWrt

With a simple hello_world.c program cross-compiled and running in the OpenWrt target, It opens up a way to build, modify and test applications using the OpenWrt toolchain without the need for a complete build system. Pre-built OpenWrt toolchain for different target architectures are also available in OpenWrt using which applications can be developed for the specific architectures.