OpenWrt's build system – About

The OpenWrt build system is a set of Makefiles and patches that allows users to easily generate both a cross-compilation toolchain and a root filesystem for embedded systems. The cross-compilation toolchain uses musl, a tiny C standard library.

A compilation toolchain is the set of tools used to compile code for your system. It consists of:

a compiler (in our case, gcc / deb: gcc)
binary utils like assembler and linker (in our case, binutils / deb: binutils )
a C standard library (for example GNU Libc, musl-libc, uClibc or dietlibc).

Using a PC, the compilation toolchain runs on an x86 processor and generates code for a x86 processor. On most Linux systems, the compilation toolchain uses the GNU libc as C standard library. It is called the "host compilation toolchain", and the machine it is running on is called the "host system". The compilation toolchain is provided by the distribution, and has nothing to do with the OpenWrt build system.

Embedded systems use a different processor and require a cross-compilation toolchain - a compilation toolchain that runs on a host system but that generates code for a target system (and target processor's instruction set architecture (ISA)). For example, if your host system uses x86 and your target system uses MIPS32, the regular compilation toolchain of your host runs on x86 and generates code for x86, while the cross-compilation toolchain runs on x86 and generates code for MIPS32.

While it is possible to manually configure and compile your own software, OpenWrt's build system automates this process to work on the instruction set architecture of most embedded systems.

While the OpenWrt's build system was designed for developers, inexperienced users can also use it to easily build their own custom firmware!

The OpenWrt Makefile has its own syntax, different from the conventional Makefile of Linux make tool. The OpenWrt Makefile defines the meta information of the package, where to download the package, how to compile, where to install the compiled binaries, etc. See How to Build OpenWrt Application Package for more detail.

OpenWrt build system – Features

Makes it easy to port software
Uses kconfig (Linux Kernel menuconfig) for configuration of features
Provides integrated cross-compiler toolchain (gcc, ld, …)
Provides abstraction for autotools (automake, autoconf), cmake, scons
Handles standard download, patch, configure, compile and packaging workflow
Provides a number of common fixups for badly behaving packages

OpenWrt build system – Make Targets

Offers a number of high level make targets for standard package workflows
Targets always in the format "component/name/action", e.g. "toolchain/gdb/compile" or "package/mtd/install"
Prepare a package source tree: package/foo/prepare
Compile a package: package/foo/compile
Clean a package: package/foo/clean

OpenWrt build system – Build sequence

tools – automake, autoconf, sed, cmake
toolchain/binutils – as, ld, …
toolchain/gcc – gcc, g++, cpp, …
target/linux – kernel modules
package – core and feed packages
target/linux – kernel image
target/linux/image – firmware image file generation

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Patch management

Many packages will not work as-is and need patches to work on the target or to even compile
OpenWrt build system integrates quilt for easy patch management
Turn package patches into quilt series: make package/foo/prepare QUILT=1
Update patches from modified series: make package/foo/update
Automatically rebase patches after an update: make package/foo/refresh

Packaging considerations

Main objective is small memory and size footprint
Features that make no sense on embedded systems get disabled through configure or are patched out
Packages must be compilable regardless of the host system, should be self contained
Shipped "configure" scripts are often faulty or unusable in a cross-compile setting, autoreconf or patching needed
Build variants and kconfig includes allow for configurable compile-time settings
There is no standard way for porting software, in many cases it "just works" but often the package build process needs tweaks

OpenWrt's build system

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