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Build systems.md

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2020-08-26_08:28:16

Build systems

A "build system" in this text is a compiler front-end / runner along with project-specific files that describe how the different parts of the project should be built. Examples of build systems are Visual Studio Solutions, Makefiles, and in some sense also CMake projects. Unreal Engine can generate project files for various build systems to make building C++ projects easier. So that one doesn't need to manually call Build.sh or Unreal Build Tool.

Project files for several build systems can be generated for a project. On Windows people seem to generally use Visual Studio. On Linux there is a bit more variability, but common options are Makefiles and CMake. Project files are generated by running the GenerateProjectFiles.sh script. It's located in the root of the Unreal Engine working copy. Pass it the path to the .uproject file and parameters controlling how and for what to generate project files. Example:

./GenerateProjectFiles.sh <PATH>/<PROJECT>.uproject -game -Makefile -CMakefile

The above will generate a Makefile and a CMakeLists.txt in <PATH>,

Unreal Engine 4.25 introduced a warning when passing project type parameters on the command line. Recommends setting these in BuildConfiguration.xml instead. On Linux this file is at ~/.config/Unreal Engine/UnrealBuildTool/.

<?xml version="1.0" encoding="utf-8" ?>
<Configuration xmlns="https://www.unrealengine.com/BuildConfiguration">
  <ProjectFileGenerator>
    <Format>Make</Format>
    <Format>CMake</Format>
  </ProjectFileGenerator>
</Configuration>

This doesn't work.

Fixing red squiggles

A bug introduced in Unreal Engine 4.25 broke IDE, e.g. CLion, integration. Someone has prepared engine source fixes patch patches for it:

In short, the problem is in Engine/Source/Programs/UnrealBuildTool/Configuration/UEBuildModuleCPP.cs and the cause is that CompileEnvironment.Definitions is cleared too early.

Symptoms are Unreal Engine macros not being recognized, missing class definitions, undeclared definitions.

Parallel compilation

From https://discord.com/channels/187217643009212416/375022233875382274/971258915906007040

If you want to control the number of jobs then you need to configure UBT by setting the appropriate options in its XML config file (which is called `BuildConfiguration.xml` and can exist in multiple locations: https://docs.unrealengine.com/4.27/en-US/ProductionPipelines/BuildTools/UnrealBuildTool/BuildConfiguration/) (edited)
    
    
Hmmm, that docs page looks to have a mistake:
    
On Linux and Mac, the following paths are used instead: - /Users//.config//Unreal Engine/UnrealBuildTool/BuildConfiguration.xml - /Users//Unreal Engine/UnrealBuildTool/BuildConfiguration.xml
    
The `/Users//` prefix there should be `/Users/$USER/` under macOS and `/home/$USER/` under Linux.

Listing compiler commands

Unreal Build Tool is supposed to be able to generate a list of native compiler invocations with UnrealBuildTool.exe -mode=GenerateClangDatabase, but I have not been able to get it to work.

Compiler flags

One can not make changes to the generated project files to add or remove build flags. That's all handled by Unreal Build Tool and it doesn't take the settings in the build files into account. There is some control over build flags through the per-module .Build.cs files. To change it globaly you must edit the engine code for Unreal Build Tool.

  • Linux platform: Engine/Source/Programs/UnrealBuildTool/Platform/Linux/LinuxToolChain.cs.
  • CMake build environment: Engine/Source/Programs/UnrealBuildTool/ProjectFiles/CMake.

Changes to .cs files part of Unreal Build Tool will be detected and the tool rebuilt when running GenerateProjectFiles.sh.

[[20200827122445]] Third-party libraries