unreal-engine error about -Wpragma-once-outside-header

slx · 2023-07-01 00:02:32

I am trying to compile unreal-engine, it worked some time ago, but since few months I got error:

[1970/17816] Compile DisplayClusterConfigurationTypes_Media.cpp
/home/DATA/Projects/ABS/BUILD/unreal-engine/unreal-engine/src/unreal-engine/Engine/Plugins/Runtime/nDisplay/Source/DisplayClusterConfiguration/Private/DisplayClusterConfigurationTypes_Media.cpp:3:9: error: #pragma once in main file [-Werror,-Wpragma-once-outside-header]
#pragma once

And of course there is "#pragma" in source. So how others can compile it? It seems that they don't have "-Wpragma-once-outside-header", but I did not set that anywhere.

MattEqualsCoder · 2023-12-24 15:45:52

Did you have any luck with this? I'm having the same problem.

burning-archer_451 · 2025-12-30 01:01:51

Hello, I have a similar issue. I have not solved it yet, but here are some relevant information:

I am manually compiling unreal engine 5.4.4 forked from Github. I tried using make -j1 and ./Engine/Build/BatchFiles/Linux/Build.sh UnrealEditor Linux Development -Progress.
One thing to note, I modified GenericPlatformProcess.cpp to always include <unistd.h> and <sched.h> instead of including them only if PLATFORM_HAS_BSD_TIME (why? some other issue causing unreal engine to crash on startup). I did not have a compile issue before this modification, and as compiling took 4 hours, I have not yet removed the changes to test if compilation works again.

Edit: the same error persist after the change is reverted. Then why did it work the first time? I don't understand either.

Onto the issue, pragma once is basically a cool header guard.

On one hand, I think epic devs must have put pragma once there on purpose. On the other hand, according to https://stackoverflow.com/questions/618 … -main-file, pragma once is supposed to only be used for header files instead of files you need to compile. This makes sense, but why then would the epic devs put pragma once in the file "Engine/Plugins/Runtime/nDisplay/Source/SharedMemoryMediaEditor/Private/ModularFeatures/SharedMemoryMediaInitializerFeature.cpp:3:9"?

Admittedly, the file for which I ran into the same error is different from your file.

Relevant error:
------ Building 6 action(s) started ------
@progress 'Compiling C++ source code...' 17%
[1/6] Link (lld) libUnrealEditor-UserToolBoxCore.so
@progress 'Compiling C++ source code...' 33%
ld.lld: error: undefined symbol: UUserToolBoxBaseBlueprint::Execute()
>>> referenced by Module.UserToolBoxCore.cpp
>>> /home/jc/Games/Unreal-Engine/UnrealEngine-fork/Engine/Plugins/Experimental/UserToolBoxCore/Intermediate/Build/Linux/x64/UnrealEditor/Development/UserToolBoxCore/Module.UserToolBoxCore.cpp.o:(vtable for UUserToolBoxBaseBlueprint)
clang++: error: linker command failed with exit code 1 (use -v to see invocation)
[2/6] Compile SharedMemoryMediaInitializerFeature.cpp
/home/jc/Games/Unreal-Engine/UnrealEngine-fork/Engine/Plugins/Runtime/nDisplay/Source/SharedMemoryMediaEditor/Private/ModularFeatures/SharedMemoryMediaInitializerFeature.cpp:3:9: error: #pragma once in main file [-Werror,-Wpragma-once-outside-header]
#pragma once

Soooo... I can either try just commenting ''#pragma once'' out of the relevant cpp files, or I can try locating where the compiler decided to add -Wpragma-once-outside-header and removing that flag.

I got a feeling that things might break after trying either of these, but I'll get back in another post if anything actually works.

Last edited by burning-archer_451 (2025-12-30 06:21:31)

burning-archer_451 · 2025-12-30 01:59:25

Additional maybe useful information: In Engine/Source/Programs/UnrealBuildTool/ToolChain/ClangWarnings.cs, there exist a function GetIntelDisabledWarnings with Arguments.Add("-Wno-pragma-once-outside-header");.

-Wno-pragma-once-outside-header would disable that pragma once warning, and it needs to be added somewhere. Maybe this function has not been called...?

schard · 2025-12-30 09:23:42

Mod note: Please don't necro-bump. Closing this old topic.

Lone_Wolf · 2025-12-30 09:40:43

I am manually compiling unreal engine 5.4.4 forked from Github.

That makes this unsuitable for this subboard, please create a new thread in "Programming & Scripting" .

Put cli/progam output in [ code ] [ /code ] tags, see https://bbs.archlinux.org/help.php#bbcode

Last edited by Lone_Wolf (2025-12-30 09:44:13)

Arch Linux

#1 2023-07-01 00:02:32

unreal-engine error about -Wpragma-once-outside-header

#2 2023-12-24 15:45:52

Re: unreal-engine error about -Wpragma-once-outside-header

#3 2025-12-30 01:01:51

Re: unreal-engine error about -Wpragma-once-outside-header

#4 2025-12-30 01:59:25

Re: unreal-engine error about -Wpragma-once-outside-header

#5 2025-12-30 09:23:42

Re: unreal-engine error about -Wpragma-once-outside-header

#6 2025-12-30 09:40:43

Re: unreal-engine error about -Wpragma-once-outside-header

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