What It Means & How to Fix It in UE5

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🧠 Why You’re Seeing This Error

This error means the linker was expecting to find a function or variable definition from a third-party library (like a .lib or .dll), but it wasn’t able to resolve it.

So your code is calling something like LibraryFunction(), but the linker can’t find where that function actually lives — because the required library:

• Isn’t linked properly in your .Build.cs file
• Isn’t included in your PublicAdditionalLibraries
• Is compiled with incompatible settings
• Is missing altogether from your include or lib folders

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💥 Example Error Message

LNK2019: unresolved external symbol "void __cdecl MyExternalFunction()" referenced in function "main"

Or:

Unresolved external symbol _MyThirdPartyFunction referenced in module MyModule.cpp.obj

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🛠️ What Usually Causes This

❌ You Declared a Function, but Didn’t Link Its Implementation

// You called a function in a third-party lib
MyLib::DoSomething(); // ❌ But Unreal doesn’t know where that implementation is

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❌ The .lib File Isn’t Being Linked

Even if you included the headers, Unreal still needs to know which .lib to link against in your .Build.cs.

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❌ You Added the Headers But Not the Library

// Included:
PublicIncludePaths.Add(Path.Combine(LibPath, "include")); // ✅
// Missing:
PublicAdditionalLibraries.Add(Path.Combine(LibPath, "lib", "MyLib.lib")); // ❌

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❌ You’re Linking the Wrong Platform or Configuration Version

Linking a Win64 library on Win32? Or a Debug library in a Shipping build? That’ll break it too.

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✅ How to Fix It – Step-by-Step

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✔️ 1. Include the Third-Party .lib File in .Build.cs

In your module’s .Build.cs:

string ThirdPartyPath = Path.Combine(ModuleDirectory, "../../ThirdParty/MyLib");

PublicIncludePaths.Add(Path.Combine(ThirdPartyPath, "include"));
PublicAdditionalLibraries.Add(Path.Combine(ThirdPartyPath, "lib", "MyLib.lib"));

✅ Make sure paths are absolute or built from ModuleDirectory
✅ Use System.IO.Path.Combine() for safe cross-platform path building

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✔️ 2. Match the Library to the Build Configuration

If you’re building in Debug:

PublicAdditionalLibraries.Add("MyLib_Debug.lib");

Or conditionally:

if (Target.Configuration == UnrealTargetConfiguration.Debug)
{
    PublicAdditionalLibraries.Add("MyLib_Debug.lib");
}
else
{
    PublicAdditionalLibraries.Add("MyLib.lib");
}

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✔️ 3. Check for C++ Function Name Mangling

If the third-party library is C-based and you’re using it in C++, wrap the declarations like so:

extern "C"
{
    void MyCFunction(); // prevents name mangling
}

Or confirm the .lib matches your calling convention and language expectations.

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✔️ 4. Double-Check Platform Compatibility

Make sure the .lib is built for your target platform:

• Win64 for UE on Windows
• x86 vs x64 mismatch
• ARM vs x64 (for mobile targets)

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✔️ 5. Clean and Rebuild the Project

Sometimes stale builds cause this error to stick around:

Delete: Binaries/, Intermediate/
→ Regenerate project files
→ Rebuild the project from scratch

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✅ Summary: How to Fix “Unresolved External Symbol from Third-Party Library” in UE5

Cause	Fix
.lib not linked in .Build.cs	Add PublicAdditionalLibraries.Add() with full path
Headers included but not implementation	Link both headers and compiled library
Platform or config mismatch	Use correct lib for Debug/Release and Win64/ARM
Missing extern "C" for C linkage	Wrap declarations to prevent C++ name mangling
Wrong file structure or stale cache	Rebuild, clean, and verify folder layout