If you’re reading this, chances are you’ve stumbled upon the frustrating Error LNK2019: Unresolved External Symbol when trying to compile your MASM (Assembly) and C++ project. Don’t worry, you’re not alone! This pesky error has plagued many a developer, but fear not, for we’re about to tackle it head-on and emerge victorious.
What is Error LNK2019?
Error LNK2019: Unresolved External Symbol is a linker error that occurs when the compiler can’t find the definition of a symbol (function, variable, or label) referenced in your code. In the context of MASM and C++ projects, this error typically arises when there’s a mismatch between the assembly code and the C++ code that’s trying to use it.
Common Causes of Error LNK2019
- Missing or incorrect extern directive in the assembly code
- Inconsistent function naming conventions between MASM and C++
- Incorrect calling convention specified in the C++ code
- Mismatched data types between MASM and C++
- Linker settings not properly configured
Step-by-Step Solution
Let’s dive into the nitty-gritty of resolving this error. Follow these steps carefully, and you’ll be well on your way to a successful compilation.
Step 1: Review Your Assembly Code
Open your MASM assembly code file (e.g., assembly.asm) and ensure that:
- The extern directive is correctly specified for the function(s) you want to call from C++.
- The function naming convention is consistent (e.g., underscore prefix, capitalization, etc.).
- The correct calling convention is specified (e.g.,
CDECL,STDCALL, etc.).
; assembly.asm
.model flat, stdcall
.stack 4096
.data
.code
PUBLIC _MyAssemblyFunction
_MyAssemblyFunction PROC C
; function code here
ret
_MyAssemblyFunction ENDP
END
Step 2: Inspect Your C++ Code
Open your C++ code file (e.g., main.cpp) and ensure that:
- The function declaration matches the assembly code’s function name and calling convention.
- The correct header file is included (e.g.,
assembly.h) that contains the function declaration.
// main.cpp
#include "assembly.h"
int main() {
_MyAssemblyFunction();
return 0;
}
Step 3: Configure the Linker
In your project settings, ensure that:
- The linker is configured to link against the object file generated by MASM (e.g.,
assembly.obj). - The correct linker options are specified (e.g.,
/link,/machine:x86, etc.).
| Project Setting | Value |
|---|---|
| Linker > Command Line | /link /machine:x86 assembly.obj |
| Linker > Input > Additional Dependencies | assembly.obj |
Step 4: Clean and Rebuild Your Project
Delete any existing object files and rebuild your project from scratch. This ensures that the linker picks up the correct object file generated by MASM.
Troubleshooting Tips
If you’re still encountering issues, try these additional troubleshooting steps:
- Verify that the object file generated by MASM is correctly named and located in the expected directory.
- Check for any typos or mismatches in function names, calling conventions, and data types between MASM and C++.
- Ensure that the C++ code is correctly including the assembly header file and declaring the function with the correct extern “C” linkage.
- Try using the
/VERBOSElinker option to get more detailed error messages.
Conclusion
Error LNK2019: Unresolved External Symbol can be a frustrating obstacle, but by methodically following these steps and troubleshooting tips, you should be able to resolve the issue and successfully compile your MASM and C++ project. Remember to double-check your code, linker settings, and project configuration to ensure a smooth compilation process.
Happy coding!
Frequently Asked Questions
Get answers to common questions about Error LNK2019 unresolved external symbol in MASM (Assembly) and C++ projects.
What is the Error LNK2019 unresolved external symbol, and why does it occur?
Error LNK2019 occurs when the linker is unable to resolve an external symbol, which means that a function or variable is declared but not defined. This can happen when there’s a mismatch between the declaration and definition of a function, or when a library or object file is missing.
How do I fix the Error LNK2019 in my MASM project?
To fix Error LNK2019 in MASM, check the following: Ensure that the symbol is correctly defined in your code; Verify that the library or object file is included in the project; Check the spelling and case of the symbol; and Make sure that the symbol is not defined more than once.
What’s the difference between a declaration and a definition in C++?
In C++, a declaration announces the existence of a variable or function, while a definition provides the actual implementation of the variable or function. The linker resolves declarations against definitions, so if a declaration is not matched with a definition, Error LNK2019 occurs.
Can I get Error LNK2019 if I’m using a third-party library?
Yes, if the third-party library is not properly linked to your project, or if the library’s dependencies are not met, you can get Error LNK2019. Ensure that you’ve included the correct library files and followed the library’s documentation for integration.
How do I troubleshoot Error LNK2019 in a mixed-language project (C++ and Assembly)?
To troubleshoot Error LNK2019 in a mixed-language project, start by examining the assembly code for any declaration or definition issues. Then, check the C++ code for any mismatches in function signatures or variable declarations. Finally, verify that the linker options and dependencies are correctly set for both languages.
