Ошибка 2 error lnk1120 1 неразрешенных внешних элементов

The «Fatal Error LNK1120: 1 unresolved externals» is a common error message in C++ development, indicating that a symbol (typically a function or variable) has been declared but not defined. This can occur for a variety of reasons, including missing library files, incorrect project settings, or errors in the source code. In this article, we will explore several solutions to this issue, including:

Method 1: Verify Library Linkage

If you are facing the C++ Fatal Error LNK1120: 1 unresolved externals error, it means that the linker is unable to find the definition of a function or variable that is declared in one module but defined in another. One way to solve this problem is by using the Verify Library Linkage feature in Visual Studio. Here are the steps to do it:

1. Open your Visual Studio project and go to the Solution Explorer.

2. Right-click on the project that is giving you the error and select Properties.

3. In the Properties window, go to the Configuration Properties > Linker > General section.

4. Set the Enable Incremental Linking option to No (/INCREMENTAL:NO).

5. Go to the Configuration Properties > Linker > Input section.

6. In the Additional Dependencies field, add the name of the library that contains the definition of the missing function or variable.

7. Go to the Configuration Properties > Linker > Debugging section.

8. Set the Generate Debug Info option to Yes (/DEBUG).

9. Build your project.

10. If the error persists, use the dumpbin tool to verify that the library is being linked correctly. Open a command prompt and navigate to the directory where the library is located. Then, run the following command:

dumpbin /exports <library_name>

This will display a list of all the exported symbols in the library. Make sure that the missing function or variable is included in the list.

1. If the missing function or variable is not included in the list, it means that the library is not being linked correctly. Check the library path and make sure that it is included in the project properties.

That’s it! By using the Verify Library Linkage feature and the dumpbin tool, you should be able to fix the C++ Fatal Error LNK1120: 1 unresolved externals error. Here are some example codes that might help you understand the process better:

// Header file: mylib.h
#pragma once

int add(int a, int b);

// Source file: mylib.cpp
#include "mylib.h"

int add(int a, int b)
{
    return a + b;
}

// Main file: main.cpp
#include "mylib.h"

int main()
{
    int result = add(3, 4);
    return 0;
}

In this example, the add() function is declared in the mylib.h header file and defined in the mylib.cpp source file. The main.cpp file uses the add() function, but if the mylib.cpp file is not linked correctly, you will get the C++ Fatal Error LNK1120: 1 unresolved externals error. By following the steps outlined above, you should be able to fix this error and successfully build your project.

Method 2: Check Source Code for Missing Definitions

To fix the C++ Fatal Error LNK1120: 1 unresolved externals, you can try checking the source code for missing definitions. Here are the steps you can follow:

1. Identify the missing definition: The first step is to identify which definition is missing. Look for the function or variable that is causing the error.

2. Check the source code: Once you have identified the missing definition, check if it is defined in the source code. Look for the function or variable declaration in the header files and the implementation in the source files.

3. Add the missing definition: If the definition is missing, add it to the source code. Make sure the function or variable is declared in the header file and implemented in the source file.

4. Rebuild the project: After adding the missing definition, rebuild the project and check if the error has been resolved.

Here is an example of how to check for a missing function definition:

// header file
#ifndef MY_HEADER_FILE_H
#define MY_HEADER_FILE_H

int add(int a, int b);

#endif

// source file
#include "my_header_file.h"

int main() {
    int result = add(1, 2);
    return 0;
}

// implementation file
int add(int a, int b) {
    return a + b;
}

In this example, the add function is declared in the header file and called in the source file. The implementation of the add function is defined in the implementation file. If the implementation file is missing or the function is not implemented correctly, you may get the LNK1120 error.

By checking the source code for missing definitions, you can easily fix the LNK1120 error and ensure that your code compiles correctly.

Method 3: Ensure Correct Project Settings

To fix the C++ Fatal Error LNK1120: 1 unresolved externals, you can ensure correct project settings. Here are the steps to follow:

1. Open your Visual Studio project and navigate to the Solution Explorer.

2. Right-click on your project and select Properties.

3. In the Properties window, navigate to Linker > Input.

4. In the Additional Dependencies field, add the name of the library that is causing the error. For example, if the error is caused by a missing reference to a function in the math library, you would add «libm.lib» to the list of dependencies.

5. Click OK to save your changes and close the Properties window.

6. Rebuild your project by selecting Build > Rebuild Solution from the menu bar.

7. If the error persists, try adding the library path to your project settings. To do this, navigate to Linker > General in the Properties window and add the path to the library files in the Additional Library Directories field.

8. Rebuild your project again and the error should be resolved.

Here’s an example of how to add the math library as a dependency:

#include <cmath>

int main()
{
    double x = 2.0;
    double y = std::sin(x);
    return 0;
}

In this example, we’re using the sin function from the math library. To ensure that the function is linked correctly, we need to add the library as a dependency in our project settings.

#include <cmath>

#pragma comment(lib, "libm.lib")

int main()
{
    double x = 2.0;
    double y = std::sin(x);
    return 0;
}

In this updated example, we’ve added the #pragma directive to link the math library. This will ensure that the sin function is resolved correctly at link time.

Method 4: Debugging with the Linker

When you encounter the C++ Fatal Error LNK1120: 1 unresolved externals, it means that the linker cannot find the definition of a symbol that is declared in your code. This error can be frustrating, but it is usually easy to fix. One way to fix this error is by using the «Debugging with the Linker» method. Here are the steps to follow:

Step 1: Identify the unresolved symbol

The first step is to identify the unresolved symbol. Look at the error message to see which symbol is causing the error. For example, if the error message says «unresolved external symbol _main», it means that the linker cannot find the main function.

Step 2: Check the declaration of the symbol

Once you have identified the unresolved symbol, check the declaration of the symbol to make sure it matches the definition. The declaration should be in a header file, and the definition should be in a source file. Make sure that the declaration and definition have the same name, return type, and parameter list.

Step 3: Check the source file for the definition

If the declaration and definition match, check the source file for the definition. Make sure that the definition is not in a different namespace or class than the declaration. Also, make sure that the definition is not commented out or conditionalized out.

Step 4: Check the project settings

If the declaration and definition match and the definition is in the correct source file, check the project settings to make sure that the source file is included in the build. Make sure that the source file is listed in the project settings under «Source Files» or «Compile».

Step 5: Check the library settings

If the declaration and definition match, the definition is in the correct source file, and the source file is included in the build, check the library settings to make sure that the library containing the definition is linked into the project. Make sure that the library is listed in the project settings under «Linker» or «Libraries».

Step 6: Check for name mangling

If the declaration and definition match, the definition is in the correct source file, the source file is included in the build, and the library containing the definition is linked into the project, check for name mangling. C++ compilers often mangle the names of symbols to support overloading. Make sure that the declaration and definition use the same name mangling scheme.

Step 7: Check for C linkage

If the declaration and definition match, the definition is in the correct source file, the source file is included in the build, the library containing the definition is linked into the project, and the name mangling is correct, check for C linkage. If the symbol is declared with C linkage, but the definition is not, the linker will not be able to find the definition. Make sure that the declaration and definition use the same linkage.

Step 8: Check for multiple definitions

If the declaration and definition match, the definition is in the correct source file, the source file is included in the build, the library containing the definition is linked into the project, the name mangling is correct, and the linkage is correct, check for multiple definitions. If the symbol is defined in more than one source file, the linker will not be able to resolve the symbol. Make sure that the symbol is defined in only one source file.

By following these steps, you should be able to fix the C++ Fatal Error LNK1120: 1 unresolved externals with Debugging with the Linker method. Here are some example code snippets to help illustrate the steps:

// Step 1: Identify the unresolved symbol
// Error message: unresolved external symbol _main
// Solution: Add a main function

// Step 2: Check the declaration of the symbol
// Declaration in header file
int main();

// Step 3: Check the source file for the definition
// Definition in source file
int main() {
    return 0;
}

// Step 4: Check the project settings
// Source file listed under "Source Files" or "Compile"

// Step 5: Check the library settings
// Library listed under "Linker" or "Libraries"

// Step 6: Check for name mangling
// Declaration and definition use the same name mangling scheme
extern "C" int _cdecl main();

// Step 7: Check for C linkage
// Declaration and definition use the same linkage
extern "C" int main();

// Step 8: Check for multiple definitions
// Symbol is defined in only one source file