--??LINK2001
學習VC＋＋時經常會遇到鏈接錯誤LNK2001，該錯誤非常討厭，因為對于
編程者來說，最好改的錯誤莫過于編譯錯誤，而一般說來發生連接錯誤時，
編譯都已通過。產生連接錯誤的原因非常多，尤其LNK2001錯誤，常常使人不
明其所以然。如果不深入地學習和理解VC＋＋，要想改正連接錯誤LNK2001非
常困難。
初學者在學習VC＋＋的過程中，遇到的LNK2001錯誤的錯誤消息主要為：
unresolved external symbol “symbol”（不確定的外部“符號”）。
如果連接程序不能在所有的庫和目標文件內找到所引用的函數、變量或
標簽，將產生此錯誤消息。一般來說，發生錯誤的原因有兩個：一是所引用
的函數、變量不存在、拼寫不正確或者使用錯誤；其次可能使用了不同版本
的連接庫。
以下是可能產生LNK2001錯誤的原因：
一．由于編碼錯誤導致的LNK2001。
1．不相匹配的程序代碼或模塊定義（.DEF）文件能導致LNK2001。例如,
如果在C＋＋ 源文件內聲明了一變量“var1”，卻試圖在另一文件內以變量
“VAR1”訪問該變量,將發生該錯誤。
2．如果使用的內聯函數是在.CPP文件內定義的，而不是在頭文件內定
義將導致LNK2001錯誤。
3．調用函數時如果所用的參數類型同函數聲明時的類型不符將會產生
LNK2001。
4．試圖從基類的構造函數或析構函數中調用虛擬函數時將會導致LNK2001。
5．要注意函數和變量的可公用性，只有全局變量、函數是可公用的。
靜態函數和靜態變量具有相同的使用范圍限制。當試圖從文件外部訪問
任何沒有在該文件內聲明的靜態變量時將導致編譯錯誤或LNK2001。
函數內聲明的變量（局部變量） 只能在該函數的范圍內使用。
C＋＋ 的全局常量只有靜態連接性能。這不同于C，如果試圖在C＋＋的
多個文件內使用全局變量也會產生LNK2001錯誤。一種解決的方法是需要時在
頭文件中加入該常量的初始化代碼，并在.CPP文件中包含該頭文件；另一種
方法是使用時給該變量賦以常數。
二．由于編譯和鏈接的設置而造成的LNK2001
1．如果編譯時使用的是/NOD（/NODEFAULTLIB）選項，程序所需要的運行
庫和MFC庫在連接時由編譯器寫入目標文件模塊， 但除非在文件中明確包含
這些庫名，否則這些庫不會被鏈接進工程文件。在這種情況下使用/NOD將導
致錯誤LNK2001。
2．如果沒有為wWinMainCRTStartup設定程序入口，在使用Unicode和MFC
時將得到“unresolved external on _WinMain@16”的LNK2001錯誤信息。
3．使用/MD選項編譯時,既然所有的運行庫都被保留在動態鏈接庫之內，
源文件中對“func”的引用，在目標文件里即對“__imp__func” 的引用。
如果試圖使用靜態庫LIBC.LIB或LIBCMT.LIB進行連接，將在__imp__func上發
生LNK2001；如果不使用/MD選項編譯，在使用MSVCxx.LIB連接時也會發生LNK2001。
4．使用/ML選項編譯時，如用LIBCMT.LIB鏈接會在_errno上發生LNK2001。
5．當編譯調試版的應用程序時，如果采用發行版模態庫進行連接也會產
生LNK2001；同樣，使用調試版模態庫連接發行版應用程序時也會產生相同的
問題。
6．不同版本的庫和編譯器的混合使用也能產生問題，因為新版的庫里可
能包含早先的版本沒有的符號和說明。
7．在不同的模塊使用內聯和非內聯的編譯選項能夠導致LNK2001。如果
創建C＋＋庫時打開了函數內聯（/Ob1或/Ob2），但是在描述該函數的相應頭
文件里卻關閉了函數內聯（沒有inline關鍵字），這時將得到該錯誤信息。
為避免該問題的發生，應該在相應的頭文件中用inline關鍵字標志內聯函數。
8．不正確的/SUBSYSTEM或/ENTRY設置也能導致LNK2001。
其實，產生LNK2001的原因還有很多，以上的原因只是一部分而已，對初
學者來說這些就夠理解一陣子了。但是，分析錯誤原因的目的是為了避免錯
誤的發生。LNK2001錯誤雖然比較困難，但是只要注意到了上述問題，還是能
夠避免和予以解決的。
LNK2019函數只有申明,沒有實現時,或是DLL中的東東沒有export啊
unresolved external symbol 'symbol' referenced in function 'function'
An undefined external symbol (symbol) was found in function. To resolve this error, provide a definition for symbol or remove the code that references it.
In Visual C++ .NET 2003, this error will be generated when /clris used and the CRT is not linked into your executable. Any object code generated by the compiler that is not built with /clr:initialAppDomain contains a reference to the _check_commonlanguageruntime_version function, which is defined in the C Runtime Library (CRT). This function provides for an error message if your application is run on version 1 of the runtime. Code generated by the current compiler is not compatible with version 1 of the common language runtime. So, if you compile without the CRT in Visual C++ .NET 2003, you should include a definition of the _check_commonlanguageruntime_version function in your code. As an alternative to using the _check_commonlanguageruntime_version function, you can link with nochkclr.obj, which contains an empty version of the function and does not provide for an error message if you run your application on version 1 of the runtime. To build an application with the current compiler version to run on the previous version of the runtime, use /clr:InitialAppDomain.
To build a pure MSIL executable (does not link with the CRT), you must define the function in your project; you cannot use nochkclr.obj (the .obj is native code). See Producing Verifiable Components with Managed Extensions for C++for more information about verifiable code. For more information on creating a pure MSIL output file from your Managed C++ project, see Converting Managed Extensions for C++ Projects from Mixed-Mode to Pure IL.
The rest of this topic discusses other causes of LNK2019.
Consider the following sample:
extern int i;
extern void g();
void f()
{
i++;
g();
}
int main()
{
}If i and g are not defined in one of the files included in the build, the linker will generate LNK2019. These definitions can be added by including the source code file that contains the definitions as part of the compilation. Alternatively, you can pass .obj or .lib files that contain the definitions to the linker.
For C++ projects from previous releases that were upgraded to the current version, if __UNICODE was defined and the entry point was WinMain, you need to change the name of the entry point function to either _tWinMain or _tmain.
Common problems that cause LNK2019 include:
- The declaration of the symbol contains a spelling mistake, such that, it is not the same name as the definition of the symbol.
- A function was used but the type or number of the parameters did not match the function definition.
- The calling convention (__cdecl, __stdcall, or __fastcall) differs on the use of the function declaration and the function definition.
- Symbol definitions are in a file that was compiled as a C program and symbols are declared in a C++ file without an extern "C" modifier. In that case, modify the declaration, for example, instead of:
extern int i;
extern void g();
use:
extern "C" int i;
extern "C" void g();
Similarly, if you define a symbol in a C++ file that will be used by a C program, use extern "C" in the definition.
- A symbol is defined as static and then later referenced outside the file.
- A static member of a class is not defined. For example, member variable
si in the class declaration below should be defined separately: #include <stdio.h>
struct X {
static int si;
};
// int X::si = 0; // uncomment this line to resolve
void main()
{
X *px = new X[2];
printf("\n%d",px[0].si); // LNK2019
}
This error can also be generated as a result of conformance work that was done for Visual Studio .NET 2003: template friends and specialization. In Visual Studio?.NET 2003, a friend declaration that declares a new non-template function must be defined.
For code that is valid in both the Visual Studio?.NET 2003 and Visual Studio?.NET versions of Visual C++, explicitly specify the friend function's template argument list.
// LNK2019.cpp
// LNK2019 expected
template<class T>
void f(T)
{
}
template<class T>
struct S
{
friend void f(T);
// Try the folowing line instead:
// friend void f<T>(T);
};
int main()
{
S<int> s;
f(1); // unresolved external
}The /VERBOSE linker option will help you see which files the linker is referencing. The /EXPORT and /SYMBOLS options of the DUMPBIN utility can also help you see which symbols are defined in your dll and object/library files.