When learning VC++, one often encounters the linker error LNK2001. This error is quite frustrating because, for programmers, the easiest errors to fix are compilation errors, and generally speaking, linker errors occur after compilation has already succeeded. There are many reasons for linker errors, especially LNK2001, which often leaves people puzzled. Without a deep understanding of VC++, correcting linker error LNK2001 can be very difficult. Beginners learning VC++ typically see the LNK2001 error message as: unresolved external symbol “symbol” (uncertain external “symbol”). This error message occurs when the linker cannot find the referenced function, variable, or label in all libraries and object files. Generally, there are two main reasons for this error: first, the referenced function or variable does not exist, is misspelled, or is used incorrectly; second, it may be due to using different versions of the linking libraries. Here are some possible causes of the LNK2001 error: 1. LNK2001 caused by coding errors. 1. Mismatched program code or module definition (.DEF) files can lead to LNK2001. For example, if a variable “var1” is declared in a C++ source file but is accessed in another file as “VAR1”, this error will occur. 2. If the inline function is defined in a .CPP file rather than in a header file, it will lead to LNK2001. 3. If the parameter types used when calling a function do not match the types declared for that function, LNK2001 will occur. 4. Attempting to call a virtual function from a base class constructor or destructor will lead to LNK2001. 5. Be aware of the accessibility of functions and variables; only global variables and functions are accessible. Static functions and static variables have the same scope limitations. Attempting to access any static variable not declared within that file from outside will lead to compilation errors or LNK2001. Variables declared within a function (local variables) can only be used within the scope of that function. C++ global constants only have static linkage. This is different from C; if you try to use global variables across multiple files in C++, it will also produce LNK2001 errors. One solution is to include the initialization code for that constant in the header file when needed and include that header file in the .CPP file; another method is to assign a constant to that variable when used. 2. LNK2001 caused by compilation and linking settings. 1. If the /NOD (/NODEFAULTLIB) option is used during compilation, the runtime libraries and MFC libraries required by the program are written into the target file module by the compiler, but unless these library names are explicitly included in the file, they will not be linked into the project file. In this case, using /NOD will lead to LNK2001. 2. If the program entry point for wWinMainCRTStartup is not set when using Unicode and MFC, the LNK2001 error message “unresolved external on _WinMain@16” will be received. 3. When compiled with the /MD option, since all runtime libraries are retained within dynamic link libraries, references to “func” in the source file will refer to “__imp__func” in the target file. If you attempt to link with the static libraries LIBC.LIB or LIBCMT.LIB, LNK2001 will occur on __imp__func; if you do not compile with the /MD option, LNK2001 will also occur when linking with MSVCxx.LIB. 4. When compiled with the /ML option, linking with LIBCMT.LIB will cause LNK2001 on _errno. 5. When compiling a debug version of an application, linking with a release version of the modal library will also produce LNK2001; similarly, linking a debug version of the modal library with a release version of the application will cause the same issue. 6. Mixing different versions of libraries and compilers can also cause problems, as newer libraries may contain symbols and definitions not present in earlier versions. 7. Using inline and non-inline compilation options across different modules can lead to LNK2001. If function inlining is enabled (/Ob1 or /Ob2) when creating a C++ library, but the corresponding header file describing that function has inlining disabled (no inline keyword), this error message will be received. To avoid this issue, the inline keyword should be used in the corresponding header file to mark inline functions. 8. Incorrect /SUBSYSTEM or /ENTRY settings can also lead to LNK2001. In fact, there are many more reasons for LNK2001; the above reasons are just a portion of them, and for beginners, these are enough to understand for a while. However, the purpose of analyzing the error causes is to avoid errors from occurring. Although LNK2001 errors can be quite challenging, by paying attention to the above issues, they can still be avoided and resolved.