C++工程编译之“error LNK2001: 无法解析的外部符号”

　　今天一整天都在折腾“error LNK2001: 无法解析的外部符号”，就在头疼不已的时候，总算是找到问题原因了：各个动态链接库的编译方式必须统一才行，要不然很容易对库函数的引用产生冲突。简单来说就是，如果使用的第三方函数库编译方式采用/MD，那么主工程也应该使用/MD。我使用了libevent，而主工程默认采用/MT，所以需要忽略一大堆的函数库，我还纳闷呢，怎么会这么奇怪！！今天总算是解决了长久以来的困惑了。

　　下面引用一篇文章的描述：[Z]VC运行库版本不同导致链接.LIB静态库时发生重复定义问题的一个案例分析和总结

　　再参考一篇：msvcprt.lib(MSVCP90.dll) : error LNK2005:已经在libcpmtd.lib(xmutex.obj) 中定义

 

Background

MSDN中对于在不同的配置下Link的LIB作了说明：

C Runtime Library：

开关	对应的库	版本
/MD	MSVCRT.LIB	多线程DLL的Release版本
/MDd	MSVCRTD.LIB	多线程DLL的Debug版本
/MT	LIBCMT.LIB	多线程静态链接的Release版本
/MTd	LIBCMTD.LIB	多线程静态链接的Debug版本
/clr	MSVCMRT.LIB	托管代码和非托管代码混合
/clr:pure	MSVCURT.LIB	纯托管代码

 C++ Standard Library：

开关	对应的库	版本
/MD	MSVCPRT.LIB	多线程DLL的Release版本
/MDd	MSVCPRTD.LIB	多线程DLL的Debug版本
/MT	LIBCPMT.LIB	多线程静态链接的Release版本
/MTd	LIBCPMTD.LIB	多线程静态链接的Debug版本

 

　　编译器会自动根据编译选项，选择对应的LIB文件。一般情况下这不会出现问题。

　　然而，在部分情况下，一旦你的程序的各个部分（LIB, OBJ…）并非由相同的编译选项编译出，而Link在一起的话，会出现各种各样的看似很难解决的问题，这类问题主要以重复定义的错误形式存在，通常的解决方法也很简单，就是选择同样的编译选项进行编译之后再Link。

Case Study

　　之前刚下载了ANTLR，在准备编译它的Example的时候发现了下面的Build错误（我自己为这个例子创建了VS的项目，当前配置为动态链接Runtime库，Debug版）：

1>msvcprtd.lib(MSVCP80D.dll) : error LNK2005: "public: __thiscall std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >::~basic_string<char,struct std::char_traits<char>,class std::allocator<char> >(void)" (??1?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@QAE@XZ) already defined in antlr.lib(CharScanner.obj) 1>msvcprtd.lib(MSVCP80D.dll) : error LNK2005: "public: class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > & __thiscall std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >::replace(unsigned int,unsigned int,char const *,unsigned int)" (?replace@?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@QAEAAV12@IIPBDI@Z) already defined in antlr.lib(CharScanner.obj) 1>msvcprtd.lib(MSVCP80D.dll) : error LNK2005: "public: void __thiscall std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >::reserve(unsigned int)" (?reserve@?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@QAEXI@Z) already defined in antlr.lib(CharScanner.obj) 1>msvcprtd.lib(MSVCP80D.dll) : error LNK2005: "public: unsigned int __thiscall std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >::length(void)const " (?length@?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@QBEIXZ) already defined in antlr.lib(CharScanner.obj) 1>msvcprtd.lib(MSVCP80D.dll) : error LNK2005: "public: class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > & __thiscall std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >::operator+=(class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > const &)" (??Y?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@QAEAAV01@ABV01@@Z) already defined in antlr.lib(CharScanner.obj) 1>msvcprtd.lib(MSVCP80D.dll) : error LNK2005: "public: __thiscall std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >(class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > const &)" (??0?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@QAE@ABV01@@Z) already defined in antlr.lib(CharScanner.obj) 1>msvcprtd.lib(MSVCP80D.dll) : error LNK2005: "public: char const & __thiscall std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >::operator[](unsigned int)const " (??A?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@QBEABDI@Z) already defined in antlr.lib(CharScanner.obj) 1>msvcprtd.lib(MSVCP80D.dll) : error LNK2005: "public: class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > & __thiscall std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >::operator=(class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > const &)" (??4?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@QAEAAV01@ABV01@@Z) already defined in antlr.lib(CharScanner.obj) 1>msvcprtd.lib(MSVCP80D.dll) : error LNK2005: "public: class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > & __thiscall std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >::operator=(char const *)" (??4?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@QAEAAV01@PBD@Z) already defined in antlr.lib(CharScanner.obj) 1>msvcprtd.lib(MSVCP80D.dll) : error LNK2005: "public: __thiscall std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >(char const *)" (??0?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@QAE@PBD@Z) already defined in antlr.lib(CharScanner.obj) 1>msvcprtd.lib(MSVCP80D.dll) : error LNK2005: "class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > __cdecl std::operator+<char,struct std::char_traits<char>,class std::allocator<char> >(class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > const &,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > const &)" (??$?HDU?$char_traits@D@std@@V?$allocator@D@1@@std@@YA?AV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@0@ABV10@0@Z) already defined in antlr.lib(MismatchedCharException.obj) 1>msvcprtd.lib(MSVCP80D.dll) : error LNK2005: "public: char const * __thiscall std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >::c_str(void)const " (?c_str@?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@QBEPBDXZ) already defined in antlr.lib(CharScanner.obj) 1>msvcprtd.lib(MSVCP80D.dll) : error LNK2005: "bool __cdecl std::operator<<char,struct std::char_traits<char>,class std::allocator<char> >(class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > const &,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > const &)" (??$?MDU?$char_traits@D@std@@V?$allocator@D@1@@std@@YA_NABV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@0@0@Z) already defined in antlr.lib(CharScanner.obj) 1>msvcprtd.lib(MSVCP80D.dll) : error LNK2005: "public: class std::basic_ostream<char,struct std::char_traits<char> > & __thiscall std::basic_ostream<char,struct std::char_traits<char> >::operator<<(class std::basic_ostream<char,struct std::char_traits<char> > & (__cdecl*)(class std::basic_ostream<char,struct std::char_traits<char> > &))" (??6?$basic_ostream@DU?$char_traits@D@std@@@std@@QAEAAV01@P6AAAV01@AAV01@@Z@Z) already defined in antlr.lib(CharScanner.obj) 1>msvcprtd.lib(MSVCP80D.dll) : error LNK2005: "public: class std::basic_ostream<char,struct std::char_traits<char> > & __thiscall std::basic_ostream<char,struct std::char_traits<char> >::operator<<(int)" (??6?$basic_ostream@DU?$char_traits@D@std@@@std@@QAEAAV01@H@Z) already defined in antlr.lib(CharScanner.obj) 1>msvcprtd.lib(MSVCP80D.dll) : error LNK2005: "class std::basic_ostream<char,struct std::char_traits<char> > & __cdecl std::endl(class std::basic_ostream<char,struct std::char_traits<char> > &)" (?endl@std@@YAAAV?$basic_ostream@DU?$char_traits@D@std@@@1@AAV21@@Z) already defined in antlr.lib(CharScanner.obj) 1>msvcprtd.lib(MSVCP80D.dll) : error LNK2005: "public: void __thiscall std::basic_ios<char,struct std::char_traits<char> >::setstate(int,bool)" (?setstate@?$basic_ios@DU?$char_traits@D@std@@@std@@QAEXH_N@Z) already defined in antlr.lib(CharScanner.obj) 1>msvcprtd.lib(MSVCP80D.dll) : error LNK2005: "public: int __thiscall std::ios_base::width(int)" (?width@ios_base@std@@QAEHH@Z) already defined in antlr.lib(CharScanner.obj) 1>msvcprtd.lib(MSVCP80D.dll) : error LNK2005: "public: int __thiscall std::basic_streambuf<char,struct std::char_traits<char> >::sputn(char const *,int)" (?sputn@?$basic_streambuf@DU?$char_traits@D@std@@@std@@QAEHPBDH@Z) already defined in antlr.lib(CharScanner.obj) 1>msvcprtd.lib(MSVCP80D.dll) : error LNK2005: "public: static bool __cdecl std::char_traits<char>::eq_int_type(int const &,int const &)" (?eq_int_type@?$char_traits@D@std@@SA_NABH0@Z) already defined in antlr.lib(CharScanner.obj) 1>msvcprtd.lib(MSVCP80D.dll) : error LNK2005: "public: static int __cdecl std::char_traits<char>::eof(void)" (?eof@?$char_traits@D@std@@SAHXZ) already defined in antlr.lib(CharScanner.obj) 1>msvcprtd.lib(MSVCP80D.dll) : error LNK2005: "public: int __thiscall std::basic_streambuf<char,struct std::char_traits<char> >::sputc(char)" (?sputc@?$basic_streambuf@DU?$char_traits@D@std@@@std@@QAEHD@Z) already defined in antlr.lib(CharScanner.obj) 1>msvcprtd.lib(MSVCP80D.dll) : error LNK2005: "public: class std::basic_streambuf<char,struct std::char_traits<char> > * __thiscall std::basic_ios<char,struct std::char_traits<char> >::rdbuf(void)const " (?rdbuf@?$basic_ios@DU?$char_traits@D@std@@@std@@QBEPAV?$basic_streambuf@DU?$char_traits@D@std@@@2@XZ) already defined in antlr.lib(CharScanner.obj) 1>msvcprtd.lib(MSVCP80D.dll) : error LNK2005: "public: char __thiscall std::basic_ios<char,struct std::char_traits<char> >::fill(void)const " (?fill@?$basic_ios@DU?$char_traits@D@std@@@std@@QBEDXZ) already defined in antlr.lib(CharScanner.obj) 1>msvcprtd.lib(MSVCP80D.dll) : error LNK2005: "public: int __thiscall std::ios_base::flags(void)const " (?flags@ios_base@std@@QBEHXZ) already defined in antlr.lib(CharScanner.obj) 1>msvcprtd.lib(MSVCP80D.dll) : error LNK2005: "public: int __thiscall std::ios_base::width(void)const " (?width@ios_base@std@@QBEHXZ) already defined in antlr.lib(CharScanner.obj) 1>msvcprtd.lib(MSVCP80D.dll) : error LNK2005: "public: static unsigned int __cdecl std::char_traits<char>::length(char const *)" (?length@?$char_traits@D@std@@SAIPBD@Z) already defined in antlr.lib(CharScanner.obj) 1>msvcprtd.lib(MSVCP80D.dll) : error LNK2005: "public: class std::basic_ostream<char,struct std::char_traits<char> > & __thiscall std::basic_ostream<char,struct std::char_traits<char> >::flush(void)" (?flush@?$basic_ostream@DU?$char_traits@D@std@@@std@@QAEAAV12@XZ) already defined in antlr.lib(CharScanner.obj) 1>msvcprtd.lib(MSVCP80D.dll) : error LNK2005: "public: class std::basic_ostream<char,struct std::char_traits<char> > * __thiscall std::basic_ios<char,struct std::char_traits<char> >::tie(void)const " (?tie@?$basic_ios@DU?$char_traits@D@std@@@std@@QBEPAV?$basic_ostream@DU?$char_traits@D@std@@@2@XZ) already defined in antlr.lib(CharScanner.obj) 1>msvcprtd.lib(MSVCP80D.dll) : error LNK2005: "public: bool __thiscall std::ios_base::good(void)const " (?good@ios_base@std@@QBE_NXZ) already defined in antlr.lib(CharScanner.obj) 1>msvcprtd.lib(MSVCP80D.dll) : error LNK2005: "public: void __thiscall std::basic_ostream<char,struct std::char_traits<char> >::_Osfx(void)" (?_Osfx@?$basic_ostream@DU?$char_traits@D@std@@@std@@QAEXXZ) already defined in antlr.lib(CharScanner.obj) 1>msvcprtd.lib(MSVCP80D.dll) : error LNK2005: "public: void __thiscall std::basic_streambuf<char,struct std::char_traits<char> >::_Lock(void)" (?_Lock@?$basic_streambuf@DU?$char_traits@D@std@@@std@@QAEXXZ) already defined in antlr.lib(CharScanner.obj) 1>msvcprtd.lib(MSVCP80D.dll) : error LNK2005: "public: void __thiscall std::basic_streambuf<char,struct std::char_traits<char> >::_Unlock(void)" (?_Unlock@?$basic_streambuf@DU?$char_traits@D@std@@@std@@QAEXXZ) already defined in antlr.lib(CharScanner.obj) 1>msvcprtd.lib(MSVCP80D.dll) : error LNK2005: "class std::basic_ostream<char,struct std::char_traits<char> > & __cdecl std::operator<<<char,struct std::char_traits<char>,class std::allocator<char> >(class std::basic_ostream<char,struct std::char_traits<char> > &,class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > const &)" (??$?6DU?$char_traits@D@std@@V?$allocator@D@1@@std@@YAAAV?$basic_ostream@DU?$char_traits@D@std@@@0@AAV10@ABV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@0@@Z) already defined in antlr.lib(LLkParser.obj) 1>libcpmt.lib(locale0.obj) : error LNK2005: "private: static class std::locale::_Locimp * __cdecl std::locale::_Getgloballocale(void)" (?_Getgloballocale@locale@std@@CAPAV_Locimp@12@XZ) already defined in msvcprtd.lib(MSVCP80D.dll) 1>libcpmt.lib(locale0.obj) : error LNK2005: "private: static void __cdecl std::locale::facet::facet_Register(class std::locale::facet *)" (?facet_Register@facet@locale@std@@CAXPAV123@@Z) already defined in msvcprtd.lib(MSVCP80D.dll) 1>libcpmt.lib(locale0.obj) : error LNK2005: "public: static void __cdecl std::_Locinfo::_Locinfo_dtor(class std::_Locinfo *)" (?_Locinfo_dtor@_Locinfo@std@@SAXPAV12@@Z) already defined in msvcprtd.lib(MSVCP80D.dll) 1>libcpmt.lib(locale0.obj) : error LNK2005: "public: static void __cdecl std::_Locinfo::_Locinfo_ctor(class std::_Locinfo *,char const *)" (?_Locinfo_ctor@_Locinfo@std@@SAXPAV12@PBD@Z) already defined in msvcprtd.lib(MSVCP80D.dll) 1>libcpmt.lib(xmutex.obj) : error LNK2005: "public: void __thiscall std::_Mutex::_Lock(void)" (?_Lock@_Mutex@std@@QAEXXZ) already defined in msvcprtd.lib(MSVCP80D.dll) 1>libcpmt.lib(xmutex.obj) : error LNK2005: "public: void __thiscall std::_Mutex::_Unlock(void)" (?_Unlock@_Mutex@std@@QAEXXZ) already defined in msvcprtd.lib(MSVCP80D.dll) 1>libcpmt.lib(xlock.obj) : error LNK2005: "public: __thiscall std::_Lockit::_Lockit(int)" (??0_Lockit@std@@QAE@H@Z) already defined in msvcprtd.lib(MSVCP80D.dll) 1>libcpmt.lib(xlock.obj) : error LNK2005: "public: __thiscall std::_Lockit::~_Lockit(void)" (??1_Lockit@std@@QAE@XZ) already defined in msvcprtd.lib(MSVCP80D.dll) 1>LIBCMT.lib(invarg.obj) : error LNK2005: __invoke_watson already defined in MSVCRTD.lib(MSVCR80D.dll) 1>LIBCMT.lib(invarg.obj) : error LNK2005: __invalid_parameter already defined in MSVCRTD.lib(MSVCR80D.dll) 1>LIBCMT.lib(setlocal.obj) : error LNK2005: __configthreadlocale already defined in MSVCRTD.lib(MSVCR80D.dll) 1>LIBCMT.lib(tidtable.obj) : error LNK2005: __encode_pointer already defined in MSVCRTD.lib(MSVCR80D.dll) 1>LIBCMT.lib(tidtable.obj) : error LNK2005: __decode_pointer already defined in MSVCRTD.lib(MSVCR80D.dll) 1>LIBCMT.lib(tolower.obj) : error LNK2005: _tolower already defined in MSVCRTD.lib(MSVCR80D.dll) 1>LIBCMT.lib(crt0dat.obj) : error LNK2005: __amsg_exit already defined in MSVCRTD.lib(MSVCR80D.dll) 1>LIBCMT.lib(crt0dat.obj) : error LNK2005: __initterm_e already defined in MSVCRTD.lib(MSVCR80D.dll) 1>LIBCMT.lib(crt0dat.obj) : error LNK2005: _exit already defined in MSVCRTD.lib(MSVCR80D.dll) 1>LIBCMT.lib(crt0dat.obj) : error LNK2005: __exit already defined in MSVCRTD.lib(MSVCR80D.dll) 1>LIBCMT.lib(crt0dat.obj) : error LNK2005: __cexit already defined in MSVCRTD.lib(MSVCR80D.dll) 1>LIBCMT.lib(lconv.obj) : error LNK2005: _localeconv already defined in MSVCRTD.lib(MSVCR80D.dll) 1>LIBCMT.lib(mlock.obj) : error LNK2005: __unlock already defined in MSVCRTD.lib(MSVCR80D.dll) 1>LIBCMT.lib(mlock.obj) : error LNK2005: __lock already defined in MSVCRTD.lib(MSVCR80D.dll) 1>LIBCMT.lib(winxfltr.obj) : error LNK2005: __XcptFilter already defined in MSVCRTD.lib(MSVCR80D.dll) 1>LIBCMT.lib(crt0init.obj) : error LNK2005: ___xi_a already defined in MSVCRTD.lib(cinitexe.obj) 1>LIBCMT.lib(crt0init.obj) : error LNK2005: ___xi_z already defined in MSVCRTD.lib(cinitexe.obj) 1>LIBCMT.lib(crt0init.obj) : error LNK2005: ___xc_a already defined in MSVCRTD.lib(cinitexe.obj) 1>LIBCMT.lib(crt0init.obj) : error LNK2005: ___xc_z already defined in MSVCRTD.lib(cinitexe.obj) 1>LIBCMT.lib(hooks.obj) : error LNK2005: "void __cdecl terminate(void)" (?terminate@@YAXXZ) already defined in MSVCRTD.lib(MSVCR80D.dll) 1>LIBCMT.lib(crt0.obj) : error LNK2005: _mainCRTStartup already defined in MSVCRTD.lib(crtexe.obj) 1>LIBCMT.lib(errmode.obj) : error LNK2005: ___set_app_type already defined in MSVCRTD.lib(MSVCR80D.dll) 1>LIBCMT.lib(_ctype.obj) : error LNK2005: _isprint already defined in MSVCRTD.lib(MSVCR80D.dll) 1>MSVCRTD.lib(MSVCR80D.dll) : error LNK2005: __stricmp already defined in LIBCMT.lib(stricmp.obj) 1>LINK : warning LNK4098: defaultlib 'MSVCRTD' conflicts with use of other libs; use /NODEFAULTLIB:library 1>LINK : warning LNK4098: defaultlib 'LIBCMT' conflicts with use of other libs; use /NODEFAULTLIB:library 1>D:\home\doc\Visual Studio 2005\Projects\tst_antlr_3\Debug\tst_antlr_3.exe : fatal error LNK1169: one or more multiply defined symbols found

 

　　分析一下错误来源，会发现：

1. 错误来源主要是重复定义的问题，而且重复定义的基本上都是VC Runtime和Standard C++ Library中的函数
2. LIBCMT和LIBCPMT为Release下的Lib，本来不应该出现在Debug版本的链接的Lib中
3. 重复定义的问题主要出现在：LIBCMT, LIBCPMT, MSVCPRTD, MSVCRTD

　　来看看出问题的LIB是那些：

1. LIBCMT：C Runtime库的多线程静态链接的Release版本
2. LIBCPMT：C++ Standard Library的多线程静态链接的Release版本
3. MSVCPRTD：C++ Standard Library的多线程DLL的Debug版本
4. MSVCRTD：C Runtime Library的多线程DLL的Debug版本

　　当前我们的配置是多线程DLL的Debug版，因此3和4是应该出现在link的列表中的，不属于多余。而后两者则是只是当多线程静态链接Release版 中才会出现。这提示我在项目中加入的ANTLR.LIB可能是造成这个问题的根源，因为静态库的编译选项很容易和主程序发生冲突，并且根据实际信息我们可 以看出ANTLR.LIB应该是用多线程静态链接的Release版本来编译的。

　　这样，解决方法就很清楚了：

1. 切换到Release，因为ANTLR.LIB是在Release下面编译的
2. 把Run Time库的版本修改成多线程静态链接

　　做了这两个修改之后编译通过。

　　还有一种方法是，自己用多线程DLL的Debug版重新编译一次ANTLR，生成一个新的ANTLRD.LIB，再link这个Lib也可以解决这个问题。

Summary

1. 知道各个不同的LIB代表的版本信息非常重要，可以帮助快速定位问题
2. 在编程的时候，一定要把所有的项目的编译选项（是静态链接Runtime库还是动态链接Runtime库，Debug/Release）配置成一样的。如果部分LIB/OBJ是由第三方提供（OBJ情况很少见），一般情况下只能调整自己的编译选项，除非你可以要求第三方提供其他版本的编译好的LIB
3. 在发布可重用的静态LIB库供其他人调用的时候，最好对应不同的编译选项，乃至VC版本，提供不同版本的LIB。VC自己的Runtime就是最好的例子。