今天在移植一个开源代码到windows的VC6工程，编译时出现了这些奇怪的LINK错误。
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msvcrt.lib(MSVCRT.dll) : error LNK2005: _toupper already defined in LIBCD.lib(toupper.obj)
msvcrt.lib(MSVCRT.dll) : error LNK2005: _tolower already defined in LIBCD.lib(tolower.obj)
msvcrt.lib(MSVCRT.dll) : error LNK2005: _isupper already defined in LIBCD.lib(_ctype.obj)
msvcrt.lib(MSVCRT.dll) : error LNK2005: _isalpha already defined in LIBCD.lib(_ctype.obj)
msvcrt.lib(MSVCRT.dll) : error LNK2005: _free already defined in LIBCD.lib(dbgheap.obj)
msvcrt.lib(MSVCRT.dll) : error LNK2005: _realloc already defined in LIBCD.lib(dbgheap.obj)
msvcrt.lib(MSVCRT.dll) : error LNK2005: _atol already defined in LIBCD.lib(atox.obj)
msvcrt.lib(MSVCRT.dll) : error LNK2005: _malloc already defined in LIBCD.lib(dbgheap.obj)
msvcrt.lib(MSVCRT.dll) : error LNK2005: _wcslen already defined in LIBCD.lib(wcslen.obj)
msvcrt.lib(MSVCRT.dll) : error LNK2005: __strdup already defined in LIBCD.lib(strdup.obj)
msvcrt.lib(MSVCRT.dll) : error LNK2005: _memmove already defined in LIBCD.lib(memmove.obj)
msvcrt.lib(MSVCRT.dll) : error LNK2005: _strrchr already defined in LIBCD.lib(strrchr.obj)
msvcrt.lib(MSVCRT.dll) : error LNK2005: _isspace already defined in LIBCD.lib(_ctype.obj)
msvcrt.lib(MSVCRT.dll) : error LNK2005: _strchr already defined in LIBCD.lib(strchr.obj)
msvcrt.lib(MSVCRT.dll) : error LNK2005: _isalnum already defined in LIBCD.lib(_ctype.obj)
msvcrt.lib(MSVCRT.dll) : error LNK2005: _isdigit already defined in LIBCD.lib(_ctype.obj)
msvcrt.lib(MSVCRT.dll) : error LNK2005: _isxdigit already defined in LIBCD.lib(_ctype.obj)
msvcrt.lib(MSVCRT.dll) : error LNK2005: _strncmp already defined in LIBCD.lib(strncmp.obj)
msvcrt.lib(MSVCRT.dll) : error LNK2005: _localeconv already defined in LIBCD.lib(lconv.obj)
msvcrt.lib(MSVCRT.dll) : error LNK2005: _sprintf already defined in LIBCD.lib(sprintf.obj)
msvcrt.lib(MSVCRT.dll) : error LNK2005: _exit already defined in LIBCD.lib(crt0dat.obj)
msvcrt.lib(MSVCRT.dll) : error LNK2005: _floor already defined in LIBCD.lib(floor.obj)
msvcrt.lib(MSVCRT.dll) : error LNK2005: __stricmp already defined in LIBCD.lib(stricmp.obj)
msvcrt.lib(MSVCRT.dll) : error LNK2005: _getenv already defined in LIBCD.lib(getenv.obj)
msvcrt.lib(MSVCRT.dll) : error LNK2005: _atoi already defined in LIBCD.lib(atox.obj)
msvcrt.lib(MSVCRT.dll) : error LNK2005: _atof already defined in LIBCD.lib(atof.obj)
msvcrt.lib(MSVCRT.dll) : error LNK2005: _fclose already defined in LIBCD.lib(fclose.obj)
msvcrt.lib(MSVCRT.dll) : error LNK2005: _strcspn already defined in LIBCD.lib(strcspn.obj)
msvcrt.lib(MSVCRT.dll) : error LNK2005: _fwrite already defined in LIBCD.lib(fwrite.obj)
msvcrt.lib(MSVCRT.dll) : error LNK2005: _fputc already defined in LIBCD.lib(fputc.obj)
msvcrt.lib(MSVCRT.dll) : error LNK2005: _strtol already defined in LIBCD.lib(strtol.obj)
msvcrt.lib(MSVCRT.dll) : error LNK2005: _strncpy already defined in LIBCD.lib(strncpy.obj)
msvcrt.lib(MSVCRT.dll) : error LNK2005: __isctype already defined in LIBCD.lib(isctype.obj)
msvcrt.lib(MSVCRT.dll) : error LNK2005: __strnicmp already defined in LIBCD.lib(strnicmp.obj)
msvcrt.lib(MSVCRT.dll) : error LNK2005: _strstr already defined in LIBCD.lib(strstr.obj)
msvcrt.lib(MSVCRT.dll) : error LNK2005: _isprint already defined in LIBCD.lib(_ctype.obj)
msvcrt.lib(MSVCRT.dll) : error LNK2005: _strtod already defined in LIBCD.lib(strtod.obj)
msvcrt.lib(MSVCRT.dll) : error LNK2005: _fopen already defined in LIBCD.lib(fopen.obj)
msvcrt.lib(MSVCRT.dll) : error LNK2005: _fflush already defined in LIBCD.lib(fflush.obj)
msvcrt.lib(MSVCRT.dll) : error LNK2005: _calloc already defined in LIBCD.lib(dbgheap.obj)
msvcrt.lib(MSVCRT.dll) : error LNK2005: _abort already defined in LIBCD.lib(abort.obj)
msvcrt.lib(MSVCRT.dll) : error LNK2005: _islower already defined in LIBCD.lib(_ctype.obj)
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从提示来看，msvcrt.lib(MSVCRT.dll)和LIBCD.lib中存在符号冲突！
导致链接器在链接时发现一个符号在两处有定义，就不知道该链接那个了，从而报出LNK2005的链接错误。
在工程设置中勾上"Force file output"即可强制生成dll或exe文件。
关于这个选项的更多说明请参考：http://msdn.microsoft.com/en-us/library/70abkas3.aspx