解决libcurl7.50.3在windows XP SP3 VC++ 6.0下编译报错 unresolved external symbol __imp__IdnToAscii@20 unresolved external symbol __imp__IdnToUnicode@20

错误重现：

--------------------Configuration: curl - Win32 LIB Debug DLL Windows SSPI DLL WinIDN--------------------

xilink6: executing 'D:\vc6.0\VC98\Bin\link.exe'
libcurld.lib(idn_win32.obj) : error LNK2001: unresolved external symbol __imp__IdnToAscii@20
libcurld.lib(idn_win32.obj) : error LNK2001: unresolved external symbol __imp__IdnToUnicode@20
..\..\..\..\build\Win32\VC6\LIB Debug - DLL Windows SSPI - DLL WinIDN\curld.exe : fatal error LNK1120: 2 unresolved externals
Error executing xilink6.exe.

curld.exe - 3 error(s), 2 warning(s)


--------------------Configuration: curl - Win32 LIB Release DLL Windows SSPI DLL WinIDN--------------------
Linking...
xilink6: executing 'D:\vc6.0\VC98\Bin\link.exe'
libcurl.lib(idn_win32.obj) : error LNK2001: unresolved external symbol __imp__IdnToAscii@20
libcurl.lib(idn_win32.obj) : error LNK2001: unresolved external symbol __imp__IdnToUnicode@20
..\..\..\..\build\Win32\VC6\LIB Release - DLL Windows SSPI - DLL WinIDN/curl.exe : fatal error LNK1120: 2 unresolved externals

根据两个API的名字google了一下

IdnToUnicode,IdnToAscii封装在normaliz.dll中，查询了本机的几个SDK(Microsoft Platform SDK February 2003,Microsoft Windows SDK v6.0A,Microsoft Windows SDK v7.0A,

Microsoft Windows SDK v7.1),只有Microsoft Platform SDK February 2003没有，心想这下坏了，不好编译了，因为Microsoft Platform SDK February 2003是Windows XP最终版SDK，翻了一下%systemroot%\system32\又可以找到normaliz.dll,查看了一下导出函数，又是有的，不懂什么软件带过来的，还是系统更新装上的。

msdn对IdnToUnicode,IdnToAscii这两个函数的使用要求如下:

Minimum supported client	Windows Vista [desktop apps | Windows Store apps]
Minimum supported server	Windows Server 2008 [desktop apps | Windows Store apps]
Redistributable	Microsoft Internationalized Domain Name (IDN) Mitigation APIs onWindows XP with SP2 and later,Windows Server 2003 with SP1
Header	Winnls.h (include Windows.h)
Library	Normaliz.lib
DLL	Normaliz.dll

  从以上表格看出，这个库一开始并不是为Windows XP准备的，只是后来在SP2以后才有，用VC++6.0编译的时候normaliz.lib指向的dll函数地址就是错误的,
于是想到用LoadLibrary和GetProcAddress函数配合，显式调用者两个函数，从这个程度上来说也可以节省内存开销，使用完立即释放掉。

解决办法:

问题的根源是idn,于是找到了CURL_FOLDER\lib\idn_win32.c进行开刀。
以下是7.50.3版本的idn_win32.c修改前的模样:

#include "curl_setup.h"

#ifdef USE_WIN32_IDN

#include "curl_multibyte.h"
#include "curl_memory.h"
#include "warnless.h"

  /* The last #include file should be: */
#include "memdebug.h"

#ifdef WANT_IDN_PROTOTYPES
#  if defined(_SAL_VERSION)
WINNORMALIZEAPI int WINAPI
IdnToAscii(_In_                           DWORD    dwFlags,
           _In_reads_(cchUnicodeChar)     LPCWSTR  lpUnicodeCharStr,
           _In_                           int      cchUnicodeChar,
           _Out_writes_opt_(cchASCIIChar) LPWSTR   lpASCIICharStr,
           _In_                           int      cchASCIIChar);
WINNORMALIZEAPI int WINAPI
IdnToUnicode(_In_                             DWORD   dwFlags,
             _In_reads_(cchASCIIChar)         LPCWSTR lpASCIICharStr,
             _In_                             int     cchASCIIChar,
             _Out_writes_opt_(cchUnicodeChar) LPWSTR  lpUnicodeCharStr,
             _In_                             int     cchUnicodeChar);
#  else
WINBASEAPI int WINAPI IdnToAscii(DWORD dwFlags,
                                 const WCHAR *lpUnicodeCharStr,
                                 int cchUnicodeChar,
                                 WCHAR *lpASCIICharStr,
                                 int cchASCIIChar);
WINBASEAPI int WINAPI IdnToUnicode(DWORD dwFlags,
                                   const WCHAR *lpASCIICharStr,
                                   int cchASCIIChar,
                                   WCHAR *lpUnicodeCharStr,
                                   int cchUnicodeChar);
#  endif
#endif

#define IDN_MAX_LENGTH 255

bool curl_win32_idn_to_ascii(const char *in, char **out);
bool curl_win32_ascii_to_idn(const char *in, char **out);

bool curl_win32_idn_to_ascii(const char *in, char **out)
{
  bool success = FALSE;

  wchar_t *in_w = Curl_convert_UTF8_to_wchar(in);
  if(in_w) {
    wchar_t punycode[IDN_MAX_LENGTH];
    int chars = IdnToAscii(0, in_w, -1, punycode, IDN_MAX_LENGTH);
    free(in_w);
    if(chars) {
      *out = Curl_convert_wchar_to_UTF8(punycode);
      if(*out)
        success = TRUE;
    }
  }

  return success;
}

bool curl_win32_ascii_to_idn(const char *in, char **out)
{
  bool success = FALSE;

  wchar_t *in_w = Curl_convert_UTF8_to_wchar(in);
  if(in_w) {
    size_t in_len = wcslen(in_w) + 1;
    wchar_t unicode[IDN_MAX_LENGTH];
    int chars = IdnToUnicode(0, in_w, curlx_uztosi(in_len),
                             unicode, IDN_MAX_LENGTH);
    free(in_w);
    if(chars) {
      *out = Curl_convert_wchar_to_UTF8(unicode);
      if(*out)
        success = TRUE;
    }
  }

  return success;
}

#endif /* USE_WIN32_IDN */

idn_win32.c修改后的模样:

#include "curl_setup.h"

#ifdef USE_WIN32_IDN

#include "curl_multibyte.h"
#include "curl_memory.h"
#include "warnless.h"

  /* The last #include file should be: */
#include "memdebug.h"

#ifdef WANT_IDN_PROTOTYPES
//#  if defined(_SAL_VERSION)
typedef int (*fnIdnToAscii)(DWORD,const WCHAR *,int,WCHAR *,int);
typedef int (*fnIdnToUnicode)(DWORD,const WCHAR *,int,WCHAR *, int);
//#  endif
#endif

#define IDN_MAX_LENGTH 255

bool curl_win32_idn_to_ascii(const char *in, char **out);
bool curl_win32_ascii_to_idn(const char *in, char **out);

bool curl_win32_idn_to_ascii(const char *in, char **out)
{
  bool success = FALSE;

  wchar_t *in_w = Curl_convert_UTF8_to_wchar(in);
 
  if(in_w) {
    wchar_t punycode[IDN_MAX_LENGTH];
    int chars = -1;
    fnIdnToAscii IdnToAscii;
    HINSTANCE hNormalizDLL = LoadLibrary("normaliz.dll");
    if (!hNormalizDLL) {
        FreeLibrary(hNormalizDLL);
        assert(hNormalizDLL);
        return FALSE;
    }
    IdnToAscii = (fnIdnToAscii)GetProcAddress(hNormalizDLL, "IdnToAscii");
    if (!IdnToAscii) {
        assert(IdnToAscii);
        return TRUE;
    }
    chars = IdnToAscii(0, in_w, -1, punycode, IDN_MAX_LENGTH);
    
    free(IdnToAscii);
    FreeLibrary(hNormalizDLL);

    free(in_w);
    if(chars) {
      *out = Curl_convert_wchar_to_UTF8(punycode);
      if(*out)
        success = TRUE;
    }
  }

  return success;
}

bool curl_win32_ascii_to_idn(const char *in, char **out)
{
  bool success = FALSE;

  wchar_t *in_w = Curl_convert_UTF8_to_wchar(in);
  if(in_w) {
    size_t in_len = wcslen(in_w) + 1;
    wchar_t unicode[IDN_MAX_LENGTH];
    int chars = -1;
    fnIdnToUnicode IdnToUnicode;
    
    HINSTANCE hNormalizDLL = LoadLibrary("normaliz.dll");
    if (!hNormalizDLL) {
        FreeLibrary(hNormalizDLL);
        assert(hNormalizDLL);
        return FALSE;
    }
    IdnToUnicode = (fnIdnToUnicode)GetProcAddress(hNormalizDLL, "IdnToUnicode");
    if (!IdnToUnicode) {
        assert(IdnToUnicode);
        return FALSE;
    }
    chars = IdnToUnicode(0, in_w, curlx_uztosi(in_len),
                             unicode, IDN_MAX_LENGTH);
    
    free(IdnToUnicode);
    FreeLibrary(hNormalizDLL);
    free(in_w);
    if(chars) {
      *out = Curl_convert_wchar_to_UTF8(unicode);
      if(*out)
        success = TRUE;
    }
  }

  return success;
}

#endif /* USE_WIN32_IDN */

 经过以上修改就可以编译成功了。