十六进制字符串转int64

#include <math.h>__int64 Str2Int64(char* str){   if(strstr(str,"0x") || strstr(str,"0X"))   {      str += 2;      __int64 tmp = 0;      int len = strlen(str);      for(int i = 0;i < len ;i++)      {         int nDecNum;         switch(str[i])         {        case 'a':        case 'A':           nDecNum = 10;           break;        case 'b':        case 'B':           nDecNum = 11;           break;         case 'c':         case 'C':            nDecNum = 12;            break;        case 'd':        case 'D':            nDecNum = 13;            break;        case 'e':        case 'E':            nDecNum = 14;            break;        case 'f':        case 'F':            nDecNum = 15;            break;        case '0':        case '1':        case '2':        case '3':        case '4':        case '5':        case '6':        case '7':        case '8':        case '9':            nDecNum = str[i] - '0';            break;        default:            return 0;

        }        tmp += nDecNum*pow(16,len-i -1);     }     return tmp;   }   else   {      return _atoi64(str);   }}