#define PORT    3456
#define BACKLOG   10
#define MAX_EPOLL_FD 65535

int CreateTcpListenSocket();
int InitEpollFd();
void UseConnectFd(int sockfd);
void setnonblocking(int sock);

static int listenfd;

int main()
{
 int epoll_fd;
 int nfds;
 int i;
 
 struct epoll_event events[50];
 struct epoll_event tempEvent;

 int sockConnect;
 struct sockaddr_in remoteAddr;
 int addrLen;

 addrLen = sizeof(struct sockaddr_in);
 epoll_fd = InitEpollFd();
 if (epoll_fd == -1)
 {
  perror("init epoll fd error.");
  exit(1);
 }

 printf("begin in loop.\n");
 while (1)
  {
  nfds = epoll_wait(epoll_fd, events, 50, 1000);
  //sleep(3);
  printf("connect num: %d\n", nfds);
  if (nfds == -1)
   {
   perror("epoll_wait error.");
   }
  for (i = 0; i < nfds; i++)
   {
   if (listenfd == events[i].data.fd)
    {
    printf("connected success\n");
    sockConnect = accept(events[i].data.fd, (struct sockaddr*)&remoteAddr, &addrLen);
    if (sockConnect == -1)
     {
     perror("accept error.");
     continue;
     }
    setnonblocking(sockConnect);
    tempEvent.events = EPOLLIN | EPOLLET;
    tempEvent.data.fd = sockConnect;
    if (epoll_ctl(epoll_fd, EPOLL_CTL_ADD, sockConnect, &tempEvent) < 0)
     {
     perror("epoll ctl error.");
     return -1;
     }
    }
   else
    {
    UseConnectFd(events[i].data.fd);
    }
   }
  }
 
}

int CreateTcpListenSocket()
{
 int sockfd;
 struct sockaddr_in localAddr;
 if ((sockfd = socket(AF_INET, SOCK_STREAM, 0)) == -1)
 {
  perror("create socket fail");
  return -1;
 }

 setnonblocking(sockfd);
 
 bzero(&localAddr, sizeof(localAddr));
 localAddr.sin_family = AF_INET;
 localAddr.sin_port = htons(PORT);
 localAddr.sin_addr.s_addr = htonl(INADDR_ANY);

 if (bind(sockfd,  (struct sockaddr*)&localAddr, sizeof(struct sockaddr)) == -1)
 {
  perror("bind error");
  return -1;
 }

 if (listen(sockfd, BACKLOG) == -1)
 {
  perror("listen error");
  return -1;
 }

 return sockfd;
}

int InitEpollFd()
{
 //epoll descriptor
 int s_epfd;
 struct epoll_event ev;
 

 listenfd = CreateTcpListenSocket();

 if (listenfd == -1)
 {
  perror("create tcp listen socket error");
  return -1;
 }

 s_epfd = epoll_create(MAX_EPOLL_FD);
 ev.events = EPOLLIN;
 ev.data.fd = listenfd;
 if (epoll_ctl(s_epfd, EPOLL_CTL_ADD, listenfd, &ev) < 0)
 {
  perror("epoll ctl error");
  return -1;
 }

 return s_epfd;
}

void UseConnectFd(int sockfd)
{
 char recvBuff[1500];
 int recvNum = 0;

 recvNum = recv(sockfd, recvBuff, 1500, 0);
 if (recvNum == -1)
  {
  perror("recv error.");
  return;
  }

 recvBuff[recvNum] = '\0';
 printf("message: %s \n", recvBuff);
}

void setnonblocking(int sock)

{

     int opts;

     opts=fcntl(sock,F_GETFL);

     if(opts<0)

     {

          perror("fcntl(sock,GETFL)");

          exit(1);

     }

     opts = opts|O_NONBLOCK;

     if(fcntl(sock,F_SETFL,opts)<0)

     {

          perror("fcntl(sock,SETFL,opts)");

          exit(1);

     }   

}

client端（VC6.0）:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <winsock2.h>
//#include <sockets.h>
//#include <unistd.h>
//#include <sys/types.h>
//#include <sys/socket.h>
//#include <arpa/inet.h>
#pragma comment(lib,"wsock32.lib")
#define PORT    3456

int main()
{
 int sockfd;
 struct sockaddr_in remoteAddr;
 char mesg[] = "This is a client for epoll test";
 WSADATA wsdata;

 memset(&remoteAddr, 0, sizeof(remoteAddr));
 remoteAddr.sin_family = AF_INET;
 remoteAddr.sin_port = htons(PORT);
 remoteAddr.sin_addr.s_addr = inet_addr("10.40.8.15");

 
 WSAStartup(0x0202,&wsdata);

 if ((sockfd = socket(AF_INET, SOCK_STREAM, 0)) == -1)
  {
  perror("create socket fail.");
  return -1;
  }
 WSAAsyncSelect(sockfd, NULL, NULL, 0);
 printf("server ip is: %s\n", (char *)inet_ntoa(remoteAddr.sin_addr));

// while (1)
// {
  if (connect(sockfd, (struct sockaddr*)&remoteAddr, sizeof(remoteAddr)) == -1)
  {
   //perror("connect error.");
   printf("Error code: %d\n", WSAGetLastError());
   return -1;
  }
  //printf("times\n");
// }

 printf("connected!\n");
 while (1)
  {
  
  send(sockfd, mesg, strlen(mesg), 0);
  printf("send over\n");
  Sleep(1000);
  }
}

       APP_PRINT("the num is: %d", num);
        return num;
      }
      利用这个函数可以得到字符串中那几个字节是一起的。因为UTF8最大只有6个字节，所以就根据返回值来处理这里我只处理了3个字节和1个字节的UTF8的编码，因为一般来说中文在UTF8中是3个字节。

//将len个字节的UTF8格式的转换成GB2312格式存放在temp预先申请好的缓冲区中
void Utf8ToGb2312(const char* utf8, int len, char *temp)
{
       APP_PRINT("utf8->unicode: \n");
       APP_PRINT("utf8: [");
       for (int k = 0; k < len; k++)
       {
              APP_PRINT("%02x ", utf8[k]);
       }
       APP_PRINT("]\n");
 
       int byteCount = 0;
       int i = 0;
       int j = 0;

       u16 unicodeKey = 0;
       u16 gbKey = 0;


      //循环解析
       while (i < len)
       {   
        switch(GetUtf8ByteNumForWord((u8)utf8[i]))
        {
          case 0:
                temp[j] = utf8[i];
                byteCount = 1;
          break;

          case 2:
          temp[j] = utf8[i];
          temp[j + 1] = utf8[i + 1];
          byteCount = 2;
          break;

         case 3:
                 //这里就开始进行UTF8->Unicode
                 temp[j + 1] = ((utf8[i] & 0x0F) << 4) | ((utf8[i + 1] >> 2) & 0x0F);
                 temp[j] = ((utf8[i + 1] & 0x03) << 6) + (utf8[i + 2] & 0x3F);

                //取得Unicode的值
                 memcpy(&unicodeKey, (temp + j), 2);
                 APP_PRINT("unicode key is: 0x%04X\n", unicodeKey);

                  //根据这个值查表取得对应的GB2312的值
                gbKey = SearchCodeTable(unicodeKey);
                APP_PRINT("gb2312 key is: 0x%04X\n", gbKey);
    
                if (gbKey != 0)
                {
                       //here change the byte
                        //不为0表示搜索到，将高低两个字节调换调成我要的形式
                       gbKey = (gbKey >> 8) | (gbKey << 8);
                       APP_PRINT("after changing, gb2312 key is: 0x%04X\n", gbKey);
                       memcpy((temp + j), &gbKey, 2);
                  }

                byteCount = 3;
          break;

          case 4:
          byteCount = 4;
          break;
         case 5:
          byteCount = 5;
          break;
         case 6:
          byteCount = 6;
          break;
    
         default:
          APP_PRINT("the len is more than 6\n");
          break;    
        }

        i += byteCount;
        if (byteCount == 1)
        {
               j++;
        }
        else
        {
               j += 2;
        }
  
       }
       APP_PRINT("utf8: [");
       for (k = 0; k < j; k++)
       {
              APP_PRINT("%02x ", temp[k]);
       }
       APP_PRINT("]\n");
}

      二、下面主要谈谈利用查表法来进行Unicode->GB2312的转换，首先下载码表，一般码表都是将GB2312的放在前面，Unicode放在后面，这样对于我们来说不方便使用，所以我转换了下，将Unicode放在前面，而且按照从小到大排好序。（这里只需要考虑都为两个字节的情况，因为前面的UTF8->Unicode并没有将单字节的ASCII转换成Unicode）
            (1)做表：(可以到这里下载：http://blog.91bs.com/?action=show&id=20，这里谢谢渣渣的猪窝)
            这个是原来的样子：
            0x8140 0x4E02 #CJK UNIFIED IDEOGRAPH
            0x8141 0x4E04 #CJK UNIFIED IDEOGRAPH
            0x8142 0x4E05 #CJK UNIFIED IDEOGRAPH
            先弄成（这个可以写个小程序来做，我就是在VC上做的，如果需要可以联系我）：
            { 0x4E02 ，0x8140 }, //CJK UNIFIED IDEOGRAPH
            { 0x4E04 ，0x8141 }, //CJK UNIFIED IDEOGRAPH
            { 0x4E05 ，0x8142 }, //CJK UNIFIED IDEOGRAPH
            这样就可以把这些放在.h文件中了，下面是我的定义：
            typedef struct unicode_gb
            {
                   unsigned short unicode;
                   unsigned short gb;
            } UNICODE_GB;

            UNICODE_GB code_table[] = 
            {
                  { 0x4E02, 0x8140 },   //CJK UNIFIED IDEOGRAPH
                  { 0x4E04, 0x8141 },  //CJK UNIFIED IDEOGRAPH
                  { 0x4E05, 0x8142 },  //CJK UNIFIED IDEOGRAPH
                  。。。。。。省略

            下面这一步也很简单，在VC中用冒泡排序法，对这个数组按照unicode值进行排序，如果需要可以联系我，把最终结果打印出来，在cmd下运行name > 1.txt就输出到文件，这样就有了一个按照unicode排好序的unicode->gb2312码表。

   int main(int argc, char *argv[])
{

    int num = 0;
    UNICODE_GB temp;
    int i = 0;
    int j = 0;

    num = sizeof(code_table) / sizeof(UNICODE_GB);

    printf("struct size: %d | total size: %d | num is: %d \n", 
    sizeof(UNICODE_GB), sizeof(code_table), num);

    for (i = 0; i < num; i++)
    {
        for (j = 1; j < num - i; j++)
        {
            if (code_table[j - 1].unicode > code_table[j].unicode)
            {
                temp.unicode = code_table[j - 1].unicode;
                temp.gb = code_table[j - 1].gb;
                code_table[j - 1].unicode = code_table[j].unicode;
                code_table[j - 1].gb = code_table[j].gb;
                code_table[j].unicode = temp.unicode;
                code_table[j].gb = temp.gb;
            }
        }
    }

    printf("here is the code table sorted by unicode\n\n");

    for (i = 0; i < num; i++)
    {
        printf("{\t0x%04X,\t0x%04X\t},\t\n", code_table[i].unicode, code_table[i].gb);
     }

       printf("\n\n print over!\n");

   //以下注释掉的其实就是我用来对原来的码表添加，{，}等用的
   /*
    char buff[100];
    char buff_1[100]; 
 
    FILE* fp = NULL;
    FILE *fp_1 = NULL;

    memset(buff, 0, 100);
    memset(buff_1, 0, 100);
 
    fp = fopen("table.txt", "rw");
    fp_1 = fopen("table_1.txt", "a+");

    if ((fp == NULL) || (fp_1 == NULL))
    {
        printf("open file error!\n");
        return 1;
    }

    while (fgets(buff, 100, fp) != NULL)
    {
        buff[8] = ',';

        fputs(buff, fp_1);
    }
 */

    return 0;
}

      最后就是搜索算法了，前面已经排好序了，现在我们把排好序的码表放在我们真正需要的.h文件中。大家应该猜我用什么算法搜索了吧，二分法。

#define CODE_TABLE_SIZE 21791
//这个表是死的，所以就直接用宏表示长度，不用每次都用size，不过这样可能对移植性不好。
u16 SearchCodeTable(u16 unicodeKey)
{
    int first = 0;
    int end = CODE_TABLE_SIZE - 1;
    int mid = 0;

    while (first <= end)
    {
        mid = (first + end) / 2;

        if (code_table[mid].unicode == unicodeKey)
        {
            return code_table[mid].gb;
        }
        else if (code_table[mid].unicode > unicodeKey)
        {
            end = mid - 1;
        }
        else 
        {
            first = mid + 1;
        }
    }
    return 0;
}
      到此，已经能够将UTF8串转换成GB2312了。是一长串哦，而不是单个汉字的编码转换。

int copy_from_back(FILE * srcfd, FILE * dstfd);

int main(int argc, char *argv[])
{
 FILE *srcfd = NULL;
 FILE *dstfd = NULL;
 
 if (argc != 3)
 {
  printf("usage: FileStudy srcFiel dstFile\n");
 }

 srcfd = fopen(argv[1], "r");
 dstfd = fopen(argv[2], "w+");

 if (copy_from_back(srcfd, dstfd) == -1)
 {
  printf("copy error: ");
  exit(EXIT_FAILURE);
 }

 return 0; 
}

int copy_from_back(FILE *srcfd, FILE *dstfd)
{
 char ch;
 int num_read = 0;
 
 if (srcfd == NULL || dstfd == NULL)
 {
  perror("fopen error: ");
  return -1;
 }

 fseek(dstfd, 0, SEEK_SET);
 
 /*指向文件末尾，如果继续读，返回EOF*/
 if (fseek(srcfd, 1, SEEK_END) != 0)
 {
  perror("fseek error");
 } 
 else
 {
  
  printf("begin to read file\n");
  while (ftell(srcfd) > 0)
  {
   char ch = fgetc(srcfd);
   if (ch == EOF)
   {
    printf("file end!\n");
    fseek(srcfd, -2, SEEK_CUR);
    continue;
   }
   putchar(ch);
   fputc(ch, dstfd);
   num_read++;
   fseek(srcfd, -2, SEEK_CUR);
  }

  char start = fgetc(srcfd);
  putchar(start);
  printf("\nread file end: %d \n", num_read);
 }

 return 0;
}

/*
int copy_from_back(FILE *srcfd, FILE *dstfd)
{
 char ch;
 
 if (srcfd == NULL || dstfd == NULL)
 {
  perror("fopen error: ");
  return -1;
 }
 
 fseek(srcfd, 0, SEEK_SET);
 
 while ((ch = fgetc(srcfd)) != EOF)
 { 
  putchar(ch);
  
  fputc(ch, dstfd); 
  fseek(dstfd, 1, SEEK_SET);
 }

 return 0;
}
*/