Socket 由浅入深系列--------- 简单实现编程（三）

socket 由浅入深 原理（一）介绍了一些，以下也就是简单实现，并未考虑其它性能化！

使用TCP的server客户机举例
server
设置一个简单的TCPserver涉及下列步骤：

调用 socket()建立套接字
调用Binding 把套接字绑定到一个监听端口上。在调用 bind()之前, 程序必须声明一个 sockaddr_in 结构体,用 memset()清除， and the sin_family (AF_INET), and fill its sin_port (the listening port, in network byte order) fields. Converting a short int to network byte order can be done by calling the function htons() (host to network short).
Preparing the socket to listen for connections (making it a listening socket), with a call to listen().
Accepting incoming connections, via a call to accept(). This blocks until an incoming connection is received, and then returns a socket descriptor for the accepted connection. The initial descriptor remains a listening descriptor, and accept() can be called again at any time with this socket, until it is closed.
Communicating with the remote host, which can be done through send() and recv() or write() and read().
Eventually closing each socket that was opened, once it is no longer needed, using close(). Note that if there were any calls to fork(), each process must close the sockets it knew about (the kernel keeps track of how many processes have a descriptor open), and two processes should not use the same socket at once.
  /* Server code in C */

  #include <sys/types.h>
  #include <sys/socket.h>
  #include <netinet/in.h>
  #include <arpa/inet.h>
  #include <stdio.h>
  #include <stdlib.h>
  #include <string.h>
  #include <unistd.h>

  int main(void)
  {
    struct sockaddr_in stSockAddr;
    int SocketFD = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP);

    if(-1 == SocketFD)
    {
      perror("can not create socket");
      exit(EXIT_FAILURE);
    }

    memset(&stSockAddr, 0, sizeof(struct sockaddr_in));

    stSockAddr.sin_family = AF_INET;
    stSockAddr.sin_port = htons(1100);
    stSockAddr.sin_addr.s_addr = INADDR_ANY;

    if(-1 == bind(SocketFD,(const struct sockaddr *)&stSockAddr, sizeof(struct sockaddr_in)))
    {
      perror("error bind failed");
      close(SocketFD);
      exit(EXIT_FAILURE);
    }

    if(-1 == listen(SocketFD, 10))
    {
      perror("error listen failed");
      close(SocketFD);
      exit(EXIT_FAILURE);
    }

    for(;;)
    {
      int ConnectFD = accept(SocketFD, NULL, NULL);

      if(0 > ConnectFD)
      {
        perror("error accept failed");
        close(SocketFD);
        exit(EXIT_FAILURE);
      }

     /* perform read write operations ... */

      shutdown(ConnectFD, SHUT_RDWR);

      close(ConnectFD);
    }

    close(SocketFD);
    return 0;
  }
 客户机
建立一个客户机连接涉及下面步骤：

调用 socket()建立套接字。
用connect()连接到server, passing a sockaddr_in structure with the sin_family set to AF_INET, sin_port set to the port the endpoint is listening (in network byte order), and sin_addr set to the IP address of the listening server (also in network byte order.)
用send() 和 recv() 或者 write() 和 read()进行通信。
用close()终止连接。假设调用fork(), 每一个进程都要用close()。
  /* Client code in C */

  #include <sys/types.h>
  #include <sys/socket.h>
  #include <netinet/in.h>
  #include <arpa/inet.h>
  #include <stdio.h>
  #include <stdlib.h>
  #include <string.h>
  #include <unistd.h>

  int main(void)
  {
    struct sockaddr_in stSockAddr;
    int Res;
    int SocketFD = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP);

    if (-1 == SocketFD)
    {
      perror("cannot create socket");
      exit(EXIT_FAILURE);
    }

    memset(&stSockAddr, 0, sizeof(struct sockaddr_in));

    stSockAddr.sin_family = AF_INET;
    stSockAddr.sin_port = htons(1100);
    Res = inet_pton(AF_INET, "192.168.1.3", &stSockAddr.sin_addr);

    if (0 > Res)
    {
      perror("error: first parameter is not a valid address family");
      close(SocketFD);
      exit(EXIT_FAILURE);
    }
    else if (0 == Res)
    {
      perror("char string (second parameter does not contain valid ipaddress");
      close(SocketFD);
      exit(EXIT_FAILURE);
    }

    if (-1 == connect(SocketFD, (const struct sockaddr *)&stSockAddr, sizeof(struct sockaddr_in)))
    {
      perror("connect failed");
      close(SocketFD);
      exit(EXIT_FAILURE);
    }

    /* perform read write operations ... */

    shutdown(SocketFD, SHUT_RDWR);

    close(SocketFD);
    return 0;
  }
使用UDP的server客户机举例
用户数据报协议（UDP）是一个不保证正确传输的无连接协议。 UDP数据包可能会乱序到达，多次到达或者直接丢失。可是设计的负载比TCP小。

UDP地址空间，也即是UDP端口，和TCP端口是没有关系的。

server
Code may set up a UDP server on port 7654 as follows:

#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <unistd.h> /* for close() for socket */
#include <stdlib.h>

int main(void)
{
  int sock = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP);
  struct sockaddr_in sa;
  char buffer[1024];
  ssize_t recsize;
  socklen_t fromlen;

  memset(&sa, 0, sizeof(sa));
  sa.sin_family = AF_INET;
  sa.sin_addr.s_addr = INADDR_ANY;
  sa.sin_port = htons(7654);

  if (-1 == bind(sock,(struct sockaddr *)&sa, sizeof(struct sockaddr)))
  {
    perror("error bind failed");
    close(sock);
    exit(EXIT_FAILURE);
  } 

  for (;;)
  {
    printf ("recv test....\n");
    recsize = recvfrom(sock, (void *)buffer, 1024, 0, (struct sockaddr *)&sa, &fromlen);
    if (recsize < 0)
      fprintf(stderr, "%s\n", strerror(errno));
    printf("recsize: %d\n ",recsize);
    sleep(1);
    printf("datagram: %s\n",buffer);
  }
}
上面的无限循环用recvfrom()接收给UDP端口7654的数据包。使用例如以下參数：

指向缓存数据指针
缓存大小
标志
地址
地址结构体大小
 客户机
用UDP数据包发送一个"Hello World!" 给地址127.0.0.1（回环地址），端口 7654 。

#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <unistd.h> /* for close() for socket */

int main(int argc, char *argv[])
{
  int sock;
  struct sockaddr_in sa;
  int bytes_sent, buffer_length;
  char buffer[200];

  buffer_length = snprintf(buffer, sizeof buffer, "Hello World!");

  sock = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP);
  if (-1 == sock) /* if socket failed to initialize, exit */
    {
      printf("Error Creating Socket");
      exit(EXIT_FAILURE);
    }

  memset(&sa, 0, sizeof(sa));
  sa.sin_family = AF_INET;
  sa.sin_addr.s_addr = htonl(0x7F000001);
  sa.sin_port = htons(7654);

  bytes_sent = sendto(sock, buffer, buffer_length, 0,(struct sockaddr*)&sa, sizeof (struct sockaddr_in));
  if (bytes_sent < 0)
    printf("Error sending packet: %s\n", strerror(errno));

  close(sock); /* close the socket */
  return 0;
}
buffer指定要发送数据的指针, buffer_length指定缓存内容的大小。

linux scoket 由浅入深 具体解释（四）