socket编程——一个简单的例子

从一个简单的使用TCP例子开始socket编程，其基本步骤如下：

server                                                  client

+++++++                                          ++++++++

创建socket                                          创建socket

+++++++                                          ++++++++

|                                                         |

|                                                         |

|                                                         |

+++++++                                          ++++++++

地址赋值（                                           地址赋值（

自己的地址）                                        服务器地址）

+++++++                                          ++++++++

|                                                         |

|                                                         |

|                                                         |

++++++++                                              |

用bind绑定                                                |

socket和地址                                             |

++++++++                                              |

|                                                         |

|                                                         |

|                                                         |

+++++++                                                 |

listen                                                         |

+++++++                                                  |

|                                                    ++++++++++

|   <------------------------------          connect 服务器

|                                                    ++++++++++

+++++++                                                  |

accept                                                        |

+++++++                                                  |

|                                                           |

|                                                    +++++++++

|                                                     recv 和send

|                                                     进行数据处理

|                                                     +++++++++

+++++++++                                                |

用accept得到                                                 |

的socket进行                                                 |

recv 和 send                                                 |

+++++++++                                                |

|                                                             |

|                                                             |

|                                                             |

+++++++++                                        +++++++++

close socket                                         close socket

+++++++++                                        +++++++++

根据以上步骤，服务器端的代码为

#include <stdio.h>
#include <string.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <stdlib.h>
#include <syslog.h>
#include <errno.h>
#define MAX_LISTEN_NUM 5
#define SEND_BUF_SIZE 100
#define RECV_BUF_SIZE 100
#define LISTEN_PORT 1010
int main()
{
  int listen_sock = ;
  int app_sock = ;
  struct sockaddr_in hostaddr;
  struct sockaddr_in clientaddr;
  int socklen = sizeof(clientaddr);
  char sendbuf[SEND_BUF_SIZE] = {};
  char recvbuf[RECV_BUF_SIZE] = {};
  int sendlen = ;
  int recvlen = ;
  int retlen = ;
  int leftlen = ;
  char *ptr = NULL;
  memset((void *)&hostaddr, , sizeof(hostaddr));
  memset((void *)&clientaddr, , sizeof(clientaddr));
  hostaddr.sin_family = AF_INET;
  hostaddr.sin_port = htons(LISTEN_PORT);
  hostaddr.sin_addr.s_addr = htonl(INADDR_ANY);
  listen_sock = socket(AF_INET, SOCK_STREAM, );
  if(listen_sock < )
  {
      syslog(LOG_ERR, "%s:%d, create socket failed", __FILE__, __LINE__);
      exit();
  }
  if(bind(listen_sock, (struct sockaddr *)&hostaddr, sizeof(hostaddr)) < )
  {
      syslog(LOG_ERR, "%s:%d, bind socket failed", __FILE__, __LINE__);
      exit();
  }
  if(listen(listen_sock, MAX_LISTEN_NUM) < )
  {
      syslog(LOG_ERR, "%s:%d, listen failed", __FILE__, __LINE__);
      exit();
  }
  while()
  {
      app_sock = accept(listen_sock, (struct sockaddr *)&clientaddr, &socklen);
      if(app_sock < )
     {
        syslog(LOG_ERR, "%s:%d, accept failed", __FILE__, __LINE__);
        exit();
     }
     sprintf(sendbuf, "welcome %s:%d here!/n", inet_ntoa(clientaddr.sin_addr.s_addr), clientaddr.sin_port);
     //send data
     sendlen = strlen(sendbuf) +;
     retlen = ;
     leftlen = sendlen;
     ptr = sendbuf;
     //while(leftlen)
     {
         retlen = send(app_sock, ptr, sendlen, );
      if(retlen < )
      {
          if(errno == EINTR)
            retlen = ;
        else
            exit();
      }
      leftlen -= retlen;
      ptr += retlen;
     }
     //receive data
     recvlen = ;
     retlen = ;
     ptr = recvbuf;
     leftlen = RECV_BUF_SIZE -;
     //do
     {
         retlen = recv(app_sock, ptr, leftlen, ) ;
      if(retlen < )
      {
          if(errno == EINTR)
            retlen = ;
        else
            exit();
      }
      recvlen += retlen;
      leftlen -= retlen;
      ptr += retlen;
     }
     //while(recvlen && leftlen);
     printf("receive data is : %s", recvbuf);
    close(app_sock);
  }
  close(listen_sock);  

  return ;  

}  

客户端代码为：

#include <stdio.h>
#include <string.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <syslog.h>
#include <errno.h>
#include <stdlib.h>
#define MAX_LISTEN_NUM 5
#define SEND_BUF_SIZE 100
#define RECV_BUF_SIZE 100
#define SERVER_PORT 1010
int main()
{
    int sock_fd = ;
    char recvbuf[RECV_BUF_SIZE] = {};
    char sendbuf[SEND_BUF_SIZE] = {};
    int recvlen = ;
    int retlen = ;
    int sendlen = ;
    int leftlen = ;
    char *ptr = NULL;
    struct sockaddr_in ser_addr;  

    memset(&ser_addr, , sizeof(ser_addr));
    ser_addr.sin_family = AF_INET;
    inet_aton("127.0.0.1", (struct in_addr *)&ser_addr.sin_addr);
    ser_addr.sin_port = htons(SERVER_PORT);
    sock_fd = socket(AF_INET, SOCK_STREAM, );
    if(sock_fd < )
    {
        syslog(LOG_ERR, "%s:%d, create socket failed", __FILE__, __LINE__);
        exit();
    }
    if(connect(sock_fd, (struct sockaddr *)&ser_addr, sizeof(ser_addr)) < )
    {
        syslog(LOG_ERR, "%s:%d, connect socket failed", __FILE__, __LINE__);
        exit();
    }
     //receive data
     recvlen = ;
     retlen = ;
     ptr = recvbuf;
     leftlen = RECV_BUF_SIZE -;
     //do
     {
         retlen = recv(sock_fd, ptr, leftlen, ) ;
      if(retlen < )
      {
          if(errno == EINTR)
            retlen = ;
        else
            exit();
      }
      recvlen += retlen;
      leftlen -= retlen;
      ptr += retlen;
     }
     //while(recvlen && leftlen);
     printf("receive data is : %s", recvbuf);
     sprintf(sendbuf, "hello server/n");
     //send data
     sendlen = strlen(sendbuf) +;
     retlen = ;
     leftlen = sendlen;
     ptr = sendbuf;
    // while(leftlen)
     {
         retlen = send(sock_fd, ptr, sendlen, );
      if(retlen < )
      {
          if(errno == EINTR)
            retlen = ;
        else
            exit();
      }
      leftlen -= retlen;
      ptr += retlen;
     }
     close(sock_fd);  

}  

现在一个简单的使用tcp的socket通信的例子已经完成了，这里有几个需要说明的问题

1）头文件：

sys/socket.h   包含了socket相关的函数，如socket，send 和recv， 以及struct sockaddr等

netinet/in.h    包含了地址结构，如struct sockaddr_in

errno.h           包含了errno 和 EINTR

syslog.h         包含了syslog相关的信息，其打印结果在/var/log/messages里面

2）socket地址

对于IPv4来说，其地址用的是struct sockaddr_in，具体结构如下

struct in_addr {
  in_addr_t   s_addr;           /* 32-bit IPv4 address */
                                /* network byte ordered */
};  

struct sockaddr_in {
  uint8_t         sin_len;      /* length of structure (16) */
  sa_family_t     sin_family;   /* AF_INET */
  in_port_t       sin_port;     /* 16-bit TCP or UDP port number */
                                /* network byte ordered */
  struct in_addr  sin_addr;     /* 32-bit IPv4 address */
                                /* network byte ordered */
  char            sin_zero[];  /* unused */
};  

其中sin_len我们一般不关注，也不填（只有在使用routing socket的时候才用到，被内核用来处理各种协议簇的地址结构）。
bind, connect, sendto, 和 sendmsg会把socket地址从程序传递给内核； 而accept, recvfrom, recvmsg, getpeername, 和
 getsockname会把地址从内核传递给程序。因为不同协议簇的地址结构是不一样的，所以必须要有一个通用的指针来传递地址，
对于ANSI C来说我们一般使用void *，但是socket产生早于ANSI C，所以也就没有使用这个机制，而是使用一个通用的地址结构
struct sockaddr来处理的

struct sockaddr {
  uint8_t      sa_len;
  sa_family_t  sa_family;    /* address family: AF_xxx value */
  char         sa_data[];  /* protocol-specific address */
}; 

IPv6的socket地址为struct sockaddr_in6

struct in6_addr {
  uint8_t  s6_addr[];          /* 128-bit IPv6 address */
                                 /* network byte ordered */
};  

#define SIN6_LEN      /* required for compile-time tests */  

struct sockaddr_in6 {
  uint8_t         sin6_len;      /* length of this struct (28) */
  sa_family_t     sin6_family;   /* AF_INET6 */
  in_port_t       sin6_port;     /* transport layer port# */
                                 /* network byte ordered */
  uint32_t        sin6_flowinfo; /* flow information, undefined */
  struct in6_addr sin6_addr;     /* IPv6 address */
                                 /* network byte ordered */
  uint32_t        sin6_scope_id; /* set of interfaces for a scope */
};  

对于sockaddr-in6来说，我们不能用通用的地址struct sockaddr来存储了，而是产用新的通用地址结构struct sockaddr_storage，
这个结构足够大可以存储任何系统支持的地址。

struct sockaddr_storage {
  uint8_t      ss_len;       /* length of this struct (implementation dependent) */
  sa_family_t  ss_family;    /* address family: AF_xxx value */
  /* implementation-dependent elements to provide:
   * a) alignment sufficient to fulfill the alignment requirements of
   *    all socket address types that the system support
   * b) enough storage to hold any type of socket address that the
   *    system supports.
   */
};  

几种常见的地址结构

3） 相关函数的的length

对于从程序传地址给内核的函数（如connect），其长度是一个整型值，告诉内核要copy的地址长度。

对于从内核传递给程序的函数（如accpt），其长度是一个整型指针，是一个value-result参数。有两个目的：一告诉内核地址结构的长度，
让内核在copy的时候不要超过这个长度；二返回内核真正copy的长度。
4）字节序
socket相关的函数都是使用网络字节序
5）地址转换函数
inet_aton, inet_ntoa, and inet_addr把IPv4字符串地址转为32位的网络字节序地址
inet_ptonand inet_ntop可以转换IPv4和IPv6的地址
6）listen中的backlog
要知道这个值的含义先用说一下，对于一个listen的socket，有两个队列：一个是incomplete connection队列（仅仅收到SYN）；
一个是complete connection队列（三次握手完成）。accept函数就是在complete connection队列中取一个socket。backlog就是指队列的个数，
但不行的是各个地方都没有明确定义这个值，没有说明究竟代表了哪个队列，或是两个队列之和。一般来说可以
同时处理的连接数是backlog的1.5倍，很多地方都用5.
7） getsockname 和 getpeername
这两个函数可以与socket关联的地址，getsockname 和 getpeername分别得到自己和对端的地址

int getsockname(int sockfd, struct sockaddr *localaddr, socklen_t *addrlen)  

int getpeername(int sockfd , struct sockaddr * peeraddr , socklen_t * addrlen );  

FROM: http://blog.csdn.net/wind19/article/details/6156339