socket编程——一个简单的样例

从一个简单的使用TCP样例開始socket编程，其基本过程例如以下：

server                                                  client

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

创建socket                                          创建socket

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

|                                                         |

|                                                         |

|                                                         |

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

地址赋值（                                           地址赋值（

自己的地址）                                        server地址）

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

|                                                         |

|                                                         |

|                                                         |

++++++++                                              |

用bind绑定                                                |

socket和地址                                             |

++++++++                                              |

|                                                         |

|                                                         |

|                                                         |

+++++++                                                 |

listen                                                         |

+++++++                                                  |

|                                                    ++++++++++

|   <------------------------------          connect server

|                                                    ++++++++++

+++++++                                                  |

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

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

依据以上步骤，server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 = 0;
int app_sock = 0;
struct sockaddr_in hostaddr;
struct sockaddr_in clientaddr;
int socklen = sizeof(clientaddr);
char sendbuf[SEND_BUF_SIZE] = {0};
char recvbuf[RECV_BUF_SIZE] = {0};
int sendlen = 0;
int recvlen = 0;
int retlen = 0;
int leftlen = 0;
char *ptr = NULL;
memset((void *)&hostaddr, 0, sizeof(hostaddr));
memset((void *)&clientaddr, 0, 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, 0);
if(listen_sock < 0)
{
syslog(LOG_ERR, "%s:%d, create socket failed", __FILE__, __LINE__);
exit(1);
}
if(bind(listen_sock, (struct sockaddr *)&hostaddr, sizeof(hostaddr)) < 0)
{
syslog(LOG_ERR, "%s:%d, bind socket failed", __FILE__, __LINE__);
exit(1);
}
if(listen(listen_sock, MAX_LISTEN_NUM) < 0)
{
syslog(LOG_ERR, "%s:%d, listen failed", __FILE__, __LINE__);
exit(1);
}
while(1)
{
app_sock = accept(listen_sock, (struct sockaddr *)&clientaddr, &socklen);
if(app_sock < 0)
{
syslog(LOG_ERR, "%s:%d, accept failed", __FILE__, __LINE__);
exit(1);
}
sprintf(sendbuf, "welcome %s:%d here!/n", inet_ntoa(clientaddr.sin_addr.s_addr), clientaddr.sin_port);
//send data
sendlen = strlen(sendbuf) +1;
retlen = 0;
leftlen = sendlen;
ptr = sendbuf;
//while(leftlen)
{
retlen = send(app_sock, ptr, sendlen, 0);
if(retlen < 0)
{
if(errno == EINTR)
retlen = 0;
else
exit(1);
}
leftlen -= retlen;
ptr += retlen;
}
//receive data
recvlen = 0;
retlen = 0;
ptr = recvbuf;
leftlen = RECV_BUF_SIZE -1;
//do
{
retlen = recv(app_sock, ptr, leftlen, 0) ;
if(retlen < 0)
{
if(errno == EINTR)
retlen = 0;
else
exit(1);
}
recvlen += retlen;
leftlen -= retlen;
ptr += retlen;
}
//while(recvlen && leftlen);
printf("receive data is : %s", recvbuf);
close(app_sock);
}
close(listen_sock);

return 0;

}

client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 = 0;
char recvbuf[RECV_BUF_SIZE] = {0};
char sendbuf[SEND_BUF_SIZE] = {0};
int recvlen = 0;
int retlen = 0;
int sendlen = 0;
int leftlen = 0;
char *ptr = NULL;
struct sockaddr_in ser_addr;

memset(&ser_addr, 0, 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, 0);
if(sock_fd < 0)
{
syslog(LOG_ERR, "%s:%d, create socket failed", __FILE__, __LINE__);
exit(1);
}
if(connect(sock_fd, (struct sockaddr *)&ser_addr, sizeof(ser_addr)) < 0)
{
syslog(LOG_ERR, "%s:%d, connect socket failed", __FILE__, __LINE__);
exit(1);
}
//receive data
recvlen = 0;
retlen = 0;
ptr = recvbuf;
leftlen = RECV_BUF_SIZE -1;
//do
{
retlen = recv(sock_fd, ptr, leftlen, 0) ;
if(retlen < 0)
{
if(errno == EINTR)
retlen = 0;
else
exit(1);
}
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) +1;
retlen = 0;
leftlen = sendlen;
ptr = sendbuf;
// while(leftlen)
{
retlen = send(sock_fd, ptr, sendlen, 0);
if(retlen < 0)
{
if(errno == EINTR)
retlen = 0;
else
exit(1);
}
leftlen -= retlen;
ptr += retlen;
}
close(sock_fd);

}

如今一个简单的使用tcp的socket通信的样例已经完毕了，这里有几个须要说明的问题

1）头文件：

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

netinet/in.h    包括了地址结构，如struct sockaddr_in

errno.h           包括了errno 和 EINTR

syslog.h         包括了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，详细结构例如以下

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[8];  /* unused */
};


当中sin_len我们一般不关注，也不填（仅仅有在使用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。由于不同协议簇的地址结构是不一样的，所以必需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 *，可是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[14];  /* protocol-specific address */
};

IPv6的socket地址为struct sockaddr_in6

struct in6_addr {
  uint8_t  s6_addr[16];          /* 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，这个结构足够大能够存储不论什么系统支持的地址。
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參数。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能够转换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（只收到SYN）；一个是complete connection队列（三次握手完毕）。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白定义这个值，没有说明到底代表了哪个队列，或是两个队列之和。一般来说能够
同一时候处理的连接数是backlog的1.5倍，非常多地方都用5.

7） getsockname 和 getpeername
这两个函数能够与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 );