socket編程——一個簡單的例子

從一個簡單的使用TCP例子開始socket編程，其基本步驟如下：

server                                                  client

 

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

建立socket                                          建立socket

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

      |                                                         |

      |                                                         |

      |                                                         |

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

地址賦值（                                           地址賦值（

自己的地址）                                        伺服器位址）

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

      |                                                         |

      |                                                         |

      |                                                         |

++++++++                                              |

用bind綁定                                                |

socket和地址                                             |

++++++++                                              |

      |                                                         |

      |                                                         |

      |                                                         |

+++++++                                                 |

listen                                                         |

+++++++                                                  |

      |                                                    ++++++++++

      |   <------------------------------          connect 伺服器         

      |                                                    ++++++++++

+++++++                                                  |

accept                                                        |

+++++++                                                  |

      |                                                           |

      |                                                    +++++++++

      |                                                     recv 和send

      |                                                     進行資料處理

      |                                                     +++++++++

+++++++++                                                |

用accept得到                                                 |

的socket進行                                                 |

recv 和 send                                                 |

+++++++++                                                |

      |                                                             |

      |                                                             |

      |                                                             |

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

close socket                                         close socket

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

 

根據以上步驟，伺服器端的代碼為

#include <stdio.h><br />#include <string.h><br />#include <sys/socket.h><br />#include <netinet/in.h><br />#include <stdlib.h><br />#include <syslog.h><br />#include <errno.h><br />#define MAX_LISTEN_NUM 5<br />#define SEND_BUF_SIZE 100<br />#define RECV_BUF_SIZE 100<br />#define LISTEN_PORT 1010<br />int main()<br />{<br /> int listen_sock = 0;<br /> int app_sock = 0;<br /> struct sockaddr_in hostaddr;<br /> struct sockaddr_in clientaddr;<br /> int socklen = sizeof(clientaddr);<br /> char sendbuf[SEND_BUF_SIZE] = {0};<br /> char recvbuf[RECV_BUF_SIZE] = {0};<br /> int sendlen = 0;<br /> int recvlen = 0;<br /> int retlen = 0;<br /> int leftlen = 0;<br /> char *ptr = NULL;<br /> memset((void *)&hostaddr, 0, sizeof(hostaddr));<br /> memset((void *)&clientaddr, 0, sizeof(clientaddr));<br /> hostaddr.sin_family = AF_INET;<br /> hostaddr.sin_port = htons(LISTEN_PORT);<br /> hostaddr.sin_addr.s_addr = htonl(INADDR_ANY);<br /> listen_sock = socket(AF_INET, SOCK_STREAM, 0);<br /> if(listen_sock < 0)<br /> {<br /> syslog(LOG_ERR, "%s:%d, create socket failed", __FILE__, __LINE__);<br /> exit(1);<br /> }<br /> if(bind(listen_sock, (struct sockaddr *)&hostaddr, sizeof(hostaddr)) < 0)<br /> {<br /> syslog(LOG_ERR, "%s:%d, bind socket failed", __FILE__, __LINE__);<br /> exit(1);<br /> }<br /> if(listen(listen_sock, MAX_LISTEN_NUM) < 0)<br /> {<br /> syslog(LOG_ERR, "%s:%d, listen failed", __FILE__, __LINE__);<br /> exit(1);<br /> }<br /> while(1)<br /> {<br /> app_sock = accept(listen_sock, (struct sockaddr *)&clientaddr, &socklen);<br /> if(app_sock < 0)<br /> {<br /> syslog(LOG_ERR, "%s:%d, accept failed", __FILE__, __LINE__);<br /> exit(1);<br /> }<br /> sprintf(sendbuf, "welcome %s:%d here!/n", inet_ntoa(clientaddr.sin_addr.s_addr), clientaddr.sin_port);<br /> //send data<br /> sendlen = strlen(sendbuf) +1;<br /> retlen = 0;<br /> leftlen = sendlen;<br /> ptr = sendbuf;<br /> //while(leftlen)<br /> {<br /> retlen = send(app_sock, ptr, sendlen, 0);<br /> if(retlen < 0)<br /> {<br /> if(errno == EINTR)<br /> retlen = 0;<br />else<br />exit(1);<br /> }<br /> leftlen -= retlen;<br /> ptr += retlen;<br /> }<br /> //receive data<br /> recvlen = 0;<br /> retlen = 0;<br /> ptr = recvbuf;<br /> leftlen = RECV_BUF_SIZE -1;<br /> //do<br /> {<br /> retlen = recv(app_sock, ptr, leftlen, 0) ;<br /> if(retlen < 0)<br /> {<br /> if(errno == EINTR)<br /> retlen = 0;<br />else<br />exit(1);<br /> }<br /> recvlen += retlen;<br /> leftlen -= retlen;<br /> ptr += retlen;<br /> }<br /> //while(recvlen && leftlen);<br /> printf("receive data is : %s", recvbuf);<br /> close(app_sock);<br /> }<br /> close(listen_sock);</p><p> return 0;</p><p>}<br />

 

 

用戶端代碼為：

#include <stdio.h><br />#include <string.h><br />#include <sys/socket.h><br />#include <netinet/in.h><br />#include <syslog.h><br />#include <errno.h><br />#include <stdlib.h><br />#define MAX_LISTEN_NUM 5<br />#define SEND_BUF_SIZE 100<br />#define RECV_BUF_SIZE 100<br />#define SERVER_PORT 1010<br />int main()<br />{<br /> int sock_fd = 0;<br /> char recvbuf[RECV_BUF_SIZE] = {0};<br /> char sendbuf[SEND_BUF_SIZE] = {0};<br /> int recvlen = 0;<br /> int retlen = 0;<br /> int sendlen = 0;<br /> int leftlen = 0;<br /> char *ptr = NULL;<br /> struct sockaddr_in ser_addr;</p><p> memset(&ser_addr, 0, sizeof(ser_addr));<br /> ser_addr.sin_family = AF_INET;<br /> inet_aton("127.0.0.1", (struct in_addr *)&ser_addr.sin_addr);<br /> ser_addr.sin_port = htons(SERVER_PORT);<br /> sock_fd = socket(AF_INET, SOCK_STREAM, 0);<br /> if(sock_fd < 0)<br /> {<br /> syslog(LOG_ERR, "%s:%d, create socket failed", __FILE__, __LINE__);<br /> exit(1);<br /> }<br /> if(connect(sock_fd, (struct sockaddr *)&ser_addr, sizeof(ser_addr)) < 0)<br /> {<br /> syslog(LOG_ERR, "%s:%d, connect socket failed", __FILE__, __LINE__);<br /> exit(1);<br /> }<br /> //receive data<br /> recvlen = 0;<br /> retlen = 0;<br /> ptr = recvbuf;<br /> leftlen = RECV_BUF_SIZE -1;<br /> //do<br /> {<br /> retlen = recv(sock_fd, ptr, leftlen, 0) ;<br /> if(retlen < 0)<br /> {<br /> if(errno == EINTR)<br /> retlen = 0;<br />else<br />exit(1);<br /> }<br /> recvlen += retlen;<br /> leftlen -= retlen;<br /> ptr += retlen;<br /> }<br /> //while(recvlen && leftlen);<br /> printf("receive data is : %s", recvbuf);<br /> sprintf(sendbuf, "hello server/n");<br /> //send data<br /> sendlen = strlen(sendbuf) +1;<br /> retlen = 0;<br /> leftlen = sendlen;<br /> ptr = sendbuf;<br /> // while(leftlen)<br /> {<br /> retlen = send(sock_fd, ptr, sendlen, 0);<br /> if(retlen < 0)<br /> {<br /> if(errno == EINTR)<br /> retlen = 0;<br />else<br />exit(1);<br /> }<br /> leftlen -= retlen;<br /> ptr += retlen;<br /> }<br /> close(sock_fd);</p><p>}<br />

 

 

現在一個簡單的使用tcp的socket通訊的例子已經完成了，這裡有幾個需要說明的問題

1）標頭檔：

sys/socket.h   包含了socket相關的函數，如socket，send 和recv， 以及struct sockaddr等

netinet/in.h    包含了地址結構，如struct sockaddr_in

errno.h           包含了errno 和 EINTR

syslog.h         包含了syslog相關的資訊，其列印結果在/var/log/messages裡面

 

2）socket地址

對於IPv4來說，其地址用的是struct sockaddr_in，具體結構如下

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[8];  /* unused */};


其中sin_len我們一般不關注，也不填（只有在使用routing socket的時候才用到，被核心用來處理各種協議簇的地址結構）。
bind, connect, sendto, 和 sendmsg會把socket地址從程式傳遞給核心； 而accept, recvfrom, recvmsg, getpeername, 和 getsockname會把地址從核心傳遞給程式。因為不同協議簇的地址結構是不一樣的，所以必須要有一個通用的指標來傳遞地址，對於ANSI C來說我們一般使用void *，但是socket產生早於ANSI C，所以也就沒有使用這個機制，而是使用一個通用的地址結構struct sockaddr來處理的
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地址為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 */};

對於sockaddr-in6來說，我們不能用通用的地址struct sockaddr來儲存了，而是產用新的通用地址結構struct sockaddr_storage，這個結構足夠大可以儲存任何系統支援的地址。
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.   */};
幾種常見的地址結構

3） 相關函數的的length
對於從程式傳地址給核心的函數（如connect），其長度是一個整型值，告訴核心要copy的地址長度。
對於從核心傳遞給程式的函數（如accpt），其長度是一個整型指標，是一個value-result參數。有兩個目的：一告訴核心地址結構的長度，讓核心在copy的時候不要超過這個長度；二返回核心真正copy的長度。

4）位元組序
socket相關的函數都是使用網路位元組序

5）地址轉換函式
inet_aton, inet_ntoa, and inet_addr把IPv4字串地址轉為32位的網路位元組序地址
inet_ptonand inet_ntop可以轉換IPv4和IPv6的地址

6）listen中的backlog
要知道這個值的含義先用說一下，對於一個listen的socket，有兩個隊列：一個是incomplete connection隊列（僅僅收到SYN）；一個是complete connection隊列（三向交握完成）。accept函數就是在complete connection隊列中取一個socket。backlog就是指隊列的個數，但不行的是各個地方都沒有明確定義這個值，沒有說明究竟代表了哪個隊列，或是兩個隊列之和。一般來說可以
同時處理的串連數是backlog的1.5倍，很多地方都用5.

7） getsockname 和 getpeername
這兩個函數可以與socket關聯的地址，getsockname 和 getpeername分別得到自己和對端的地址

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