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 1010int 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 1010int 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 );