获取服务名、服务端口号、服务所用协议——getservent(),getservbyname(),getservbyport()

Services are represented by the port number portion of the address. Each service is offered on a unique, well-known port number. We can map a service name to a port number withgetservbyname, map a port number to a service name withgetservbyport, or scan the services database sequentially withgetservent.(APUE-2e)

地址的端口部分和服务是一一对应的，getservbyname()和 getservbyport()能将二者相互映射（map）。

/* Description of data base entry for a single service.  */struct servent{  char *s_name;/* Official service name.  */  char **s_aliases;/* Alias list.  */  int s_port;/* Port number.  */  char *s_proto;/* Protocol to use.  */};struct servent *getservent(void);struct servent *getservbyname(const char *name, const char *proto);struct servent *getservbyport(int port, const char *proto);void setservent(int stayopen);void endservent(void);

说明：

(1) 这几个函数的原理是查询 /etc/services 文件(ubuntu下)

(2) s_port is in network byte order, you should usentohs()(not htohl(), I don't know why) to convert it to host byte order.

/** * getservent() * OS: Ubuntu 11.04 Server */#include <stdio.h>#include <stdlib.h>#include <netdb.h>#include <arpa/inet.h>static void printserv(struct servent *serv);int main(){struct servent *serv = NULL;setservent(1);while( (serv = getservent()) != NULL ){printserv(serv);printf("\n");}endservent();return 0;}static void printserv(struct servent *serv){char **p = NULL;printf("service name: %s\n", serv->s_name);for(p = serv->s_aliases; *p != NULL; p++){printf("aliase: %s\n", *p);}printf("port number: %d\n", ntohs(serv->s_port));// Note: the port here is in network byte order.//printf("port number: %x\n", serv->s_port);//printf("port number: %d\n", ntohl(serv->s_port));// Note: use ntohs(), not ntohl(), I don't know why.printf("protocol: %s\n", serv->s_proto);}/*output:service name: tcpmuxport number: 256protocol: tcpservice name: echoport number: 1792protocol: tcpservice name: echoport number: 1792protocol: udp.../etc/services:tcpmux          1/tcp                           # TCP port service multiplexerecho            7/tcpecho            7/udp...*/