Non reentrant information about gethostbyname()

Recently, I found that many little friends are learning by themselves Linux The book of network programming , There may be many puzzling doubts in the middle , Some parts of the book are not explained very thoroughly , I happened to have taken this course before , Today, share a problem you have encountered .

First of all, please read 229 page ,8.4.4 function gethostbyname Examples of non reentrant

Let's take a look at this code ：

#include <netdb.h>
#include <string.h>
#include <stdio.h>
#include <arpa/inet.h>
int main(int argc, char *argv[])
{
	struct hostent *ht=NULL;
	
	char host[]="www.sina.com.cn";				/* Inquire about sina Host domain name */
	char host1[]="www.sohu.com";				/* Inquire about sohu Host domain name */
	char str[30];
	struct hostent *ht1=NULL, *ht2=NULL;
	
	ht1 = gethostbyname(host);					/* Inquire about "www.sina.com.cn"*/
	ht2 = gethostbyname(host1);					/* Inquire about "www.sohu.com"*/
	int j = 0; 	
	for(j = 0;j<2;j++){
		if(j == 0)
			ht = ht1;								/*sina*/
		else
			ht =ht2;								/*sohu*/
		
		if(ht){
			int i = 0;
			printf("get the host:%s addr\n",host);			/* Original domain name */
			printf("name:%s\n",ht->h_name);				/* name */
			
			/* Protocol family AF_INET by IPv4 perhaps AF_INET6 by IPv6*/
			printf("type:%s\n",ht->h_addrtype==AF_INET?"AF_INET":"AF_INET6"); 		
			
			printf("legnth:%d\n",ht->h_length);			/*IP The length of the address */
			/* Print IP Address */
			for(i=0;;i++){
				if(ht->h_addr_list[i] != NULL){	/* No IP End of address array */
					printf("IP:%s\n",inet_ntop(ht->h_addrtype,ht->h_addr_list[i],str,30));							/* Print IP Address */
				}	else{							/* Reach the end */
					break;							/* sign out for loop */
				}
			}
			
			/* Print domain name and address */
			for(i=0;;i++){/* loop */
				if(ht->h_aliases[i] != NULL){		/* The end of the domain name array was not reached */
					printf("alias %d:%s\n",i,ht->h_aliases[i]);															/* Print domain name */
				}	else{							/* ending */
					break;							/* Exit loop */
				}
			}
		}
	}
	return 0;
}

This is the result of this code , The result of the second line is what my friend asked me the most incomprehensible ： Why? ht Of h_name The output is sohu instead of sina.

This involves gethostbyname How to handle functions , Just need to know a few points ：

First of all ：gethostbyname The result returned when processing data is stored in a static structure variable , We all know static variables , As a local global variable , Initialize only once , in other words , The next time you operate on this variable , Based on the results of this operation

second ：ht1 and ht2 This structure pointer is running gethostbyname This function points to the same address （ Above gethostbyname Store the variable address of the returned result ）（ You can perform partial execution in the compiler to check the change of variable value ）

（ The expression may not be very accurate , You can understand what I mean or make comments ）

After knowing these two points , We can know ,（14 and 15 That's ok ） At the first time gethostbyname After running , The value of the variable that stores the returned result is the original domain name host Domain name of （ namely www.sina.com.cn）, here ht1 Also point to the address where the variable is stored , But after the second run , This value has changed to （www.sohu.com）, And at this point ht1 and ht2 The address pointed to is the address of this result , So when the second line outputs the result, even at this time ht The assignment is ht1,（25 and 26 That's ok ） The result is still www.sohu.com, You can share your views or put forward your opinions .