inet_ Pton () and INET_ Detailed explanation of ntop() function

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1. hold ip The address is converted into a binary value for network transmission

int inet_aton(const char *cp, struct in_addr *inp);

inet_aton() transformation Network host address ip( Such as 192.168.1.10) by Binary values , And stored in struct in_addr In structure , That's the second parameter *inp, Function returns non 0 Express cp The host is efficient , return 0 Indicates that the host address is invalid .（ This conversion cannot be used for network transmission , You also need to call htons or htonl Function to convert host byte order to network byte order ）

 

in_addr_t inet_addr(const char *cp);

inet_addr Function conversion Network host address （ Such as 192.168.1.10) by Network byte order binary value , If parameters char *cp Invalid , The function returns -1(INADDR_NONE), This function is dealing with address 255.255.255.255 When you go back to －1,255.255.255.255 Is a valid address , however inet_addr Unable to deal with ;

 

2. Convert the binary value of network transmission into dot decimal ip Address

char *inet_ntoa(struct in_addr in);

inet_ntoa function Translate network byte sort address by The standard ASCII Addresses separated by dots , This function returns the address of the string pointing to the point separated （ Such as 192.168.1.10) The pointer to , Space allocated for the static string , This means that when the function is called the second time , The last call will be overridden （ Covering ）, So if you need to save the string, you can copy it and manage it yourself ！

 

How do we output a dot decimal IP Well ？ Let's take a look at the following program ：

#include <stdio.h>   
#include <sys/socket.h>   
#include <netinet/in.h>   
#include <arpa/inet.h>   
#include <string.h>   
int main()   
{   
	struct in_addr addr1,addr2;   
	ulong l1,l2;   
	l1= inet_addr("192.168.0.74");   
	l2 = inet_addr("211.100.21.179");   
	memcpy(&addr1, &l1, 4);   
	memcpy(&addr2, &l2, 4);   
	printf("%s : %s\n", inet_ntoa(addr1), inet_ntoa(addr2)); // Pay attention to the result of this sentence    
	printf("%s\n", inet_ntoa(addr1));   
	printf("%s\n", inet_ntoa(addr2));  
	return 0;   
}   

The actual operation results are as follows ：　

192.168.0.74 : 192.168.0.74          // You can see it here ,printf Inside inet_ntoa Only run once .　　

192.168.0.74　　

211.100.21.179　　

inet_ntoa Return to one char *, And this char * The space is in inet_ntoa It's statically allocated , therefore inet_ntoa Subsequent calls will override the previous call . The first sentence printf The result can only show that in printf Variable parameters in it It's evaluated from right to left Of , That's it .

 

3. New network address translation function inet_pton and inet_ntop
These two functions follow IPv6 Functions that appear , about IPv4 Address and IPv6 All addresses apply , Function p and n Each represents the expression （presentation) And numbers （numeric). Address is usually expressed in the form of ASCII character string , The numeric format is a binary value stored in the socket address structure .

#include <arpa/inet.h>
int inet_pton(int family, const char *strptr, void *addrptr);     // Will be dotted decimal ip The address is converted into a numeric format for network transmission 
         Return value ： If successful 1, If the input is not a valid expression 0, In case of error -1
 
const char * inet_ntop(int family, const void *addrptr, char *strptr, size_t len);     // Convert numerical format to dot decimal ip Address format 
         Return value ： If successful, a pointer to the structure , In case of error NULL

（1） Of these two functions family The parameter can be either AF_INET（ipv4） It can also be AF_INET6（ipv6）. If , Take an unsupported address family as family Parameters , Both functions return an error , And will errno Set as EAFNOSUPPORT.

（2） The first function attempts to convert from strptr The string that the pointer points to , And pass addrptr The pointer holds the binary result , If successful, the return value is 1, Otherwise, if specified family The input string is not a valid expression format , So the return value is zero 0.

（3）inet_ntop Do the opposite conversion , From the numerical format （addrptr） Convert to expression （strptr).inet_ntop Functional strptr Parameter cannot be a null pointer . The caller must allocate memory for the target storage unit and specify its size , When the call succeeds , This pointer is the return value of the function .len Parameter is the size of the target storage unit , To prevent the function from overflowing its caller's buffer . If len Too small , Not enough to hold expression results , Then return a null pointer , Juxtaposition as errno by ENOSPC.

4. Example

inet_pton(AF_INET, ip, &foo.sin_addr);   //   Instead of  foo.sin_addr.addr=inet_addr(ip);
 
 
char str[INET_ADDRSTRLEN];
char *ptr = inet_ntop(AF_INET,&foo.sin_addr, str, sizeof(str));      //  Instead of  ptr = inet_ntoa(foo.sin_addr)