I'm trying to print an ip address from inet_ntop, but the output seems pretty weird.
The program seems to work well, I succeed to connect the socket but it print this:
H��H9�u�H�[]A\A]A^A_�ff.�
Here is my code:
#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <netdb.h>
#include <string.h>
#include <arpa/inet.h>
int main(int argc, char *argv[]){
int sock = socket(AF_INET, SOCK_STREAM, 0);
struct addrinfo hints, *result;
memset(&hints, 0, sizeof(struct addrinfo));
hints.ai_family = AF_INET;
hints.ai_socktype = SOCK_STREAM;
hints.ai_protocol = 0;
hints.ai_flags = 0;
int s = getaddrinfo("irc.root-me.org", "6667", &hints, &result);
if( s != 0){
printf("erreur\n");
exit(EXIT_FAILURE);
}
int f = connect(sock, result->ai_addr, result->ai_addrlen);
if(f != 0){
printf("erreur connect\n");
}
struct sockaddr_in *sockin;
sockin = (struct sockaddr_in *)result->ai_addr;
char *dst;
inet_ntop(AF_INET, &sockin->sin_addr, dst, sizeof(char *));
printf("%s\n", dst);
freeaddrinfo(result);
exit(0);
}
Two issues here:
The 3rd parameter to inet_ntop() should be a char-array or a pointer to the 1st element of a char-array.
The 4th parameter to inet_ntop() should be the size of the destination buffer, whose address is passed as 3rd parameter.
What you do is to pass the uninitialised char-pointer dst as destination and tell the function it would point to sizeof(char*) bytes memory.
An AF_INET address (which has the form xxx.xxx.xxx.xxx) uses a maximum of 4x3 characters plus 3 delimiting . chars which sums up to 15 chars plus 1 additional char used as 0-terminator to make it a C-"string", so the corrected code would look like this:
char dst[16] = ""; /* inet_ntop() does not necessarily 0-terminates the result. */
inet_ntop(AF_INET, &sockin->sin_addr, dst, sizeof dst);
As pointed out by Filipe Gonçalves in his comment one can use INET_ADDRSTRLEN (if available) instead of the hard coded "magic number" 16 to define the buffer size for the textual representation of a IPv4 address. It's a good thing.
Documentation for inet_ntop() is here:
The problem is with the size of the destination char* you're trying to save the result in, and the fact that it is uninitialized, what you want to do is save it in a char[].
sizeof(char*) on x86 is 4B and 8B on x64, IP Addresses are usually larger than that (an IPv4 address is between 7 to 15 bytes) + 1 for null terminator.
You can fix the code as follows:
char dst[16] = {0};
inet_ntop(AF_INET, &sockin->sin_addr, dst, sizeof(dst));
After the fix:
$ ./main
212.83.153.145
If you want the full fixed main.c, I've uploaded it to github.