How to handle inet_ntop() failure?

Question

First of all, my code example:

cout << "bla1" << endl;
struct addrinfo hints, *info;
int status;

memset(&hints, 0, sizeof hints);

char ip4[INET_ADDRSTRLEN];
char ip6[INET6_ADDRSTRLEN];

hints.ai_family = AF_INET;
hints.ai_socktype = SOCK_STREAM;

cout << "bla2" << endl;

status = getaddrinfo(url.c_str(), NULL, &hints, &info);

cout << "bla3" << endl;

if(!inet_ntop(AF_INET, &((const sockaddr_in *)info->ai_addr)->sin_addr , ip4, INET_ADDRSTRLEN)) {
  return ERROR_PAR;
}

cout << "bla4" << endl;

url variable contains the adress to be resolved (I'm working on simple client/server DNS resolver). If it can be resolved, everything works fine, however when the url can't be resolved, my output is only

bla1 bla2 bla3

The code above is in forked child so it doesn't stop the whole script, it just goes back to the parent process, no error though (I'm testing the return value, in this case it should be ERROR_PAR = 1 so the error message should appear).

Is there something wrong about the way I use these functions or the problem must be somewhere else?

EDIT: It's important to check getaddrinfo return value before any other functions. So the problem is solved.

Have you checked `errno` when the function fails? It should give you an error code that tells you why it fails. — Some programmer dude, Mar 07 '12 at 14:34
Also, are you sure the first address returned by `getaddrinfo` is an IPv4 address? — Some programmer dude, Mar 07 '12 at 14:35
Oh, how could I forget to check the getaddrinfo return value? I'm trying to solve it wrong way here. Thanks, the problem is "solved"! — milanseitler, Mar 07 '12 at 14:40

score 1 · Answer 1 · answered Nov 12 '17 at 19:18

To formally answer this, check the manual:

On success, inet_ntop() returns a non-null pointer to dst. NULL is returned if there was an error, with errno set to indicate the error.

So you would do something like:

#include <arpa/inet.h>
#include <stdio.h>                                                                                                                                                                                                 
#include <string.h>
#include <errno.h>

int main(void) {
    char *ip = "127.0.0.1";
    uint32_t src;
    inet_pton(AF_INET, ip, &src);

    char dst[INET_ADDRSTRLEN];
    if (inet_ntop(AF_INET, &src, dst, INET_ADDRSTRLEN)) {
        printf("converted value = %s \n", dst);   
        return 0;                                                                                                                                        
    } else {
        printf("inet_ntop conversion error: %s\n", strerror(errno));
        return 1;
    }
}

How to handle inet_ntop() failure?

1 Answers1