I noticed the same problem. I used a similar fix. If I did the example with http://www.google.com, the original code, printed “0.0.0.0”. It appears the last entry given by default name server is broken. The first entry is correct.

Actually, having learned a little more, I’ve reverted to the newer inet_ntop. Simpler to use, I’m unsure if any power is lost from using it. I think it will be alright. Just thought I would leave evidence of my trail.

Your getaddrinfo example continuously overwrites the “ip” C-string, until interating through the linked list is finished. Why not simply write to it once at end of the loop, if only getting the last entry is what you REALLY want, but that’s hardly useful! What you want is to make sure you’re using an IP that works!

“man getaddrinfo” from the terminal. On my OSX, it has nice example of iterating through the list and trying to test a socket connection for TCP. I have turned that example into usable code, integrated with your tutorial’s code:

http://pastebin.com/cfUw5sG6

For UDP you can do similar things by implementing a “test” msg for the server to receive and acknowledge with adaptive retransmission timeout.

i am not sure how AF_UNSPEC is supposed to behave exactly. It probably checks for both and based on the availability and priority it will return one of the addresses.

you can check out these links for more information on it

http://beej.us/guide/bgnet/output/html/singlepage/bgnet.html#ip4to6

http://stackoverflow.com/questions/8149601/what-would-be-the-disadvantages-risks-of-using-af-unspec

Thanks for your patient , while ,what makes me confuse is …

I mean,
if the ai_family’s value is “AF_UNSPEC”,

how the code judge using IPv4 or IPv6 ?

For example , if only my pc support IPv6 , will the code trying return the IPv6 address ?

for using ipv6, the server, your isp, your computer – all 3 should support ipv6

Thank you, It works !
In the manual,
“The value AF_UNSPEC indicates that getaddrinfo() should return socket addresses for any address family (either IPv4 or IPv6, for example) that can be used with node and service.”
What is the basis of the judgments using IPv4 or IPv6?

The server support ipv6 ？ or just my pc support IPv6?

or call the getaddrinfo function like this

getaddrinfo( hostname , NULL , NULL , &servinfo);

that should give the correct ip of the hostname.

try changing the following line

hints.ai_family = AF_UNSPEC; // use AF_INET6 to force IPv6

to this

hints.ai_family = AF_INET;

[yu@argcandargv-com gethostbyname]$ ./gethostbyname http://www.google.com

http://www.google.com resolved to 173.194.72.99

[yu@argcandargv-com gethostbyname]$ ./getaddrinfo http://www.google.com

http://www.google.com resolved to 0.0.0.0

[yu@argcandargv-com gethostbyname]$ ./getaddrinfo http://www.msn.com

http://www.msn.com resolved to 131.253.13.140

[yu@argcandargv-com gethostbyname]$

WHY?

check your internet connectivity by pinging the host first.
use the following command at terminal/console

ping hostname

if ping replies come correctly, then try the program again and let me know the results.

for more information on how to get ip address of local interface check this post

http://www.binarytides.com/get-local-ip-c-linux/