casting sockaddr_storage as sockaddr_in for inet_ntop

Question

I am trying to cast a sockaddr_storage to a sockadd_in, so that i can print out the source ip address of a datagram packet, i do not seem to be able to get the cast correct,

struct sockaddr_storage  peer_addr;


getnameinfo((struct sockaddr *) &peer_add
                peer_addrlen,
                hostbuff, sizeof(hostbuff),
                NULL, 0, NI_NAMEREQD);


inet_ntop(AF_INET, (((struct sockaddr_in *)peer_addr).sin_addr), ipbuff, INET_ADDRSTRLEN);

when i try to cast the structure to a sockaddr_in, i either get 'cannot convert to pointer', or when i remove the dereferance i get 'conversion to non scaler type requested'.

I have tried alot of combinations and simply don't understand where i am going wrong.

Martin R · Accepted Answer · 2013-11-22T12:30:12.933

11

inet_ntop(peer_addr->ss_family, &(((struct sockaddr_in *)peer_addr)->sin_addr), ipbuff, INET_ADDRSTRLEN);

should work. But consider to use getnameinfo() instead, which is the more modern interface:

char host[NI_MAXHOST];
getnameinfo((struct sockaddr *)peer_addr, peer_addr->ss_len, host, sizeof(host), NULL, 0, NI_NUMERICHOST);

works for both IPv4 and IPv6 addresses.

Update according to the changed types in the question: This is a complete example that should compile without warnings:

int socket = ...;

struct sockaddr_storage peer_addr;
socklen_t peer_addrlen;
char host[NI_MAXHOST];

ssize_t amount;
char buffer[1000];
amount = recvfrom(socket, buffer, sizeof(buffer), 0, (struct sockaddr *)&peer_addr, &peer_addrlen);
getnameinfo((struct sockaddr *)&peer_addr, peer_addrlen, host, sizeof(host), NULL, 0, NI_NUMERICHOST);

Or, using inet_ntop():

char ipbuff[INET_ADDRSTRLEN];
inet_ntop(peer_addr.ss_family, &(((struct sockaddr_in *)&peer_addr)->sin_addr), ipbuff, INET_ADDRSTRLEN);

edited Nov 22 '13 at 12:30

answered Nov 21 '13 at 19:56

Martin R

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i thought getnameinfo() returned the hostname of the address? – Babbleshack Nov 21 '13 at 20:00
@Babbleshack: See "man getnameinfo" for the NI_NUMERICHOST flag: Return the address in numeric form, as if calling inet_ntop(3), instead of a host name. – Martin R Nov 21 '13 at 20:02
The portable way is not to use `peer_addr->ss_family` (which isn't even known at least on my system), but to use the real size of the allocated structure. – glglgl Nov 21 '13 at 23:41
@Babbleshack: May I ask why you "unaccepted" the answer? Does it not work anymore? - Of course glglgl is right that using the actual sockaddr length (e.g. as returned by recvfrom()) is the more portable way, but that should not make my answer completely wrong. - And it confusing if you change the question. Originally, peer_addr was a *pointer* to a struct sockaddr_storage, therefore my answer assumed that. – Martin R Nov 22 '13 at 11:49
@MartinR Thanks your answer reflects my working code. I have reaccepted your answer. – Babbleshack Nov 22 '13 at 13:09

score 2 · Answer 2 · answered Nov 21 '13 at 23:40

struct sockaddr_storage  *  peer_addr;


getnameinfo((struct sockaddr *) &peer_add
                peer_addrlen,
                hostbuff, sizeof(hostbuff),
                NULL, 0, NI_NAMEREQD);

Here you are mixing up stuff.

getnameinfo() indeed takes a struct sockaddr* as its first parameter, but what you try to do here won't work: peer_addr is a struct sockaddr_storage *, you take its address - which is a struct sockaddr_storage **, and try to cast this. That won't work.

I don't know where your peer_addr comes from, but

either it should be a struct sockaddr_storage (I don't think there is a need to have a pointer to a struct sockaddr_storage somewhere)
or it is really a pointer, and you should pass (struct sockaddr *) peer_addr - without the & - to getnameinfo().

Another point: The second parameter for getnameinfo() is supposed to be the "real" size of the address struct you are inputting.

when i try to do that i get a compilation error, when i use my original code, my program works as expected. does this mean i have 'undefined behaviour' — Babbleshack, Nov 21 '13 at 23:57
@Babbleshack I don't know what that meas as I don't know which compilation error you get. But it is very likely that you get UB, as a cast might hide and shadow problems. — glglgl, Nov 22 '13 at 08:43
When i remove the dereferance from sockaddr, i get 'conversion to non scaler type' error — Babbleshack, Nov 22 '13 at 11:06

casting sockaddr_storage as sockaddr_in for inet_ntop

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