C socket get IP address from filedescriptor returned from accept

Question

I know this question seems typical and multiple times answered but I think if you read the details it is not so common (I did not find it).

The point is that I am developing a unix service in c that opens a socket and waits for connections, when I have a connection I create a new process to treat it, so there can be multiple connections opened at the same time.

int newfd = accept(sockfd, (struct sockaddr *)&clientaddr, (socklen_t*)&clientaddr_size);

Later on (after and inside some other methods and code) the child process save the connection information to the BBDD and I need also, in that precise moment, to get the IP address that opened that connection being treated.

As there can be multiple connections at the same time and the variable struct sockaddr_in clientaddr that I pass to the accept method is shared for all the process I am not sure that later on is a good idea to get the IP address information from that way because then I could get the IP address from another connection opened.

I would like to be able to access the IP address from the file descriptor int newfd that I get from the accept method (the returned integer). Is it possible? Or I misunderstood the file descriptor function?

Solution 1

Ok. Thanks to @alk and @rileyberton I found the correct method to use, the getpeername:

int sockfd;

void main(void) {
    //[...]
    struct sockaddr_in clientaddr;
    socklen_t clientaddr_size = sizeof(clientaddr);
    int newfd = accept(sockfd, (struct sockaddr *)&clientaddr, &clientaddr_size);
    //fork() and other code
    foo(newfd);
    //[...]
}
void foo(int newfd) {
    //[...]
    struct sockaddr_in addr;
    socklen_t addr_size = sizeof(struct sockaddr_in);
    int res = getpeername(newfd, (struct sockaddr *)&addr, &addr_size);
    char *clientip = new char[20];
    strcpy(clientip, inet_ntoa(addr.sin_addr));
    //[...]
}

So now in a different process I can get the IP address (in the "string" clientip) of the client that originated the connection only carrying the file descriptor newfd obtained with the accept method.

Solution 2

You would use getsockname() (http://linux.die.net/man/2/getsockname) to get the IP of the bound socket.

Also answered before, here: C - Public IP from file descriptor

Comments

[Aleix]

Thanks @rileyberton, the correct method it worked for me is getpeername() (linux.die.net/man/2/getpeername) I can see is so similar the post but a little bit confusing the question with so much talk between the processes :P

[alk]

Do not declare addr_size as int, but as socklen_t. Casting the int* to socklen_t* in the call to getpeername() hides possible issues in case socklen_t would be of different size then int. Same for the call to accept().

[Aleix]

Done! and also I changed it in the call to accept() method. Thank you very much for the suggestion:)

[user207421]

The IP address of the bound socket is not what he asked for. Read the question.