sockaddr value changes unexpectedly after calling getaddrinfo()

Question

I am programming an UDP client. I want to bind the socket to a given port on the client machine, so that the same port is always used for all sends. I get the sockaddr for the server using getaddrinfo, and I do the same to get the sockaddr which I pass to the call to getaddrinfo. However, after the second call to getaddrinfo the address of the server machine changes, and I end up sending the packet from the client machine to the client machine itself.

The following code is a standalone example that reproduces the error:

#include <stdio.h>
#include <stdint.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>
#include <time.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <netinet/in.h>
#include <arpa/inet.h>

#define SERVER_HOST "www.google.com"
#define UDP_PORT "4000"

static struct sockaddr_in *destination_addr = NULL;
static int client_port;

int main(){

    uint8_t bytes[5] = { 0xaa, 0xab, 0xac, 0xad, 0xaf}; //some data to send

    uint16_t length = 5;
    int status;


    //initialize socket and bind
    if (destination_addr == NULL) {
        struct addrinfo hints;
        struct addrinfo *servinfo, *p;
        srand(time(NULL));
        memset(&hints, 0, sizeof hints);
        hints.ai_family = AF_INET;
        hints.ai_socktype = SOCK_DGRAM;
        hints.ai_flags = AI_PASSIVE;

        if ((status = getaddrinfo(SERVER_HOST, UDP_PORT, &hints, &servinfo)) != 0) {
            printf("Unable to send UDP. Reason: %s", gai_strerror(status));
            return 0;
        }


        for (p = servinfo; p != NULL; p = p->ai_next) {
            if (p->ai_addr != NULL)
                destination_addr = (struct sockaddr_in *) p->ai_addr;
        }
        client_port = 1027 + rand()%50000;
        freeaddrinfo(servinfo);
        printf("Created destination_addr with IP %s\n",  inet_ntoa(destination_addr->sin_addr));
    }


    int send_socket_fd = socket(AF_INET, SOCK_DGRAM, 0);
    if (send_socket_fd == -1) {
        printf("Unable to create UDP socket. Reason: %s", strerror(errno));
        return 0;
    }
    printf("IP after socket creation is %s\n", inet_ntoa(destination_addr->sin_addr));
    int yes = 1;
    if (setsockopt(send_socket_fd, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof (int)) == -1) {
        perror("setsockopt");
        return 0;
    }
    printf("IP after sockopt is %s\n", inet_ntoa(destination_addr->sin_addr));

    // bind to local address
    char str_client_port[6];
    snprintf(str_client_port, 5, "%d", client_port);
    struct addrinfo *source_addr_info;
    struct addrinfo hints;
    hints.ai_family = AF_INET;
    hints.ai_socktype = SOCK_DGRAM;


    // ***** destination_addr changes after this call *****
    getaddrinfo (NULL, str_client_port, &hints, &source_addr_info); 


    printf("IP after getaddrinfo is %s\n", inet_ntoa(destination_addr->sin_addr));
    bind(send_socket_fd, source_addr_info->ai_addr, source_addr_info->ai_addrlen);
    printf("IP after binding is %s\n", inet_ntoa(destination_addr->sin_addr));

    // send
    int bytes_sent = sendto(send_socket_fd, bytes, length, 0, (struct sockaddr *)destination_addr, sizeof *destination_addr);
    printf("Sent to IP %s\n", inet_ntoa(destination_addr->sin_addr));
    if (bytes_sent != length){
        if (bytes_sent == -1){
            printf("UDP send failed. Reason: %s", strerror(errno));
        }
        else {
            printf("UDP: not all bytes could be sent.");
        }
    }


    close(send_socket_fd);
    return 1;

}

The output generated by the execution of this program in my machine is:

Created destination_addr with IP 64.233.167.105
IP after socket creation is 64.233.167.105
IP after sockopt is 64.233.167.105
IP after getaddrinfo is 0.0.0.0
IP after binding is 0.0.0.0
Sent to IP 0.0.0.0

I am rather new to socket programming in C, and pretty sure I am doing some silly mistake, but after googling a lot and trying many things, I am still stuck with this. Any idea?

Well of course it changes. That's what the function is suppose to do. You should bind with the old address before you get the new address. Or use two address structures. Surely this is obvious? — user207421, Sep 30 '15 at 07:45
I doubt that `p->ai_addr` (a.k.a. `destination_addr`) is a valid pointer after `freeaddrinfo`. — molbdnilo, Sep 30 '15 at 07:51
@EJP I am using two address structures, `destination_addr` and `source_addr_info->ai_addr`. There should be no reason why calling `getaddrinfo` on `source_addr_info` should change the value of `destination_addr`. Unless, as @molbdnilo points out, the memory freed by `freeaddrinfo` is overwritten by the call to `getaddrinfo`. I will follow that path to solve the problem. — joanlofe, Sep 30 '15 at 08:42
I really can not understand why this question has been downvoted. — joanlofe, Sep 30 '15 at 10:22

score 1 · Accepted Answer · answered Sep 30 '15 at 09:11

Solved. As @molbdnilo pointed out, the error was caused by the call to freeaddrinfo. To fix it I now copy the value pointed by p->ai_addr, so that it is not lost when freeing. I substituted:

if (p->ai_addr != NULL)
    destination_addr = (struct sockaddr_in *) p->ai_addr;

with

if (p->ai_addr != NULL){
    destination_addr = malloc(sizeof *destination_addr);
    memcpy(destination_addr, (struct sockaddr_in *)p->ai_addr, sizeof *p->ai_addr);
}

and it worked.

sockaddr value changes unexpectedly after calling getaddrinfo()

1 Answers1