Get local ip in C on linux

/*
 * Find local ip used as source ip in ip packets.
 * Read the /proc/net/route file
 */
 
#include<stdio.h> //printf
#include<string.h>    //memset
#include<errno.h> //errno
#include<sys/socket.h>
#include<netdb.h>
#include<ifaddrs.h>
#include<stdlib.h>
#include<unistd.h>
 
int main ( int argc , char *argv[] )
{
    FILE *f;
    char line[100] , *p , *c;
     
    f = fopen("/proc/net/route" , "r");
     
    while(fgets(line , 100 , f))
    {
        p = strtok(line , " \t");
        c = strtok(NULL , " \t");
         
        if(p!=NULL && c!=NULL)
        {
            if(strcmp(c , "00000000") == 0)
            {
                printf("Default interface is : %s \n" , p);
                break;
            }
        }
    }
     
    //which family do we require , AF_INET or AF_INET6
    int fm = AF_INET;
    struct ifaddrs *ifaddr, *ifa;
    int family , s;
    char host[NI_MAXHOST];
 
    if (getifaddrs(&ifaddr) == -1) 
    {
        perror("getifaddrs");
        exit(EXIT_FAILURE);
    }
 
    //Walk through linked list, maintaining head pointer so we can free list later
    for (ifa = ifaddr; ifa != NULL; ifa = ifa->ifa_next) 
    {
        if (ifa->ifa_addr == NULL)
        {
            continue;
        }
 
        family = ifa->ifa_addr->sa_family;
 
        if(strcmp( ifa->ifa_name , p) == 0)
        {
            if (family == fm) 
            {
                s = getnameinfo( ifa->ifa_addr, (family == AF_INET) ? sizeof(struct sockaddr_in) : sizeof(struct sockaddr_in6) , host , NI_MAXHOST , NULL , 0 , NI_NUMERICHOST);
                 
                if (s != 0) 
                {
                    printf("getnameinfo() failed: %s\n", gai_strerror(s));
                    exit(EXIT_FAILURE);
                }
                 
                printf("address: %s", host);
            }
            printf("\n");
        }
    }
 
    freeifaddrs(ifaddr);
     
    return 0;
}

















/*
 * Find local ip used as source ip in ip packets.
 * Use getsockname and a udp connection
 */
 
#include<stdio.h> //printf
#include<string.h>    //memset
#include<errno.h> //errno
#include<sys/socket.h>    //socket
#include<netinet/in.h> //sockaddr_in
#include<arpa/inet.h> //getsockname
#include<unistd.h>    //close
 
int main ( int argc , char *argv[] )
{
    const char* google_dns_server = "8.8.8.8";
    int dns_port = 53;
     
    struct sockaddr_in serv;
     
    int sock = socket ( AF_INET, SOCK_DGRAM, 0);
     
    //Socket could not be created
    if(sock < 0)
    {
        perror("Socket error");
    }
     
    memset( &serv, 0, sizeof(serv) );
    serv.sin_family = AF_INET;
    serv.sin_addr.s_addr = inet_addr( google_dns_server );
    serv.sin_port = htons( dns_port );
 
    int err = connect( sock , (const struct sockaddr*) &serv , sizeof(serv) );
     
    struct sockaddr_in name;
    socklen_t namelen = sizeof(name);
    err = getsockname(sock, (struct sockaddr*) &name, &namelen);
         
    char buffer[100];
    const char* p = inet_ntop(AF_INET, &name.sin_addr, buffer, 100);
         
    if(p != NULL)
    {
        printf("Local ip is : %s \n" , buffer);
    }
    else
    {
        //Some error
        printf ("Error number : %d . Error message : %s \n" , errno , strerror(errno));
    }
 
    close(sock);
     
    return 0;
}