Obtaining the ip address of an interface dynamically

Obtaining the ip address of an interface dynamically

[Glover George]

Hi, I'm working on a little module here and was wondering what the BEST
way to get an interface's current ip address by using it's name (i.e.,
eth0) is.  I was trying one way using sockets and it just doesn't seem
to reliably give it to me every time.  I was using these two functions
below, but I it would return NULL some of the time from
IPCon_GetIpAddrByStr sometimes, so I'm not sure.  I need a way to be
able to reliably obtain this ip at any give time (due to dhcp, etc). Any
pointers?  TIA.


struct in_addr * IPCon::IPCon_GetIpAddr(void)
{
        struct ifreq ifr;
        struct sockaddr_in *saddr;
        int fd;
        fd = get_sockfd();
        if (fd >= 0 )
        {
                strcpy(ifr.ifr_name, m_ifname);
                ifr.ifr_addr.sa_family = AF_INET;
                if (ioctl(fd, SIOCGIFADDR, &ifr) == 0)
                {
                        saddr = (sockaddr_in *)&ifr.ifr_addr;
                        return &saddr->sin_addr;
                }
                else
                {
                        close(fd);
                        return NULL;
                }
        }
        return NULL;
}

char * IPCon::IPCon_GetIpAddrStr(void)
{
        /*struct in_addr *adr;
        adr = IPCon_GetIpAddr();
        if (adr == NULL)
                return NULL;
        else
                return inet_ntoa(*adr);*/
        return "208.164.149.42";
}

Re: Obtaining the ip address of an interface dynamically

[Glynn Clements]

Glover George wrote:

> Hi, I'm working on a little module here and was wondering what the BEST
> way to get an interface's current ip address by using it's name (i.e.,
> eth0) is.  I was trying one way using sockets and it just doesn't seem
> to reliably give it to me every time.  I was using these two functions
> below, but I it would return NULL some of the time from
> IPCon_GetIpAddrByStr sometimes, so I'm not sure.  I need a way to be
> able to reliably obtain this ip at any give time (due to dhcp, etc). Any
> pointers?  TIA.
> 
> struct in_addr * IPCon::IPCon_GetIpAddr(void)
> {
>         struct ifreq ifr;
>         struct sockaddr_in *saddr;
>         int fd;
>         fd = get_sockfd();
>         if (fd >= 0 )
>         {
>                 strcpy(ifr.ifr_name, m_ifname);
>                 ifr.ifr_addr.sa_family = AF_INET;
>                 if (ioctl(fd, SIOCGIFADDR, &ifr) == 0)
>                 {
>                         saddr = (sockaddr_in *)&ifr.ifr_addr;
>                         return &saddr->sin_addr;

You are returning a pointer to data which is on the stack, and which
ceases to be valid once you've returned from the function.

You should either have the caller supply the buffer, e.g.

	bool IPCon::IPCon_GetIpAddr(struct in_addr *result)

or allocate the memory dynamically, with malloc() or new.

-- 
Glynn Clements <glynn.clements@virgin.net>

Re: Obtaining the ip address of an interface dynamically

[Bernd Eckenfels]

In article <001401c1f066$cce6c210$0300a8c0@yellow> you wrote:
> IPCon_GetIpAddrByStr sometimes, so I'm not sure.

You should look for errno/return value of ictl to be able to provide more
info where the problem is. 

BTW: you version leaks file descriptors.