Re: [PLUG] determining my IP address in C

[Tobias DiPasquale <toby@cbcg.net>]

On Wed, 2004-03-03 at 08:14, Jeff Abrahamson wrote:
> I want to determine my own IP address.
> 
> A kludgy way is to bind to a high port, send myself (on 127.0.0.1) a
> packet containing only a long random number, and look at the from
> address.  Even then I might see 127.0.0.1, though.
> 
> I bet there's a better way.  Anyone know?
> 
> If the box has multiple interfaces, the above fails.  Is their a way
> to get all the addresses that are me?

A quick trip into UNIX Network Programming tells all...

<code_sample>
[...more #includes here...]
#include	<sys/param.h>

const char *Inet_ntop( int family, 
		       const void *addrptr, 
		       char *strptr, 
		       size_t len)
{
        const char *ptr;

	/* check for old code */
        if (strptr == NULL)
                err_quit( "NULL 3rd argument to inet_ntop");
        if ((ptr = inet_ntop( family, addrptr, strptr, len)) == NULL)
                err_sys( "inet_ntop error");
        return( ptr);
}

char **my_addrs( int *addrtype)
{
	struct hostent	*hptr;
	char		myname[MAXHOSTNAMELEN];

	if (gethostname(myname, sizeof(myname)) < 0)
		return(NULL);

	if ((hptr = gethostbyname(myname)) == NULL)
		return(NULL);

	*addrtype = hptr->h_addrtype;
	return(hptr->h_addr_list);
}

int main( int argc, char **argv)
{
        int     addrtype;
        char    **pptr, buf[INET6_ADDRSTRLEN];

        if ((pptr = my_addrs( &addrtype)) == NULL)
                err_quit( "my_addrs error");

        for (; *pptr != NULL; pptr++)
                printf( "\taddress: %s\n",
                        Inet_ntop( addrtype, *pptr, buf, sizeof( buf)));

        exit(0);
}
</snip>

NOTES: You will have to add some #include macros to make this code work.
Also, you will have to replace err_quit() calls with something that
works (e.g. fprintf( stderr, ...)).

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-- 
Tobias DiPasquale, www.cbcg.net
202A 04C4 2CE6 B985 8520  88D6 CD25 1A6C B9B5 1595

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