Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!samsung!rex!ames!excelan!murthy
From: murthy@la.excelan.com (M.D. Srinivas Murthy)
Newsgroups: comp.unix.questions
Subject: Re: How to know if the network supports broadcasting ?
Keywords: broadcast, ioctl, SIOCGIFFLAGS
Message-ID: <1620@excelan.COM>
Date: 30 Jul 90 18:25:27 GMT
References: <2588@sequent.cs.qmw.ac.uk>
Sender: news@excelan.COM
Reply-To: murthy@la.excelan.com (M.D. Srinivas Murthy)
Organization: Excelan, Inc., San Jose, Califonia
Lines: 37

In article <2588@sequent.cs.qmw.ac.uk> kinawi@cs.qmw.ac.uk (Husam Kinawi) writes:
>Hello,
>SIOCGIFFLAGS,  the parameter to be of type "ifreq", and the file descriptor to
>
>My Questions are:
>i) Is this the right way to detect ifa network supports broadcast, and if yes,
>   what makes my ioctl fail to work ?

>ii)If  I  am  not  on  the  right  way, then is there a possible way to detect
>   whether the network used supports broadcast ?
>
> if(ioctl(SocketId, SIOCGIFADDR, p) == -1){
>        perror("Ioctl");
>        printf("Errno = %d\n", errno);
> }

The approach is right. But -

1. The ioctl code should be SIOCGIFFLAGS.
2. The third parameter has to be a pointer to ifreq structure.
3. You have to specify the interface name (p.ifr_name) for which you want
   the flags.

here is the part of the code which worked on a SUN system.

 strcpy(p.ifr_name,"ie0");
 if(ioctl(SocketId, SIOCGIFFLAGS, &p) == -1){
        perror("Ioctl");
        printf("Errno = %d\n", errno);
 }

I hardcoded the interface name in the example. But you can get the interface
name of all the interfaces using the SIOCGIFCONF ioctl. 
SIOCGIFCONF take an array of ifreq structures and fills them with interface
name and interface address.

murthy