12.3.11 Finding the Link Used for a Virtual Circuit

If you let Solstice X.25 select the link for an outgoing call, or make an accept call that accepts incoming calls from any link, you may use the X25_GET_LINK ioctl to obtain the identifier of the link used for the call:

int s, error;
 int linkid; /* link identifier */

 error = ioctl(s, X25_GET_LINK, &linkid);

If this call is made before connection establishment and you have not explicitly selected a link, linkid will be set to -1 on return from the call. After connection establishment, linkid will have a value in the range zero through one less than the maximum number of links configured.

An important use for this ioctl arises when the called side determines the remote address in order to call back the remote DTE. In this situation, the remote address is presented in exactly the form it arrived in the Call Request. For some PSDNs, this may not contain a DNIC. Hence, the only way you can call the remote DTE back is by finding out the link id for the call using the X25_GET_LINK ioctl, and explicitly selecting this link using the X25_SET_LINK ioctl when calling the remote DTE back. In this situation, you should not set the ANY_LINK bit in the hostlen field of the CONN_DB parameter to the connect call.