public override PacketType NextPacket()
{
// Code here is not just a simple call to impl.nextPacket(). For a KernelLink, nextPacket() returns a
// wider set of packet constants than the MathLink C API itself. We want nextPacket() to work on the
// non-packet types of heads the kernel and FE send back and forth.
// Because for some branches below we seek back to the start before returning, it is not guaranteed
// that when nextPacket() returns, the current packet has been "opened". Thus it is not safe to write
// a J/Link program that loops calling nextPacket()/newPacket(). You need to call handlePacket() after
// nextPacket() to ensure that newPacket() will throw away the curent "packet".
// createMark will fail on an unconnected link, and the user might call this
// before any reading that would connect the link.
if (!linkConnected)
{
Connect();
linkConnected = true;
}
PacketType pkt;
ILinkMark mark = CreateMark();
try {
pkt = impl.NextPacket();
} catch (MathLinkException e) {
if (e.ErrCode == 23 /* MLEUNKNOWNPACKET */)
{
ClearError();
SeekMark(mark);
int argCount;
string f = GetFunction(out argCount);
if (f == "ExpressionPacket")
{
pkt = PacketType.Expression;
}
else if (f == "BoxData")
{
// Probably more than just BoxData will need to join this group. I thought that kernel
// always sent cell contents (e.g. Print output) wrapped in ExpressionPacket, but not so.
// "Un-wrapped" BoxData is also possible, perhaps others. We need to treat this case
// specially, since there is no packet wrapper.
SeekMark(mark);
pkt = PacketType.Expression;
}
else
{
// Note that all other non-recognized functions get labelled as PacketType.FrontEnd. I could perhaps be
// more discriminating, but then I risk not recognizing a legitimate communication
// intended for the front end. This means that PacketType.FrontEnd is not a specific indication that
// a front-end-related packet is on the link. Because there is no diagnostic packet head,
// we need to seek back to before the function was read. That way, when programs call
// methods to read the "contents" of the packet, they will in fact get the whole thing.
SeekMark(mark);
pkt = PacketType.FrontEnd;
}
}
else
{
// The other MathLink error from MLNextPacket is MLENEXTPACKET, which is a common one saying
// that you have called MLNextPacket when more data is in the current packet.
throw e;
}
} finally {
DestroyMark(mark);
}
return(pkt);
}