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Re: Chain of packets in MathLink: are packets always strictly ordered?

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  • Subject: [mg116668] Re: Chain of packets in MathLink: are packets always strictly ordered?
  • From: "Alexey Popkov" <lehin.p at>
  • Date: Wed, 23 Feb 2011 05:25:09 -0500 (EST)

Thank you for the answers.

For now, the only headache is correct parsing of Message-related packets
generated by the slave kernel. I have not found any strightforward way to do
it both when $Output has FomatType->OutputForm and FomatType->StandardForm.
I wonder how this is done in the FrondEnd? What is the convenient way for
generating Message Cell from a packet with the content of the Message?

----- Original Message ----- 
From: "John Fultz" <jfultz at>
To: <lehin.p at>; <mathgroup at>
Sent: Wednesday, February 23, 2011 6:01 AM
Subject: [mg116668] Re: [mg116592] Chain of packets in MathLink: are packets always
strictly ordered?

1) Here's example code which returns two ReturnExpressionPackets...

lnk=LinkLaunch["mathkernel -mathlink"];lr:=LinkRead[lnk];lr;



(* then LinkRead[] away *)

2) Yes, if you wrap, e.g., StandardForm[] or TraditionalForm[] around the

3) Not that I'm aware of, although I didn't look deeply into the question.

As far as PostScript is concerned, the additional rules to note are...

* You can receive a DisplayPacket any time before an InputNamePacket, as
long as
doing so doesn't violate one of the other parts of the contract I outlined

* After you receive a DisplayPacket, the only packet you can receive is
DisplayPacket or a DisplayEndPacket.  The final PostScript cell is built
the concatenated strings of all of the DisplayPackets plus the final


John Fultz
jfultz at
User Interface Group
Wolfram Research, Inc.

On Tue, 22 Feb 2011 07:07:50 +0500, Alexey Popkov wrote:
> Thanks for the detailed response! Things are getting clearer.
> I have some additional questions:
> 1) I tried to get multilple output with EnterExpressionPacket and
> ExpressionPacket but I have not found a way to get it. In which cases it
> is produced?
> 2) Can ExpressionPacket be returned when expression is sent with
> EnterExpressionPacket and in the slave kernel $Output has attribute
> FormatType -> OutputForm? If it is, in which cases it is returned?
> 3) Is there a way to get Messages and Prints to be ExpressionPackets (or
> just to be returned in a way suitable for good-looking printing in a
> Notebook) when in the slave kernel $Output has attribute FormatType ->
> OutputForm? The text-based version of complex expressons looks ugly.
> P.S. I am interested in support of legacy versions of Mathematica as the
> slaves and the FrontEnd v.7.0.1 (which I use) supports legacy PostScript.
> So I'm interested in correct handling packets related to PostScript, too.
> ----- Original Message -----
> From: "John Fultz" <jfultz at>
> To: <lehin.p at> <mathgroup at>
> Sent: Monday, February 21, 2011 8:44 PM
> Subject: Re: [mg116592] Chain of packets in MathLink: are packets always
> strictly ordered?
> On Mon, 21 Feb 2011 04:20:41 -0500 (EST), Alexey wrote:
>> Hello,
>> The Documentation does not state clear the order of packets returned
>> by slave kernel via MathLink. It is natural to assume that (when
>> sending an input expression with head EnterExpressionPacket):
>> 1) the last packet before the next InputNamePacket is always
>> ReturnExpressionPacket
>> 2) there may be always only one ReturnExpressionPacket and one
>> OutputNamePacket for one EnterExpressionPacket
>> 3) ReturnExpressionPacket always goes after OutputNamePacket
>> 4) after MessagePacket the next packet is always TextPacket with
>> contents of that message.
>> 5) when working in standard mode there may be only 7 types of returned
>> packets: InputNamePacket, OutputNamePacket, ReturnExpressionPacket,
>> DisplayPacket, DisplayEndPacket, MessagePacket, TextPacket.
>> Are these assumptions true?
> No, they are not.  I'll pick them apart one by one...
> 1) If the output is suppressed by a semicolon, then you won't get any
> output-related packets.  Also, you might get a ReturnTextPacket if the
> output is requested in a textual format such as OutputForm or InputForm.
> 2) False and false, for the same reason as above.
> 3) *If* you get an OutputNamePacket, it will be followed by *either* a
> ReturnExpressionPacket or a ReturnTextPacket.  If an input produces
> multiple outputs, then each OutputNamePacket is followed immediately by
> its corresponding return packet.
> 4) True.
> 5) Also, ExpressionPacket.  And if you accept arbitrary Mathematica
> input, you really should be prepared to accept a ReturnTextPacket as
> well.  There's also the PostScript-related packets, but you might be able
> to safely assume no legacy behavior and ignore those.
> Perhaps answering your specific assumptions isn't a very good way of
> summarizing the rules, so let me summarize them.  These are the rules of
> the state machine for a front end to the kernel:
> * Main loop evaluations (ones that assign In/Out) may begin with an
> EnterExpressionPacket or an EnterTextPacket.
> * Main loop evaluations are always terminated by exactly one
> InputNamePacket, regardless of how many individual inputs the enter
> packet contained.
> * If the main loop evaluation returns output(s), it(they) will be in the
> form of an OutputNamePacket[] followed by a ReturnTextPacket or
> ReturnExpressionPacket.
> * Messages are TextPackets (and theoretically could be ExpressionPackets,
> although that doesn't happen now) which are identified by being preceded
> by a MessagePacket.
> * All unidentified Text/ExpressionPackets are considered Print cells.
> * You can send a non-main-loop evaluation using EvaluatePacket.  The
> kernel always responds to an EvaluatePacket with a ReturnPacket (this
> rule is slightly more complex for *the* Mathematica front end, but it
> suffices for custom front ends).  Such evaluations should not be sent
> while you're waiting for an InputNamePacket.
> All of these rules can be observed without too much effort by launching a
> kernel from a kernel using, e.g.,
> lnk=LinkLaunch["mathkernel -mathlink"]
> and then using LinkRead/LinkWrite to communicate with the link.  Although
> one needs to be careful with EvaluatePackets that their contents evaluate
> where you intended them to.  I sometimes will do
> SetAttributes[EvaluatePacket, {HoldAll}] when I'm doing some sort of
> demonstration or experiment to make sure that LinkWrite isn't evaluating
> the expression before it sends it.
> Sincerely,
> John Fultz
> jfultz at
> User Interface Group
> Wolfram Research, Inc.

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