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Two-Way Communication with External Programs

When you install a MathLink-compatible external program using Install, the program is set up to behave somewhat like a simplified Mathematica kernel. Every time you call a function in the external program, a CallPacket is sent to the program, and the program responds by sending back a result wrapped in a ReturnPacket.
This installs an external program, returning the LinkObject used for the connection to that program.
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The function ExternalCall sends a CallPacket to the external program.
You can send the CallPacket explicitly using LinkWrite. The first argument of the CallPacket specifies which function in the external program to call.
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Here is the response to the CallPacket from the external program.
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If you use Install several times on a single external program, Mathematica will open several MathLink connections to the program. Each connection will however always correspond to a unique LinkObject.
$CurrentLinkthe MathLink connection to the external program currently being run

Identifying different instances of a single external program.

:Function: addto
This gives $CurrentLink as an argument to addto.
:Pattern:       addto[$CurrentLink, n_Integer]
:Arguments:     {n}
:ArgumentTypes: {Integer}
:ReturnType: Integer
This zeros the global variable counter every time the program is started.
int counter = 0;
int addto(int n) {
counter += n;
return counter;
This installs one instance of the external program containing addto.
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This installs another instance.
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This adds 10 to the counter in the first instance of the external program.
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This adds 15 to the counter in the second instance of the external program.
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This operates on the first instance of the program again.
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If an external program maintains information about its state then you can use different instances of the program to represent different states. $CurrentLink then provides a way to refer to each instance of the program.
The value of $CurrentLink is temporarily set every time a particular instance of the program is called, as well as when each instance of the program is first installed.
send input to Mathematica but return no results

Sending a string for evaluation by Mathematica.

The two-way nature of MathLink connections allows you not only to have Mathematica call an external program, but also to have that external program call back to Mathematica.
In the simplest case, you can use the MathLink function MLEvaluateString() to send a string to Mathematica. Mathematica will evaluate this string, producing whatever effects the string specifies, but it will not return any results from the evaluation back to the external program.
To get results back you need explicitly to send an EvaluatePacket to Mathematica, and then read the contents of the ReturnPacket that comes back.
This starts an EvaluatePacket.
MLPutFunction(stdlink, "EvaluatePacket", 1);
This constructs the expression Factorial[7] or 7!.
  MLPutFunction(stdlink, "Factorial", 1);
MLPutInteger32(stdlink, 7);
This specifies that the packet you are constructing is finished.
This checks the ReturnPacket that comes back.
MLCheckFunction(stdlink, "ReturnPacket", &n);
This extracts the integer result for 7! from the packet.
MLGetInteger32(stdlink, &ans);
MLEndPacket(stdlink)specify that a packet is finished and ready to be sent to Mathematica

Sending a packet to Mathematica.

When you can send Mathematica an EvaluatePacket[input], it may in general produce many packets in response, but the final packet should be ReturnPacket[output]. "Manipulating Expressions in External Programs" will discuss how to handle sequences of packets and expressions whose structure you do not know in advance.