This is documentation for Mathematica 6, which was
based on an earlier version of the Wolfram Language.
View current documentation (Version 11.2)

Running Mathematica from Within an External Program

To run Mathematica from within an external program requires making use of many general features of MathLink. The first issue is how to establish a MathLink connection to Mathematica.
When you use MathLink templates to create external programs that can be called from Mathematica, source code to establish a MathLink connection is automatically generated, and all you have to do in your external program is to call MLMain(argc, argv). But in general you need to call several functions to establish a MathLink connection.
MLENVMLInitialize(0)initialize MathLink library functions
MLINKMLOpenArgcArgv(MLENV env,int argc,char**argv,int*errno)
open a MathLink connection taking parameters from an argv array
MLINKMLOpenString(MLENV env,char*string,int*errno)
open a MathLink connection taking parameters from a single character string
intMLActivate(MLINK link)activate a MathLink connection, waiting for the program at the other end to respond
voidMLClose(MLINK link)close a MathLink connection
voidMLDeinitialize(MLENV env)deinitialize MathLink library functions

Opening and closing MathLink connections.

Include the standard MathLink header file.
#include "mathlink.h"
int main(int argc, char *argv[]) {
   MLENV env;
MLINK link;
int errno;
This initializes MathLink library functions.
   env = MLInitialize(0);
This opens a MathLink connection, using the same arguments as were passed to the main program.
   link = MLOpenArgcArgv(env, argc, argv, &errno);
This activates the connection, waiting for the other program to respond.
Often the argv that you pass to MLOpenArgcArgv() will come directly from the argv that is passed to main() when your whole program is started.
The elements in the argv array are character strings which mirror the arguments and options used in the Mathematica functions LinkLaunch, LinkCreate and LinkConnect.
"-linklaunch"operate like LinkLaunch["name"]
"-linkcreate"operate like LinkCreate["name"]
"-linkconnect"operate like LinkConnect["name"]
"-linkname","name"give the name to use
"-linkprotocol","protocol"give the link protocol to use (TCPIP, Pipes, etc.)

Possible elements of the argv array passed to MLOpenArgcArgv().

As an alternative to MLOpenArgcArgv() you can use MLOpenString(), which takes parameters concatenated into a single character string with spaces in between.
Once you have successfully opened a MathLink connection to the Mathematica kernel, you can then use standard MathLink functions to exchange data with it.
intMLEndPacket(MLINK link)indicate the end of a packet
intMLNextPacket(MLINK link)find the head of the next packet
intMLNewPacket(MLINK link)skip to the end of the current packet

Functions often used in communicating with the Mathematica kernel.

Once you have sent all the pieces of a packet using MLPutFunction() etc., MathLink requires you to call MLEndPacket() to ensure synchronization and consistency.
One of the main issues in writing an external program which communicates directly with the Mathematica kernel is handling all the various kinds of packets that the kernel can generate.
The function MLNextPacket() finds the head of the next packet that comes from the kernel, and returns a constant that indicates the type of the packet.
Mathematica packet
ReturnPacket[expr]RETURNPKTresult from a computation
ReturnTextPacket["string"]RETURNTEXTPKTtextual form of a result
InputNamePacket["name"]INPUTNAMEPKTname of an input line
OutputNamePacket["name"]OUTPUTNAMEPKTname of an output line
TextPacket["string"]TEXTPKTtextual output from functions like Print
MessagePacket[symb,"tag"]MESSAGEPKTname of a message generated by Mathematica
InputPacket["prompt"]INPUTPKTrequest for a response to an Input function
CallPacket[i,list]CALLPKTrequest for a call to an external function

Some packets recognized by MLNextPacket().

This keeps on reading data from a link, discarding it until an error or a ReturnPacket is found.
while ((p = MLNextPacket(link)) && p  RETURNPKT)
If you want to write a complete front end to Mathematica, you will need to handle all of the possible types of packets that the kernel can generate. Typically you can do this by setting up an appropriate switch on the value returned by MLNextPacket().
The MathLink Developer Kit contains sample source code for several simple but complete front ends.
intMLReady(MLINK link)test whether there is data waiting to be read on a link
int MLReadyParallel (MLENV e, MLINK *links, int n, mltimeval t)
test in parallel whether there is data to be read from a list of links
intMLFlush(MLINK link)flush out buffers containing data waiting to be sent on a link

Flow of data on links.

One feature of more sophisticated external programs such as front ends is that they may need to perform operations while they are waiting for data to be sent to them by Mathematica. When you call a standard MathLink library function such as MLNextPacket() your program will normally block until all the data needed by this function is available.
You can avoid blocking by repeatedly calling MLReady(), and only calling functions like MLNextPacket() when MLReady() no longer returns 0. MLReady() is the analog of the Mathematica function LinkReadyQ.
Note that MathLink sometimes buffers the data that you tell it to send. To make sure that all necessary data has been sent you should call MLFlush(). Only after doing this does it make sense to call MLReady() and wait for data to be sent back.