Running the Wolfram System from within an External Program

To run the Wolfram System from within an external program requires making use of many general features of WSTP (Wolfram Symbolic Transfer Protocol). The first issue is how to establish a WSTP connection to the Wolfram System.
When you use WSTP templates to create external programs that can be called from the Wolfram Language, source code to establish a WSTP connection is automatically generated, and all you have to do in your external program is to call WSMain(argc,argv). But in general you need to call several functions to establish a WSTP connection.
WSENV WSInitialize(0)
initialize WSTP library functions
WSLINK WSOpenArgcArgv(WSENV env,int argc,char**argv,int*errno)
open a WSTP connection taking parameters from an argv array
WSLINK WSOpenString(WSENV env,char*string,int*errno)
open a WSTP connection taking parameters from a single character string
int WSActivate(WSLINK link)
activate a WSTP connection, waiting for the program at the other end to respond
void WSClose(WSLINK link)
close a WSTP connection
void WSDeinitialize(WSENV env)
deinitialize WSTP library functions
Opening and closing WSTP connections.
Include the standard WSTP header file. 
#include "wstp.h"
int main(int argc, char *argv[]) {
   WSENV env;
   WSLINK link;
   int errno;
This initializes WSTP library functions. 
   env = WSInitialize(0);
This opens a WSTP connection, using the same arguments as were passed to the main program. 
   link = WSOpenArgcArgv(env, argc, argv, &errno);
This activates the connection, waiting for the other program to respond.
   WSActivate(link);
   ...
}
Often the argv that you pass to WSOpenArgcArgv() 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 that mirror the arguments and options used in the Wolfram Language 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 WSOpenArgcArgv().
As an alternative to WSOpenArgcArgv() you can use WSOpenString(), which takes parameters concatenated into a single character string with spaces in between.
Once you have successfully opened a WSTP connection to the Wolfram Language kernel, you can then use standard WSTP functions to exchange data with it.
int WSEndPacket(WSLINK link)
indicate the end of a packet
int WSNextPacket(WSLINK link)
find the head of the next packet
int WSNewPacket(WSLINK link)
skip to the end of the current packet
Functions often used in communicating with the Wolfram Language kernel.
Once you have sent all the pieces of a packet using WSPutFunction() etc., WSTP requires you to call WSEndPacket() to ensure synchronization and consistency.
One of the main issues in writing an external program that communicates directly with the Wolfram Language kernel is handling all the various kinds of packets that the kernel can generate.
The function WSNextPacket() finds the head of the next packet that comes from the kernel, and returns a constant that indicates the type of the packet.
Wolfram Language packet
constant
ReturnPacket[expr]RETURNPKT
result from a computation
ReturnTextPacket["string"]RETURNTEXTPKT
textual form of a result
InputNamePacket["name"]INPUTNAMEPKT
name of an input line
OutputNamePacket["name"]OUTPUTNAMEPKT
name of an output line
TextPacket["string"]TEXTPKT
textual output from functions like Print
MessagePacket[symb,"tag"]MESSAGEPKT
name of a message generated by the Wolfram Language
InputPacket["prompt"]INPUTPKT
request for a response to an Input function
CallPacket[i,list]CALLPKT
request for a call to an external function
Some packets recognized by WSNextPacket().
This keeps on reading data from a link, discarding it until an error or a ReturnPacket is found. 
while ((p = WSNextPacket(link)) && p != RETURNPKT)
    WSNewPacket(link);
If you want to write a complete front end to the Wolfram System, 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 WSNextPacket().
The WSTP Developer Kit contains sample source code for several simple but complete front ends.
int WSReady(WSLINK link)
test whether there is data waiting to be read on a link
int WSReadyParallel (WSENV e, WSLINK *links, int n, mltimeval t)
test in parallel whether there is data to be read from a list of links
int WSFlush(WSLINK 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 the Wolfram Language. When you call a standard WSTP library function such as WSNextPacket(), your program will normally block until all the data needed by this function is available.
You can avoid blocking by repeatedly calling WSReady(), and only calling functions like WSNextPacket() when WSReady() no longer returns 0. WSReady() is the analog of the Wolfram Language function LinkReadyQ.
Note that WSTP sometimes buffers the data that you tell it to send. To make sure that all necessary data has been sent you should call WSFlush(). Only after doing this does it make sense to call WSReady() and wait for data to be sent back.