S7-cpp-for-Snap7/building_note_for_snap7 at master · reyanvaldes/S7-cpp-for-Snap7 · GitHub

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The libsnap7 is an excellent work!!! developed made by Davide Nardella, http://snap7.sourceforge.net/

A bug was identified that showing in heavy busy network where the packages can get a delay in the receiver.
The changing in src/core/s7_isotcp.cpp helps or protect it to make sure the received package match the sending PDU (Identifier) make it the data communication and parsing more secure and avoid access violation.
In other words, this helps parsing the package that we actually requested, making bullet proof the communication.
This was hitting me for along time, and we didn't know what to do until Stéphane Rousseau found a solution for it. (Thanks!)

It is based on https://sourceforge.net/p/snap7/discussion/bugfix/thread/de352e4cc2/

for building from source code:  see https://snap7.sourceforge.net/snap7_source.html

download snap7-full-1.4.2 or higher from https://sourceforge.net/projects/snap7
unzip snap7-full-1.4.2 or that higher version

Need to fix a bug in the reading isotcp packages to guarantee the reponse match the query on snap7, this is very important in heavy network traffic
https://sourceforge.net/p/snap7/discussion/bugfix/thread/de352e4cc2/
////////   MODIFICATION OF SNAP7 TO FIX ACCESS VIOLATION ISSUE

Before build the library, need to modify src/core/s7_isotcp.cpp

#include "s7_types.h"  // Added on 7/5/2024
#include <iostream>    // Added on 7/5/2024

//*********************************************************************
int TIsoTcpSocket::isoExchangeBuffer(void *Data, int &Size)
{
	/*int Result;

    ClrIsoError();
	Result =isoSendBuffer(Data, Size);
	if (Result==0)
		Result =isoRecvBuffer(Data, Size);
	return Result; */

	// Improved based on the solution: https://sourceforge.net/p/snap7/discussion/bugfix/thread/de352e4cc2/
  // Provided by 	Stéphane Rousseau
	// Changed on 7/5/2024 by Reyan Valdes
	int Result;

	ClrIsoError();
	PS7ReqHeader pduheader = (PS7ReqHeader)&PDU.Payload;
	int PDUTypeSend = pduheader->PDUType;
	int PDUTypeSequence = pduheader->Sequence;
	Result = isoSendBuffer(Data, Size);
	if (Result == 0)
	{
	   bool SequenceMatch = false;
	   while (!SequenceMatch && Result == 0)
	   {
		   Result = isoRecvBuffer(Data, Size);
		   if (Result == 0)
		   {
			   if (pduheader->PDUType == PduType_response &&
							   pduheader->Sequence == PDUTypeSequence)
			   {
				   SequenceMatch = true;
			   }
			   else
			   {
				  std::cout << "[PROFINET] PDU Rejected didn't match response with request" << std::endl;

				  // TRACE("PDU Rejected ! : pduheader->PDUType %d pduheader->Sequence %d waiting for sequence %d for PDUType %d\r\n", pduheader->PDUType, pduheader->Sequence, PDUTypeSequence, PDUTypeSend);
			   }
		   }
	   }
	}
	return Result;
}

//*********************************************************************

For bulding library just follow the regular steps

cd build/unix
make –f <architecture>_linux.mk all to rebuild the library
make –f <architecture>_linux.mk clean to clean the project
make –f <architecture>_linux.mk install to rebuild and copy the library in usr/lib

for the third option you need be root or use sudo make …

Where <architecture> can be x86_64.

if you want to rebuild the library and copy it into /mylib instead of /usr/lib the above example becames:

make –f x86_64_linux.mk install LibInstall=/home/sdc/

copy snap7.so in source core code from /home/sdc/libsnap7.so to your project libs/