variable in a header file shared between different projects - global-variables

Question

I have a solution which includes three projects. one is creating static library i.e .lib file. It contains one header file main.h and one main.cpp file. cpp file contains the definition of functions of header file.
second project is .exe project which includes the header file main.h and calls a function of header file.
third project is also a .exe project which includes the header file and uses a variable flag of header file.
Now both .exe projects are creating different instance of the variable. But I want to share same instance of the variable between the projects dynamically. as I have to map the value generated by one project into other project at the same instant.
Please help me as I am nearing my project deadline.
Thanks for the help.
Here are some part of the code.
main.cpp and main.h are files of .lib project
main.h
extern int flag;
extern int detect1(void);
main.cpp
#include<stdio.h>
#include"main.h"
#include <Windows.h>
#include <ShellAPI.h>
using namespace std;
using namespace cv;
int flag=0;
int detect1(void)
{
int Cx=0,Cy=0,Kx=20,Ky=20,Sx=0,Sy=0,j=0;
//create the cascade classifier object used for the face detection
CascadeClassifier face_cascade;
//use the haarcascade_frontalface_alt.xml library
face_cascade.load("E:\\haarcascade_frontalface_alt.xml");
//System::DateTime now = System::DateTime::Now;
//cout << now.Hour;
//WinExec("E:\\FallingBlock\\FallingBlock\\FallingBlock\\bin\\x86\\Debug\\FallingBlock.exe",SW_SHOW);
//setup video capture device and link it to the first capture device
VideoCapture captureDevice;
captureDevice.open(0);
//setup image files used in the capture process
Mat captureFrame;
Mat grayscaleFrame;
//create a window to present the results
namedWindow("capture", 1);
//create a loop to capture and find faces
while(true)
{
//capture a new image frame
captureDevice>>captureFrame;
//convert captured image to gray scale and equalize
cvtColor(captureFrame, grayscaleFrame, CV_BGR2GRAY);
equalizeHist(grayscaleFrame, grayscaleFrame);
//create a vector array to store the face found
std::vector<Rect> faces;
//find faces and store them in the vector array
face_cascade.detectMultiScale(grayscaleFrame, faces, 1.1, 3, CV_HAAR_FIND_BIGGEST_OBJECT|CV_HAAR_SCALE_IMAGE, Size(30,30));
//draw a rectangle for all found faces in the vector array on the original image
for(unsigned int i = 0; i < faces.size(); i++)
{
Point pt1(faces[i].x + faces[i].width, faces[i].y + faces[i].height);
Point pt2(faces[i].x, faces[i].y);
rectangle(captureFrame, pt1, pt2, cvScalar(0, 255, 0, 0), 1, 8, 0);
if(faces.size()>=1)
j++;
Cx = faces[i].x + (faces[i].width / 2);
Cy = faces[i].y + (faces[i].height / 2);
if(j==1)
{
Sx=Cx;
Sy=Cy;
flag=0;
}
}
if(Cx-Sx > Kx)
{
flag = 1;
printf("%d",flag);
}
else
{
if(Cx-Sx < -Kx)
{
flag = 2;
printf("%d",flag);
//update(2);
}
else
{
if(Cy-Sy > Ky)
{
flag = 3;
printf("%d",flag);
//update(3);
}
else
{
if(Cy-Sy < -Ky)
{
flag = 4;
printf("%d",flag);
//update(4);
}
else
if(abs(Cx-Sx) < Kx && abs(Cy-Sy)<Ky)
{
flag = 0;
printf("%d",flag);
//update(0);
}
}
}
}
2nd project's code
face.cpp
#include"main.h"
#include<stdio.h>
int main()
{
detect1();
}
3rd project's code
tetris.cpp
#include"main.h"
int key;
key = flag;
if(key==0)
{
MessageBox(hwnd,"Space2","TetRiX",0);
}
if(key==4)
{
tetris.handleInput(1);
tetris.drawScreen(2);
//MessageBox(hwnd,"Space2","TetRiX",0);
}

Answer 1

You need to look up how to do inter-process communication in the operating system under which your applications will run. (At this point I assume that the processes are running on the same computer.) It looks like you're using Windows (based on seeing a call to "MessageBox") so the simplest means would be for both processes to use RegisterWindowMessage create a commonly-understood message value, and then send the data via LPARAM using either PostMessage or SendMessage. (You'll need each of them to get the window handle of the other, which is reasonably easy.) You'll want to have some sort of exclusion mechanism (mutex or critical section) in both processes to ensure that the shared value can't be read and written at the same time. If both processes can do the "change and exchange" then you'll have an interesting problem to solve if both try to do that at the same time, because you'll have to deal with the possibility of deadlocks over that shared value.
You can also use shared memory, but that's a bit more involved.
If the processes are on different computers you'll need to do it via TCP/IP or a protocol on top of TCP/IP. You could use a pub-sub arrangement--or any number of things. Without an understanding of exactly what you're trying to accomplish, it's difficult to know what to recommend.
(For the record, there is almost no way in a multi-process/multi-threaded O/S to share something "at the same instant." You can get arbitrarily close, but computers don't work like that.)
Given the level of difficulty involved, is there some other design that might make this cleaner? Why do these processes have to exchange information this way? Must it be done using separate processes?