Allow wxThread to access global data Topic is solved

[parad0x13]

I'm having this bothersome problem when I create a thread and try to access global data. Basically I create the wxThread passing a pointer to a global class with a variable, and in the Entry() function I try to access that variable.

The problem I get every time is "Access Violation" saying that it cannot access that global class' data

- Please and thank you

[illnatured]

This is probably because two threads try to access the variable at the same time. A simple solution is to use wxCriticalSection or wxMutex.

[Auria]

Can you post some code? It's near to impossible to tell without code. Access violation generally means that your pointers are screwed up.

[parad0x13]

Absolutely, thank you for your attention

Simple format of the code I'm talking about:

part of main.cpp

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	Pointers *pointers = new Pointers();
	pointers->ScreenResolution = ::wxGetDisplaySize();

	pointers->tempAccess = new temp;

Pointers.h

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#ifndef _POINTERS_
#define _POINTERS_

class RenderGLCanvas;
class temp;

class Pointers
{
public:
	RenderGLCanvas *RenderCanvas;
	temp *tempAccess;

	wxSize ScreenResolution;
};

#endif

CalculateThread.h

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#ifndef _CALCULATETHREAD_
#define _CALCULATETHREAD_

#include "wx.h"
#include "Pointers.h"

#include "temp.h"

DECLARE_EVENT_TYPE(wxEVT_THREAD, -1)

class CalculateThread : public wxThread
{
public:
	CalculateThread(wxEvtHandler *ehParent, Pointers *pointer, int equation){m_ehParent = ehParent;Equation = equation;};
	~CalculateThread(){};

private:
	int Equation;
	void *Entry();
	Pointers *global;
	wxEvtHandler *m_ehParent;
};

#endif

CalculateThread.cpp

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#include "CalculateThread.h"
#include "enum.h"

void *CalculateThread::Entry()
{
	// Why will this not write to memory!
	//IFS.SierpinskiTriangleCoords();

	int test = global->tempAccess->value;
	//global->tempAccess->value = 5;
	//if(temp.value == 2)
	//	exit(0);

	wxCommandEvent evt(wxEVT_THREAD, wxID_ANY);
	evt.SetId(ID_Thread_Render);
	m_ehParent->AddPendingEvent(evt);

	return NULL;
};

some function somewhere

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global->tempAccess->value = 2;
wxThread *test = new CalculateThread(this, global, ID_SierpinskiTriangle);
		test->Create();
		test->Run();

Now, the global variable to the PointerClass has been passed to the CalculateThread initializer seen in the last bit of code that I pasted, this thread runs Entry() which tries to access the variable data in the global temp class by means of assigning integer 'test' that value, this is where everything goes wrong... oh so very wrong

[Auria]

Can you show what "global" is? Beyond being quite an ugly way to do things, I also suspect you did the same mistake as in the other thread : statics in a header

EDIT:
Seems like the thread receives the object as argument but doesn't do anything with it :

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CalculateThread(wxEvtHandler *ehParent, Pointers *pointer, int equation){m_ehParent = ehParent;Equation = equation;}; 

[parad0x13]

global is just a variable that points to the global Pointer class that is declared in main.cpp

main.cpp

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#include "main.h"
#include "wxVertigo_Frm.h"
#include "Pointers.h"
#include "rand.h"

#include "temp.h"

IMPLEMENT_APP(MainApp)

bool MainApp::OnInit()
{
	initImageHandlers();
	seed();	// Seed random generator

	Pointers *pointers = new Pointers();
	pointers->ScreenResolution = ::wxGetDisplaySize();

	pointers->tempAccess = new temp;

	wxVertigo_Frm *wxVertigo = new wxVertigo_Frm(wxT("wxVertigo"), *pointers);
	wxVertigo->Show(true);
	wxVertigo->Maximize(true);

	return true;
}

void MainApp::initImageHandlers()
{
	wxImage::AddHandler(new wxPNGHandler());
};

I'd like to know a cleaner not-so-ugly way of doing it, but this is the only way I know how!

The thread reads the global made class "temp" from the global pointer and tries to read it, but can't.

Please show me a better way of doing this!

Edit*

And I see what you mean by not initializing global from the initializer that was passed to the thread, I did such:

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CalculateThread(wxEvtHandler *ehParent, Pointers *pointer, int equation)
	{
		m_ehParent = ehParent;
		global = pointer;
		Equation = equation;
	};

and now it wont cry when I try to access the variable, but it still wont write to it

[Auria]

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        wxVertigo_Frm *wxVertigo = new wxVertigo_Frm(wxT("wxVertigo"), *pointers); 

What is the signature of the wxVertigo_Frm constructor? I don't quite get why you are passing "*pointers", are you passing by copy?

[parad0x13]

signature? I'm not quite sure what you mean. The only way I know how to pass global variables, like the Pointers class that I initiated in main.cpp, would be by pointer assigning when I call a function

If there's a better way please tell!

[frank_frl]

Hi parad0x13,

there is a nice class in wxWidgets to make things like this easy.
http://docs.wxwidgets.org/stable/wx_wxthreadhelper.html

Frank

[Auria]

signature? I'm not quite sure what you mean.

The function with its list or arguments (what you'd put in a header)