my problem is: i have a function that i pass a pointer to so that it can update a variable for me. when this function is called normally the variable updates and everything works fine.

BUT when i call this function *as a seperate thread*, and pass it the address of the variable, the function executes completely, but when it returns the variable is NOT updated.

*NOTE that this variable is a global variable*

now i was thinking (obviously dont know for sure though cause im new to multithreading) that i may be related to the fact that a new thread has its own stack created, and hence unable to access the variable?
also, could it maybe be that some OpenGL functions which i am calling in this new thread are not working because they are being called from a seperate thread to the one that OpenGL is mainly working/drawing??
could any of these be correct ?? and if so, how do i get around this?? or otherwise, does anyone have any idea why this is happening??

any help whatsoever would be so so so appreciated. thanks in advance.

Edited by skp888: n/a

7 Years
Discussion Span
Last Post by skp888

theres a lot of code, too much to post, so ill post the functions that are related to the thread:

the main working function. basically converts a .jpg into a bitmap to be rendered as a texture in opengl/glut:

//    Pass the filename, and the glTexture to bind it to
int BuildTexture(char *szPathName, GLuint* texid)						// Load Image And Convert To A Texture
    std::cout << "debug:  TextureBuild Starting..." << std::endl;

	HDC			hdcTemp;												// The DC To Hold Our Bitmap
	HBITMAP		hbmpTemp;												// Holds The Bitmap Temporarily
	IPicture	*pPicture;												// IPicture Interface
	OLECHAR		wszPath[MAX_PATH+1];									// Full Path To Picture (WCHAR)
	char		szPath[MAX_PATH+1];										// Full Path To Picture
	long		lWidth;													// Width In Logical Units
	long		lHeight;												// Height In Logical Units
	long		lWidthPixels;											// Width In Pixels
	long		lHeightPixels;											// Height In Pixels
	GLint		glMaxTexDim ;											// Holds Maximum Texture Size

	if (strstr(szPathName, "http://"))									// If PathName Contains http:// Then...
		strcpy(szPath, szPathName);										// Append The PathName To szPath
	else																// Otherwise... We Are Loading From A File
		GetCurrentDirectory(MAX_PATH, szPath);							// Get Our Working Directory
		strcat(szPath, "\\");											// Append "\" After The Working Directory
		strcat(szPath, szPathName);										// Append The PathName

	MultiByteToWideChar(CP_ACP, 0, szPath, -1, wszPath, MAX_PATH);		// Convert From ASCII To Unicode
	HRESULT hr = OleLoadPicturePath(wszPath, 0, 0, 0, IID_IPicture, (void**)&pPicture);

	if(FAILED(hr))														// If Loading Failed
		return FALSE;													// Return False

	hdcTemp = CreateCompatibleDC(GetDC(0));								// Create The Windows Compatible Device Context
	if(!hdcTemp)														// Did Creation Fail?
		pPicture->Release();											// Decrements IPicture Reference Count
		return FALSE;													// Return False (Failure)

	glGetIntegerv(GL_MAX_TEXTURE_SIZE, &glMaxTexDim);					// Get Maximum Texture Size Supported

	pPicture->get_Width(&lWidth);										// Get IPicture Width (Convert To Pixels)
	lWidthPixels	= MulDiv(lWidth, GetDeviceCaps(hdcTemp, LOGPIXELSX), 2540);
	pPicture->get_Height(&lHeight);										// Get IPicture Height (Convert To Pixels)
	lHeightPixels	= MulDiv(lHeight, GetDeviceCaps(hdcTemp, LOGPIXELSY), 2540);

	// Resize Image To Closest Power Of Two
	if (lWidthPixels <= glMaxTexDim) // Is Image Width Less Than Or Equal To Cards Limit
		lWidthPixels = 1 << (int)floor((log((double)lWidthPixels)/log(2.0f)) + 0.5f);
	else  // Otherwise  Set Width To "Max Power Of Two" That The Card Can Handle
		lWidthPixels = glMaxTexDim;

	if (lHeightPixels <= glMaxTexDim) // Is Image Height Greater Than Cards Limit
		lHeightPixels = 1 << (int)floor((log((double)lHeightPixels)/log(2.0f)) + 0.5f);
	else  // Otherwise  Set Height To "Max Power Of Two" That The Card Can Handle
		lHeightPixels = glMaxTexDim;

	//	Create A Temporary Bitmap
	BITMAPINFO	bi = {0};												// The Type Of Bitmap We Request
	DWORD		*pBits = 0;												// Pointer To The Bitmap Bits

	bi.bmiHeader.biSize			= sizeof(BITMAPINFOHEADER);				// Set Structure Size
	bi.bmiHeader.biBitCount		= 32;									// 32 Bit
	bi.bmiHeader.biWidth		= lWidthPixels;							// Power Of Two Width
	bi.bmiHeader.biHeight		= lHeightPixels;						// Make Image Top Up (Positive Y-Axis)
	bi.bmiHeader.biCompression	= BI_RGB;								// RGB Encoding
	bi.bmiHeader.biPlanes		= 1;									// 1 Bitplane

	//	Creating A Bitmap This Way Allows Us To Specify Color Depth And Gives Us Imediate Access To The Bits
	hbmpTemp = CreateDIBSection(hdcTemp, &bi, DIB_RGB_COLORS, (void**)&pBits, 0, 0);

	if(!hbmpTemp)														// Did Creation Fail?
		DeleteDC(hdcTemp);												// Delete The Device Context
		pPicture->Release();											// Decrements IPicture Reference Count
		return FALSE;													// Return False (Failure)

	SelectObject(hdcTemp, hbmpTemp);									// Select Handle To Our Temp DC And Our Temp Bitmap Object

	// Render The IPicture On To The Bitmap
	pPicture->Render(hdcTemp, 0, 0, lWidthPixels, lHeightPixels, 0, lHeight, lWidth, -lHeight, 0);

	// Convert From BGR To RGB Format And Add An Alpha Value Of 255
	for(long i = 0; i < lWidthPixels * lHeightPixels; i++)				// Loop Through All Of The Pixels
		BYTE* pPixel	= (BYTE*)(&pBits[i]);							// Grab The Current Pixel
		BYTE  temp		= pPixel[0];									// Store 1st Color In Temp Variable (Blue)
		pPixel[0]		= pPixel[2];									// Move Red Value To Correct Position (1st)
		pPixel[2]		= temp;											// Move Temp Value To Correct Blue Position (3rd)

		// This Will Make Any Black Pixels, Completely Transparent		(You Can Hardcode The Value If You Wish)
		if ((pPixel[0]==0) && (pPixel[1]==0) && (pPixel[2]==0))			// Is Pixel Completely Black
			pPixel[3]	=   0;											// Set The Alpha Value To 0
		else															// Otherwise
			pPixel[3]	= 255;											// Set The Alpha Value To 255

	glGenTextures(1, texid);											// Create The Texture

	// Typical Texture Generation Using Data From The Bitmap
	glBindTexture(GL_TEXTURE_2D, *texid);								// Bind To The Texture ID
	glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_LINEAR);		// (Modify This For The Type Of Filtering You Want)
	glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_LINEAR);     // (Modify This For The Type Of Filtering You Want)
	glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, lWidthPixels, lHeightPixels, 0, GL_RGBA, GL_UNSIGNED_BYTE, pBits);	// (Modify This If You Want Mipmaps)

	DeleteObject(hbmpTemp);												// Delete The Object
	DeleteDC(hdcTemp);													// Delete The Device Context

	pPicture->Release();												// Decrements IPicture Reference Count

    std::cout << "        TextureBuild Complete:" << std::endl;
	return TRUE;														// Return True (All Good)

The thread function, which calls the above function:

DWORD WINAPI createBackFaceTextureThread(LPVOID lpParam) {
/* random code to figure out backFaceIndex*/

BuildTexture(_fileList[_uptoFileNumber], &(_face_textures[backFaceIndex]));

    return 0;

and the function that starts the thread:

void startTextureBuildingThread(void) {

    HANDLE hRenderThread = NULL;
    int unusedData = 0;

    hRenderThread = CreateThread(NULL, 0, createBackFaceTextureThread, &unusedData, 0, NULL);


   // ****************************************
    // ************ READ **********************
    // ******************************************
    // if you comment out the above thread creation, and
    // instead uncomment this (ie call the same function but
   // just not as a thread, then everything works, and the
   // global variable is updated
   // ******************************************
   // ******************************************
   // ******************************************


thats all the functions that are linked to the problem

should be noted though, that openGL at the same time is also drawing etc via its glutIdleFunc(..), not sure if thats relevant or not. i dont think it is cause i tried passing in some basic integer global variables into the image loading function, and to just increment them by one (ie are not used by the opengl functions) and they also failed to be incremented... so dont think its opengl functions but i dont knwo for sure.

thanks heaps

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