Hi,

From http://doc.trolltech.com/4.4/threads.html, I found that
Note that QCoreApplication::exec() must always be called from the main thread (the thread that executes main()), not from a QThread. In GUI applications, the main thread is also called the GUI thread because it's the only thread that is allowed to perform GUI-related operations.

but in the attached application, I tried creating a linux thread from main and tries to call exec from the spawned thread and exec works.
I guess as per above line, exec() should only be calling from thread running main() and not from the spawned thread, am I right?
My question is is any thread a main thread (even though it's not running main()) and therefore can call exec() so long it's not spawned by QThread api?

Qt Code: 
  1. #include <pthread.h>
  2. #include <Qt>
  3. #include <QtCore>
  4. #include <QtGui>
  5. #include <QPainter>
  6. #include <QWidget>
  7. #include <QImage>
  8.  
  9. class QPen;
  10. class QBrush;
  11. class QImage;
  12.  
  13. #define SCREEN_X_RESOLUTION 800
  14. #define SCREEN_Y_RESOLUTION 600
  15.  
  16. struct qtimagedesc {
  18. };
  19. typedef struct qtimagedesc QtImageDesc;
  20.  
  21. QtImageDesc *QtImageDescPool = NULL;
  22. bool initDone = false;
  23.  
  24. #define qtApp	((QtApplication *)qApp)
  25. void threadfunc(void *arg);
  26. void AWT_drawline(int x1, int y1, int x2, int y2);
  27. void AWT_drawrect(int x, int y, int w, int h);
  28. void getQtColorModel();
  29. void Qtexec();
  30.  
  31.  
  32. QImage img(SCREEN_X_RESOLUTION,SCREEN_Y_RESOLUTION,QImage::Format_ARGB32);
  33.  
  34. class QtWindow : public QWidget
  35. {
  36.        QBitmap transMask;
  37.  
  38. public:
  39.         QtWindow(int flags,
  40. 		const char *name = "Sun AWT/Qt",
  41.              QWidget *parent = NULL) ;
  42. 	virtual void setMask(const QBitmap &);
  43. 	virtual void paintEvent(QPaintEvent *);
  44.  
  45. };
  46.  
  47. class QtApplication : public QApplication {
  48. public :
  49. 	QtApplication(int &argc, char **argv);
  50. 	int exec(); //overloadded exec from QApplication
  51. };
  52.  
  53. class QtScreen
  54. {
  55. protected :
  56.     int m_x;
  57.     int m_y;
  58.     int m_width;
  59.     int m_height;
  60.     QtWindow *m_window;
  61.     bool m_bounds_restricted;
  62. /*    virtual char **getArgs(int *argc);
  63.     virtual void computeBounds();
  64.     virtual void createQtWindow();
  65.     virtual void windowShown();
  66. */
  67.  
  68. public :
  69.     QtScreen();
  70.     bool init();
  71.  /*   void close();
  72.     void showWindow();
  73.     void setMouseCursor(int cursor);
  74.     void beep();
  75.     int width();
  76.     int height();
  77.     int dotsPerInch();
  78.     QtWindow *window();
  79.     int x();
  80.     int y();
  81. */
  82. };
  83.  
  84. class QtScreenFactory
  85. {
  86. 	static QtScreen *theScreen;
  87. public:
  88. 	static QtScreen *getScreen();
  89. };
  90.  
  91.  
  92. QtWindow *m_window;
  93. QtScreen *QtScreenFactory::theScreen = NULL;
  94. QtApplication::QtApplication(int &argc, char **argv) : QApplication(argc, argv) {
  95. }
  96.  
  97. int
  98. QtApplication::exec(void) {
  99. 	printf("QApplication exec called %x\n", pthread_self());
  100. 	QApplication::exec();
  101. 	printf("QApplication exec done\n");
  102. }
  103.  
  104. QtWindow::QtWindow(int flags, const char *name, QWidget *parent) :
  105. 	QWidget(parent, (Qt::WindowFlags)flags)
  106. {
  107. 	setMouseTracking(true);
  108. }
  109.  
  110. void QtWindow::setMask(const QBitmap &bm)
  111. {
  112. 	printf("QtWindow::setMask\n");
  113. }
  114.  
  115. void
  116. QtWindow::paintEvent(QPaintEvent *event)
  117. {
  118. 	printf("paintEvent called\n");
  119. #if 0
  120. 	QPen *qp = new QPen();
  121. 	QColor c(0xffff0000);
  122. 	qp->setColor(c);
  123. 	QBrush *qb = new QBrush();
  124. 	QPainter p(this);
  125. 	p.setCompositionMode(QPainter::CompositionMode_SourceOver);
  126. 	p.setPen(*qp);
  127. 	p.setClipRect(0,0,800,600);
  128. 	p.drawLine(30,135,790,135);
  129. #endif
  130. 	QPainter p(QtImageDescPool[0].qpd);
  131. 	QPoint pt(0,0);
  132. 	p.drawImage(pt, img);
  133. }
  134.  
  135. int main(int argc, char **argv)
  136. {
  137. 	pthread_t tid;
  138. 	printf("main thread %x, argc %d, argv %s\n",pthread_self(), argc, *argv);
  139. #if 1
  140. 	pthread_create(&tid, NULL, (void *(*)(void *))threadfunc, NULL);
  141. #endif
  142. 	while(!initDone);
  143. 	AWT_drawline(0,599,799,599);
  144. 	usleep(1000);
  145. 	AWT_drawrect(30,135,190,100);
  146. 	while(true) {}
  147. //	 img = new QImage(800,600,QImage::Format_ARGB32);
  148. }
  149.  
  150. QtScreen *
  151. QtScreenFactory::getScreen() {
  152. 	printf("QtScreenFactory::getScreen()\n");
  153. 	if ( QtScreenFactory::theScreen == NULL ) {
  154.         QtScreenFactory::theScreen = new QtScreen();
  155.     	    if(QtScreenFactory::theScreen != NULL) {
  156. 	    	    QtScreenFactory::theScreen->init();
  157. 			}
  158. 	}
  159. 	return QtScreenFactory::theScreen;
  160. }
  161.  
  162. QtScreen::QtScreen()
  163. {
  164. 	printf("QtScreen::QtScreen()\n");
  165. 	QtImageDescPool = (QtImageDesc *) calloc(sizeof(QtImageDesc), 128);	
  166. }
  167.  
  168. void getQtColorModel()
  169. {
  170. 	int depth = 0;
  171. 	int amask=0,rmask=0,gmask=0,bmask=0;
  172.  
  173. 	QWidget *d = QApplication::desktop();
  174. 	depth = d->depth();	
  175. 	printf ("Depth = %d\n", depth);
  176. 	switch(depth) {
  177. 		case 32:
  178. 			amask = 0xff000000;
  179. 		case 24:
  180. 			rmask = 0x00ff0000;
  181. 			gmask = 0x0000ff00;
  182. 			bmask = 0x000000ff;
  183. 				break;
  184. 	}
  185. }
  186.  
  187. void Qtexec()
  188. {
  189. 	qtApp->exec();
  190. }
  191.  
  192. bool
  193. QtScreen::init()
  194. {
  195. 	printf("QtScreen::init() %x\n", pthread_self());
  196. 	int m_x = 0,m_y=0,m_width=SCREEN_X_RESOLUTION,m_height=SCREEN_Y_RESOLUTION;
  197.  
  198. 	int argc = 1;
  199. //	char *argv[3] = {"a.out", "-qws", NULL};
  200.  
  201. 	char **argv = (char**)malloc(sizeof(char**) * 2);
  202. 	argv[0] = (char *)malloc(sizeof(char) * 128);
  203. 	argv[1] = NULL;
  204. 	strcpy(argv[0], "cvm");
  205.  
  206. 	new QtApplication(argc, argv);
  207. 	QRect screen_bounds = QApplication::desktop()->frameGeometry();
  208. 	m_window = new QtWindow(Qt::FramelessWindowHint|Qt::Window);
  209. 	printf("m_window %x\n", this->m_window);
  210. 	QBitmap bitmap(m_width, m_height);
  211. 	m_window->setMask(bitmap);
  212. 	qtApp->setActiveWindow(this->m_window);
  213. 	this->m_window->move(0,0);
  214. 	this->m_window->setFixedSize(SCREEN_X_RESOLUTION, SCREEN_Y_RESOLUTION);
  215. 	this->m_window->show();
  216.  
  217. 	QtImageDescPool[0].qpd = this->m_window;
  218.  
  219. 	getQtColorModel();
  220. 	initDone = true;
  221.  
  222. }
  223.  
  224.  
  225. void threadfunc(void *arg)
  226. {
  227. 	printf("threadfunc called\n");
  228. 	QtScreen *screen = QtScreenFactory::getScreen();
  229. 				Qtexec();
  230. }
  231.  
  232. void
  233. AWT_drawline(int x1, int y1, int x2, int y2)
  234. {
  235. #if 0
  236. 	QPen *qp = new QPen();
  237. 	QBrush *qb = new QBrush();
  238. 	QPainter p(qpd);
  239. 	p.setCompositionMode(QPainter::CompositionMode_SourceOver);
  240. 	p.setPen(*qp);
  241. 	p.setClipRect(0,0,800,600);
  242. 	p.drawLine(30,135,790,135);
  243. #endif
  244. 	printf("drawline called\n");
  245. 	QPainter p(&img);
  246. 	p.setCompositionMode(QPainter::CompositionMode_SourceOver);
  247. 	QPen *qp = new QPen();
  248. 	QColor c(0xffff0000);
  249. 	qp->setColor(c);
  250. 	QBrush *qb = new QBrush();
  251. 	p.setPen(*qp);
  252. 	p.setClipRect(0,0,SCREEN_X_RESOLUTION,SCREEN_Y_RESOLUTION);
  253. 	p.drawLine(x1, y1, x2, y2);
  254. 	printf("m_window %x\n", ((QtWindow *)QtImageDescPool[0].qpd));
  255. 	//((QtWindow *)arg)->update(0,0,800,600);
  256. 	((QtWindow *)QtImageDescPool[0].qpd)->update(0,0,SCREEN_X_RESOLUTION,SCREEN_Y_RESOLUTION);
  257. }
  258.  
  259. void
  260. AWT_drawrect(int x, int y, int w, int h)
  261. {
  262. 	printf("drawrect called\n");
  263. 	QPainter p(&img);
  264. 	p.setCompositionMode(QPainter::CompositionMode_SourceOver);
  265. 	QPen *qp = new QPen();
  266. 	QColor c(0xffff0000);
  267. 	qp->setColor(c);
  268. 	p.setPen(*qp);
  269. 	p.setClipRect(0,0,800,600);
  270. 	p.drawRect(x, y, w, h);
  271. 	printf("m_window %x\n", ((QtWindow *)QtImageDescPool[0].qpd));
  272. 	//((QtWindow *)arg)->update(0,0,800,600);
  273. 	((QtWindow *)QtImageDescPool[0].qpd)->update(0,0,SCREEN_X_RESOLUTION,SCREEN_Y_RESOLUTION);
  274. }
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