Prevent segfaults in out of memory conditions by checking the pointer returned from XGetImage.

git-svn-id: svn://svn.code.sf.net/p/rdesktop/code/trunk/rdesktop@1305 423420c4-83ab-492f-b58f-81f9feb106b5

doc/HACKING

@@ -26,3 +26,12 @@ rdesktop.c
 RDP resources
 -------------
 http://dev.remotenetworktechnology.com/refdata.htm
+
+
+Checking for out of memory conditions
+-------------------------------------
+Try to handle out of memory conditions gracefully. Use the xmalloc
+routines from rdesktop.c, instead of calling malloc manually. Also,
+remember that several Xlib functions can return NULL. This includes
+XGetImage. Use exit_if_null to verify returned pointers. 
+

proto.h

@@ -109,6 +109,7 @@ BOOL pstcache_init(uint8 cache_id);
 int main(int argc, char *argv[]);
 void generate_random(uint8 * random);
 void *xmalloc(int size);
+void exit_if_null(void *ptr);
 char *xstrdup(const char *s);
 void *xrealloc(void *oldmem, int size);
 void xfree(void *mem);

rdesktop.c

@@ -1079,6 +1079,17 @@ xmalloc(int size)
 	return mem;
 }
 
+/* Exit on NULL pointer. Use to verify result from XGetImage etc */
+void
+exit_if_null(void *ptr)
+{
+	if (ptr == NULL)
+	{
+		error("unexpected null pointer. Out of memory?\n");
+		exit(1);
+	}
+}
+
 /* strdup */
 char *
 xstrdup(const char *s)

xwin.c

@@ -3192,12 +3192,14 @@ ui_desktop_save(uint32 offset, int x, int y, int cx, int cy)
 	if (g_ownbackstore)
 	{
 		image = XGetImage(g_display, g_backstore, x, y, cx, cy, AllPlanes, ZPixmap);
+		exit_if_null(image);
 	}
 	else
 	{
 		pix = XCreatePixmap(g_display, g_wnd, cx, cy, g_depth);
 		XCopyArea(g_display, g_wnd, pix, g_gc, x, y, cx, cy, 0, 0);
 		image = XGetImage(g_display, pix, 0, 0, cx, cy, AllPlanes, ZPixmap);
+		exit_if_null(image);
 		XFreePixmap(g_display, pix);
 	}