{"id":31003,"date":"2006-06-01T10:00:06","date_gmt":"2006-06-01T10:00:06","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/oldnewthing\/2006\/06\/01\/what-does-the-cs_owndc-class-style-do\/"},"modified":"2006-06-01T10:00:06","modified_gmt":"2006-06-01T10:00:06","slug":"what-does-the-cs_owndc-class-style-do","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20060601-06\/?p=31003","title":{"rendered":"What does the CS_OWNDC class style do?"},"content":{"rendered":"

\nRecall that window DCs are most commonly used only temporarily.\nIf you need to draw into a window, you call\nBeginPaint<\/code>\nor, if outside of a paint cycle, GetDC<\/code>,\nalthough painting outside of a paint cycle is generally to be avoided.\nThe window manager produces a DC for the window and returns it.\nYou use the DC, then restore it to its original state and return\nit to the window manager with\nEndPaint<\/code> (or ReleaseDC<\/code>).\nInternally, the window manager keeps a small cache of DCs which it\ndips into when people come asking for a window DC,\nand when the DC is returned, it goes back into the cache.\nSince window DCs are used only temporarily, the number of outstanding DCs\nis typically not more than a handful, and a small cache is sufficient\nto satisfy DC demands in a normally-running system.\n<\/p>\n

\nIf you register a window class and include the CS_OWNDC<\/code>\nflag in the class styles,\nthen the window manager creates a DC for the window and puts it into\nthe DC cache with a special tag that means\n“Do not purge this DC from the DC cache\nbecause it’s the CS_OWNDC<\/code> for this window.”\nIf you call BeginPaint<\/code> or GetDC<\/code> to get\na DC for a CS_OWNDC<\/code> window,\nthen that DC will always be found and returned\n(since it was marked as “never purge”).\nThe consequences of this are good, bad, and worse.\n<\/p>\n

\nThe good part is that since the DC has been created specially for\nthe window and is never purged,\nyou don’t have to worry about “cleaning up the DC” before\nreturning it to the cache.\nWhenever you call BeginPaint<\/code> or GetDC<\/code>\nfor a CS_OWNDC<\/code> window,\nyou always get that special DC back.\nIndeed, that’s the whole point of CS_OWNDC<\/code> windows:\nYou can create a CS_OWNDC<\/code> window,\nget its DC, set it up the way you like it\n(selecting fonts, setting colors, etc<\/i>.), and even if you\nrelease the DC and get it again later, you will get that\nsame DC back and it will be just the way you left it.\n<\/p>\n

\nThe bad part is that you’re taking something that was meant to be\nused only temporarily (a window DC) and using it permanently.\nEarly versions of Windows had a very low limit for DCs (eight or so),\nso it was crucial that DCs be released as soon as they weren’t needed.\nThat limit has since been raised significantly, but the underlying\nprinciple remains: DCs should not be allocated carelessly.\nYou may have noticed that the implementation of CS_OWNDC<\/code>\nstill uses the DC cache; it’s just that those DCs get a special marking\nso the DC manager knows to treat them specially.\nThis means that a large number of CS_OWNDC<\/code> DCs end up\n“polluting” the DC cache, slowing down future calls to\nfunctions like BeginPaint<\/code> and ReleaseDC<\/code>\nthat need to search through the DC cache.\n<\/p>\n

\n(Why wasn’t the DC manager optimized to handle the case of\na large number of CS_OWNDC<\/code> DCs?\nFirst, as I already noted, the original DC manager didn’t have\nto worry about the case of a large number of DCs since the system\nsimply couldn’t even create that many in the first place.\nSecond, even after the limit on the number of DCs was raised,\nthere wasn’t much point in rewriting the DC manager to optimize\nthe handling of CS_OWNDC<\/code> DCs since programmers were\nalready told to use CS_OWNDC<\/code> sparingly.\nThis is one of the practicalities of software engineering:\nYou can do only so much.\nEverything you decide to do comes at the expense of something else.\nIt’s hard to justify optimizing a scenario that programmers were told\nto avoid and which they in fact were already avoiding.\nYou don’t optimize for the case where somebody is abusing your system.\nIt’s like spending time designing a car’s engine so it maintained\ngood gas mileage when the car has no oil.)\n<\/p>\n

\nThe worse part is that most windowing framework libraries and\nnearly all sample code\nassume that your windows are not CS_OWNDC<\/code> windows.\nConsider the following code that draws text in two fonts,\nusing the first font to guide the placement of characters in the second.\nIt looks perfectly fine, doesn’t it?\n<\/p>\n

\nvoid FunnyDraw(HWND hwnd, HFONT hf1, HFONT hf2)\n{\n HDC hdc1 = GetDC(hwnd);\n HFONT hfPrev1 = SelectFont(hdc1, hf1);\n UINT taPrev1 = SetTextAlign(hdc1, TA_UPDATECP);\n MoveToEx(hdc1, 0, 0, NULL);\n HDC hdc2 = GetDC(hwnd);\n HFONT hfPrev2 = SelectFont(hdc2, hf2);\n for (LPTSTR psz = TEXT(\"Hello\"); *psz; psz++) {\n  POINT pt;\n  GetCurrentPositionEx(hdc1, &pt);\n  TextOut(hdc2, pt.x, pt.y + 30, psz, 1);\n  TextOut(hdc1, 0, 0, psz, 1);\n }\n SelectFont(hdc1, hfPrev1);\n SelectFont(hdc2, hfPrev2);\n SetTextAlign(hdc1, taPrev1);\n ReleaseDC(hwnd, hdc1);\n ReleaseDC(hwnd, hdc2);\n}\n<\/pre>\n
\nWe get two DCs for the window.\nIn the first we select our first font; in the second, we select the second.\nIn the first DC, we also set the text alignment to TA_UPDATECP<\/code>\nwhich means that the coordinates passed to the TextOut<\/code>\nfunction will be ignored.\nInstead the text will be drawn starting at the “current position”\nand the “current position” will be updated to the end of the string,\nso that the next call to TextOut<\/code> will resume where the\nprevious one left off.\n<\/p>\n
\nOnce the two DCs are set up, we draw our string one character at a time.\nWe query the first DC for the current position and draw the character\nin the second font at that same x<\/i>-coordinate (but a bit lower),\nthen we draw the character in the first font (which also advances the\ncurrent position).\n<\/p>\n
\nAfter the text drawing loop is done, we restore the states of the two\nDCs as part of the standard bookkeeping.\n<\/p>\n
\nThe intent of the function is to draw something like this,\nwhere the first font is bigger than the second.\n<\/p>\n\n\n\n
H<\/td>\ne<\/td>\nl<\/td>\nl<\/td>\no<\/td>\n<\/tr>\n
H<\/td>\ne<\/td>\nl<\/td>\nl<\/td>\no<\/td>\n<\/tr>\n<\/table>\n
\nAnd if the window is not CS_OWNDC<\/code> that’s what you get.\nYou can try it out by calling it from our scratch program:\n<\/p>\n
\nHFONT g_hfBig;\nBOOL\nOnCreate(HWND hwnd, LPCREATESTRUCT lpcs)\n{\n LOGFONT lf;\n GetObject(GetStockFont(ANSI_VAR_FONT),\n           sizeof(lf), &lf);\n lf.lfHeight *= 2;\n g_hfBig = CreateFontIndirect(&lf);\n return g_hfBig != NULL;\n}\nvoid\nOnDestroy(HWND hwnd)\n{\n if (g_hfBig) DeleteObject(g_hfBig);\n PostQuitMessage(0);\n}\nvoid\nPaintContent(HWND hwnd, PAINTSTRUCT *pps)\n{\n FunnyDraw(hwnd, g_hfBig,\n                 GetStockFont(ANSI_VAR_FONT));\n}\n<\/pre>\n
\nBut if the window is CS_OWNDC<\/code>, then bad things happen.\nTry it yourself by changing the line\nwc.style = 0;<\/code>\nto\nwc.style = CS_OWNDC;<\/code>\nYou get the following unexpected output:\n<\/p>\n\n\n
HHeelllloo<\/td>\n<\/tr>\n<\/table>\n
\nOf course, if you understand how CS_OWNDC<\/code> works,\nthis is hardly unexpected at all.\nThe key to understanding is remembering that when the window\nis CS_OWNDC<\/code> then GetDC<\/code> just returns the\nsame DC back no matter how many times you call it.\nNow all you have to do is walk through the\nFunnyDraw<\/code> function remembering that\nhdc1<\/code> and hdc2<\/code> are in fact\nthe same thing<\/strong>.\n<\/p>\n
\nvoid FunnyDraw(HWND hwnd, HFONT hf1, HFONT hf2)\n{\n HDC hdc1 = GetDC(hwnd);\n HFONT hfPrev1 = SelectFont(hdc1, hf1);\n UINT taPrev1 = SetTextAlign(hdc1, TA_UPDATECP);\n MoveToEx(hdc1, 0, 0, NULL);\n<\/pre>\n
\nSo far, execution of the function is pretty normal.\n<\/p>\n
\n HDC hdc2 = GetDC(hwnd);\n<\/pre>\n
\nSince the window is a CS_OWNDC<\/code> window,\nthe DC that is returned in hdc2<\/code> is the same\none that was returned in hdc1<\/code>.\nIn other words, hdc1 == hdc2<\/code>!\nNow things get exciting.\n<\/p>\n
\n HFONT hfPrev2 = SelectFont(hdc2, hf2);\n<\/pre>\n
\nSince hdc1 == hdc2<\/code>,\nwhat this really does is deselect the font hf1<\/code>\nfrom the DC and select the font hf2<\/code> instead.\n<\/p>\n
\n for (LPTSTR psz = TEXT(\"Hello\"); *psz; psz++) {\n  POINT pt;\n  GetCurrentPositionEx(hdc1, &pt);\n  TextOut(hdc2, pt.x, pt.y + 30, psz, 1);\n  TextOut(hdc1, 0, 0, psz, 1);\n }\n<\/pre>\n
\nNow this loop completely falls apart.\nAt the first iteration, we retrieve the current position from\nthe DC, which returns (0, 0) since we haven’t moved it yet.\nWe then draw the letter “H” at position (0, 30) into the\nsecond DC.\nBut since the second DC is the same as the first one, what\nreally happens is that we are calling TextOut<\/code> into\na DC that is in TA_UPDATECP<\/code> mode.\nThus, the coordinates are ignored, the letter “H” is displayed\n(in the second font), and the current position is updated to\nbe after the “H”.\nFinally, we draw the “H” into the first DC (which is the same as\nthe second).\nWe think we’re drawing it with the first font, but in fact\nwe’re drawing with the second font.\nWe think we’re drawing at (0, 0), but in fact we’re\ndrawing at (x<\/i>, 0), where x<\/i> is the width of\nthe letter “H”,\nbecause the call to TextOut(hdc2, ...)<\/code>\nupdated the current position.\n<\/p>\n
\nThus, each time through the loop, the next character in the\nstring is displayed twice, all in the second font.\n<\/p>\n
\nBut wait, the disaster isn’t finished yet.\nLook at our cleanup code:\n<\/p>\n
\n SelectFont(hdc1, hfPrev1);\n<\/pre>\n
\nThis restores the original font into the DC.\n<\/p>\n
\n SelectFont(hdc2, hfPrev2);\n<\/pre>\n
\nThis re-selects the first font!\nWe failed to restore the DC to its original state\nand ended up putting a “corrupted” DC into the cache.\n<\/p>\n
\nThat’s why I described CS_OWNDC<\/code> as “worse”.\nIt takes code that used to work and breaks it\nby violating assumptions that most people make (usually\nwithout realizing it) about DCs.\n<\/p>\n
\nAnd you thought CS_OWNDC<\/code> was bad.\nNext time I’ll talk about the disaster that is known as\nCS_CLASSDC<\/code>.<\/p>\n","protected":false},"excerpt":{"rendered":"
Recall that window DCs are most commonly used only temporarily. If you need to draw into a window, you call BeginPaint or, if outside of a paint cycle, GetDC, although painting outside of a paint cycle is generally to be avoided. The window manager produces a DC for the window and returns it. You use […]<\/p>\n","protected":false},"author":1069,"featured_media":111744,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[25,2],"class_list":["post-31003","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code","tag-history"],"acf":[],"blog_post_summary":"
Recall that window DCs are most commonly used only temporarily. If you need to draw into a window, you call BeginPaint or, if outside of a paint cycle, GetDC, although painting outside of a paint cycle is generally to be avoided. The window manager produces a DC for the window and returns it. You use […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/31003","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/users\/1069"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/comments?post=31003"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/31003\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media\/111744"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media?parent=31003"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=31003"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=31003"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}