{"id":16073,"date":"2009-11-11T07:00:00","date_gmt":"2009-11-11T07:00:00","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/oldnewthing\/2009\/11\/11\/trying-to-avoid-double-destruction-and-inadvertently-triggering-it\/"},"modified":"2009-11-11T07:00:00","modified_gmt":"2009-11-11T07:00:00","slug":"trying-to-avoid-double-destruction-and-inadvertently-triggering-it","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20091111-00\/?p=16073","title":{"rendered":"Trying to avoid double-destruction and inadvertently triggering it"},"content":{"rendered":"

\nWe saw some time ago the importance of\n\nartificially bumping an object’s reference count during destruction\nto avoid double-destruction<\/a>.\nHowever, one person’s attempt to avoid this problem ended up triggering it.\n<\/p>\n

\nULONG MyObject::Release()\n{\n LONG cRef = InterlockedDecrement(&m_cRef);\n if (cRef > 0) return cRef;\n m_cRef = MAXLONG; \/\/ avoid double-destruction\n delete this;\n return 0;\n}\n<\/pre>\n
\nThe explanation for the line m_cRef = MAXLONG<\/code>\nwas that it was done\nto avoid the double-destruction problem if the object receives\na temporary AddRef\/Release<\/code> during destruction.\n<\/p>\n
\nWhile it’s true that you should set the reference count to an artificial\nnon-zero value,\nchoosing MAXLONG<\/code> has its own problem:\ninteger overflow.\n<\/p>\n
\nSuppose that during the object’s destruction,\nthe reference count is temporarily incremented twice and decremented\ntwice.\n<\/p>\n\n\n\n\n\n\n\n\n\n
Action<\/th>\nm_cRef<\/th>\n<\/tr>\n
Just before call to Release()<\/td>\n1<\/td>\n<\/tr>\n
InterlockedDecrement<\/td>\n0<\/td>\n<\/tr>\n
m_cRef = MAXLONG<\/td>\n2147483647<\/td>\n<\/tr>\n
destructor does temporary AddRef()<\/code><\/td>\n−2147483648 (integer overflow)<\/td>\n<\/tr>\n
destructor does temporary AddRef()<\/code><\/td>\n−2147483647<\/td>\n<\/tr>\n
destructor does temporary Release()<\/code><\/td>\n−2147483648<\/td>\n<\/tr>\n
since m_cRef<\/code> < 0, we re-destruct<\/td>\n<\/tr>\n<\/table>\n
\nSure, choosing a huge DESTRUCTOR_REFCOUNT<\/code>\nmeans that you have absolutely no chance of decrementing\nthe reference count back to zero prematurely.\nHowever, if you choose a value too high, you introduce the\nrisk of incrementing<\/i> the reference count\nso high that it overflows.\n<\/p>\n
\nThat’s why the most typical values for DESTRUCTOR_REFCOUNT<\/code>\nare 1, 42, and 1000.\nThe value 1 is really all you need to avoid double-destruction.\nSome people choose 42 because it’s cute,\nand other people choose 1000 because it’s higher than any “normal”\nrefcount, so it makes it easier to spot during debugging.\nBut even then, the “high” value of 1000 still leaves room for over\ntwo billion AddRef()<\/code>s before overflowing the\nreference count.\n<\/p>\n
\nOn the other hand, if you choose a value like MAXLONG<\/code>\nor MAXDWORD<\/code>,\nthen you’re taking something that previously never happened\n(reference count integer overflow)\nand turning it into an almost certainty.<\/p>\n","protected":false},"excerpt":{"rendered":"
We saw some time ago the importance of artificially bumping an object’s reference count during destruction to avoid double-destruction. However, one person’s attempt to avoid this problem ended up triggering it. ULONG MyObject::Release() { LONG cRef = InterlockedDecrement(&m_cRef); if (cRef > 0) return cRef; m_cRef = MAXLONG; \/\/ avoid double-destruction delete this; return 0; } […]<\/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],"class_list":["post-16073","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
We saw some time ago the importance of artificially bumping an object’s reference count during destruction to avoid double-destruction. However, one person’s attempt to avoid this problem ended up triggering it. ULONG MyObject::Release() { LONG cRef = InterlockedDecrement(&m_cRef); if (cRef > 0) return cRef; m_cRef = MAXLONG; \/\/ avoid double-destruction delete this; return 0; } […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/16073","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=16073"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/16073\/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=16073"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=16073"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=16073"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}