{"id":92892,"date":"2016-01-20T07:00:00","date_gmt":"2016-01-20T22:00:00","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/oldnewthing\/?p=92892"},"modified":"2019-03-13T10:29:09","modified_gmt":"2019-03-13T17:29:09","slug":"20160120-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20160120-00\/?p=92892","title":{"rendered":"So how bad is it that I’m calling RegOpenKey instead of RegOpenKeyEx?"},"content":{"rendered":"

A customer had some code that called the Reg­Open­Key<\/code> function and was concerned by the remark in MSDN: <\/p>\n

Note<\/b> This function is provided only for compatibility with 16-bit versions of Windows. Applications should use the RegOpenKeyEx<\/b> function. <\/p><\/blockquote>\n

What are the dire consequences of using this old function instead of the new one? <\/p>\n

In general, not much. <\/p>\n

If you call Reg­Open­Key<\/code>, then some compatibility stuff kicks in, and then it goes ahead and behaves as if you had called Reg­Open­Key­Ex<\/code>. <\/p>\n

In the specific case of Reg­Open­Key<\/code>, the compatibility stuff is mentioned in the parameter documentation of Reg­Open­Key<\/code>: <\/p>\n

lpSubKey<\/i>: If this parameter is NULL<\/b> or a pointer to an empty string, the function returns the same handle that was passed in. <\/p><\/blockquote>\n

This is different from Reg­Open­Key­Ex<\/code>, which always returns a new key. It means that if you pass NULL<\/code> as the lpSub­Key<\/code>, then the returned registry key is the same as the one that you passed in, and therefore it does not<\/i> create a new obligation to call Reg­Close­Key<\/code>. In other words, this code has a potential bug: <\/p>\n

\nvoid DoSomething(HKEY hkey, PCSTR subkeyName)\n{\n  HKEY subkey;\n  if (RegOpenKey(hkey, subkeyName, &subkey) == ERROR_SUCCESS) {\n    \/\/ do something\n    RegCloseKey(subkey);\n  }\n}\n<\/pre>\n
The bug occurs if subkeyName<\/code> is NULL<\/code> or \"\"<\/code>. In that case, the special 16-bit compatibility behavior<\/a> kicks in, and subkey<\/code> is set to a copy of hkey<\/code>. This means that when you do Reg­Close­Key(subkey)<\/code>, you are closing the original hkey<\/code><\/i>, and the caller will probably be rather upset that you closed a key out from under it. <\/p>\n
If you know that subkeyName<\/code> is never NULL<\/code> or \"\"<\/code>, then you can safely close the key. Otherwise, you either need to check against this special case or (better) just switch to Reg­Open­Key­Ex<\/code> so you don’t have to deal with the special case in the first place. <\/p>\n","protected":false},"excerpt":{"rendered":"
Mostly not bad, except for the gotcha.<\/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-92892","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
Mostly not bad, except for the gotcha.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/92892","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=92892"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/92892\/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=92892"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=92892"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=92892"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}