{"id":104048,"date":"2020-08-06T07:00:00","date_gmt":"2020-08-06T14:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=104048"},"modified":"2020-08-06T07:31:30","modified_gmt":"2020-08-06T14:31:30","slug":"20200806-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20200806-00\/?p=104048","title":{"rendered":"What was so horrible about the FORMAT_MESSAGE_ALLOCATE_BUFFER flag that the Windows Store disallowed it for so long?"},"content":{"rendered":"

Last time<\/a>, we learned about the tumultuous history of the FORMAT_<\/code>MESSAGE_<\/code>ALLOCATE_<\/code>BUFFER<\/code> flag in Windows Store UWP apps.<\/p>\n

But why was this flag disallowed for so long?<\/p>\n

It’s nothing particularly profound. Rather, it was just bad luck.<\/p>\n

The buffer allocated by the FORMAT_<\/code>MESSAGE_<\/code>ALLOCATE_<\/code>BUFFER<\/code> flag needs to be freed by calling Local\u00adFree<\/code>, but Local\u00adFree<\/code> was not one of the functions that can be called from a Windows Store app.<\/p>\n

Why not?<\/p>\n

Because Local\u00adAlloc<\/code> and Local\u00adFree<\/code> are legacy functions that hang around for backward compatibility with 16-bit Windows. New programs shouldn’t be using them. It’s not like your new program needs to be backward compatible with 16-bit Windows 3.1.<\/p>\n

But this left the FORMAT_<\/code>MESSAGE_<\/code>ALLOCATE_<\/code>BUFFER<\/code> flag in a bit of a pickle, because despite being something new for Win32, the flag continues to use that old and busted legacy function for memory allocation.<\/p>\n

There was some discussion within the team about how to address the problem. One school of thought was to document enough of the internals of the Local\u00adFree<\/code> function so that you could call Heap\u00adFree<\/code> to free it. You can see remnants of this approach in the comments of the winbase.h<\/code> header file:<\/p>\n

\/\/\r\n\/\/ FORMAT_MESSAGE_ALLOCATE_BUFFER requires use of HeapFree\r\n\/\/\r\n<\/pre>\n
For a while, there was also this paragraph of the FORMAT_<\/code>MESSAGE_<\/code>ALLOCATE_<\/code>BUFFER<\/code> documentation that says a bunch of stuff in a rather confusing way.<\/p>\n
Windows 10<\/b>: LocalAlloc()<\/b> has different options: LMEM_FIXED<\/b>, and LMEM_MOVABLE<\/b>. FormatMessage()<\/b> uses LMEM_FIXED<\/b>, so HeapFree<\/b> can be used. If LMEM_MOVABLE<\/b> is used, HeapFree<\/b> cannot be used.<\/p><\/blockquote>\n
(Fortunately, the confusing paragraph isn’t there any more.)<\/p>\n
The decision that won the day was to accept that legacy code will never die. The team just held their nose and added LocalAlloc<\/code> and LocalFree<\/code> to the list of functions that are permitted to be called by Windows Store app.<\/p>\n
But please, promise to use it only for situations that absolutely require it for compatibility purposes, like freeing the message string allocated by Format\u00adMessage<\/code>. Don’t use it as your go-to memory allocation function.<\/p>\n","protected":false},"excerpt":{"rendered":"
Letting in the riffraff.<\/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":[2],"class_list":["post-104048","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-history"],"acf":[],"blog_post_summary":"
Letting in the riffraff.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/104048","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=104048"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/104048\/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=104048"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=104048"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=104048"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}