\n| H<\/td>\n | e<\/td>\n | l<\/td>\n | l<\/td>\n | o<\/td>\n | \\0<\/td>\n | w<\/td>\n | o<\/td>\n | r<\/td>\n | l<\/td>\n | d<\/td>\n | \\0<\/td>\n | \\0<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n \nBut what about the edge cases?\nWhat if you want to build a double-null-terminated string\nwith no strings?\n<\/p>\n \nLet’s step back and look at the double-null-terminated string\nwith two strings in it.\nBut I’m going to insert line breaks to highlight the structure.\n<\/p>\n \n\n\n| H<\/td>\n | e<\/td>\n | l<\/td>\n | l<\/td>\n | o<\/td>\n | \\0<\/td>\n<\/tr>\n | \n| w<\/td>\n | o<\/td>\n | r<\/td>\n | l<\/td>\n | d<\/td>\n | \\0<\/td>\n<\/tr>\n | \n| \\0<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n \nNow I’m going to move the lines around.\n<\/p>\n \n\n\n\nHello\\0<\/span>\n<\/td>\n<\/tr>\n\n\nworld\\0<\/span>\n<\/td>\n<\/tr>\n\n| \n\\0<\/span>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n \nThis alternate way of writing the double-null-terminated string\nis the secret.\nInstead of viewing the string as something terminated by\ntwo consecutive null terminators, let’s view it as a\nlist of null-terminated strings, with a zero-length string at the end.\nAlternatively, think of it as a packed array of null-terminated\nstrings, with a zero-length string as the terminator.\n<\/p>\n \nThis type of reinterpretation happens a lot in advanced\nmathematics.\nYou have some classical definition of an object,\nand then you invent a new interpretation which agrees\nwith the classical definition, but which gives you a\ndifferent perspective on the system and even generalizes\nto cases the classical definition didn’t handle.\n<\/p>\n \nFor example, this “modern reinterpretation” of\ndouble-null-terminated strings provides another answer\nto a standard question:\n<\/p>\n \nHow do I build a double-null-terminated string with an empty\nstring as one of the strings in the list?\n<\/p>\n \nYou can’t, because the empty string is treated as the end of the\nlist.\nIt’s the same reason why you can’t put a null character\ninside a null-terminated string:\nThe null character is treated as the terminator.\nAnd in a double-null-terminated string,\nan empty string is treated as the terminator.\n<\/p>\n \n\n\n\nOne\\0<\/span>\n<\/td>\n<\/tr>\n\n\n\\0<\/span>\n<\/td>\n<\/tr>\n\n<\/tr>\n \n\nThree\\0<\/span>\n<\/td>\n<\/tr>\n\n| \n\\0<\/span>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n \nIf you try to put a zero-length string in your list,\nyou end up accidentally terminating it prematurely.\nUnder the classical view, you can see the two consecutive\nnull terminators:\nThey come immediately after the\n\"One\"<\/code>.\nUnder the reinterpretation I propose,\nit’s more obvious,\nbecause the zero-length string is itself the terminator.\n<\/p>\n\nIf you’re writing a helper class to manage double-null-terminated strings,\nmake sure you watch out for these empty strings.\n<\/p>\n \nThis reinterpretation of a double-null-terminated string as really\na list of strings with an empty string as the terminator<\/i>\nmakes writing code to walk through a double-null-terminated string\nquite straightforward:\n<\/p>\nfor (LPTSTR pszz = pszzStart; *pszz; pszz += lstrlen(pszz) + 1) {
... do something with pszz ...
}
<\/pre>\n\nDon’t think about looking for the double null terminator.\nInstead, just view it as a list of strings,\nand you stop when you find a string of length zero.\n<\/p>\n \nThis reinterpretation also makes it clear how you express a list\nwith no strings in it at all:\nAll you have is the zero-length string terminator.\n<\/p>\n \n\n\n| \\0<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n \nWhy do we even have double-null-terminated strings at all?\nWhy not just pass an array of pointers to strings?\n<\/p>\n \nThat would have worked, too,\nbut it makes allocating and freeing the array more complicated,\nbecause the memory for the array and the component strings are\nnow scattered about.\n(Compare absolute and self-relative security descriptors.)\nA double-null-terminated string occupies a single block of memory\nwhich can be allocated and freed at one time,\nwhich is very convenient when you have to serialize and deserialize.\nIt also avoids questions like\n“Is it legal for two entries in the array to point to the same string?”\n<\/p>\n \nKeeping it in a single block of memory reduces the number of selectors\nnecessary to represent the data in 16-bit Windows.\n(And this data representation was developed long before the 80386\nprocessor even existed.)\nAn array of pointers to 16 strings would require 17 selectors,\nif you used GlobalAlloc<\/code> to allocate the memory:\none for the array itself,\nand one for each string.\nSelectors were a scarce resource in 16-bit Windows;\nthere were only 8192 of them available in the entire system.\nYou don’t want to use 1% of your system’s entire allocation\njust to represent an array of 100 strings.\n<\/p>\n\nOne convenience of double-null-terminated strings is that you can load one\ndirectly out of\nyour resources with a single call to LoadString<\/code>:\n<\/p>\nSTRINGTABLE
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<\/pre>\n\nThis is very handy because it allows new filters to be added\nby simply changing a resource.\nIf the filter were passed as an array of pointers to strings,\nyou would probably put each string in a separate resource,\nand then the number of strings becomes more difficult to update.\n<\/p>\n \nBut there is a gotcha in the above code,\nwhich we will look at next time.\n<\/p>\n \nBonus Gotcha<\/b>:\nEven though you may know how double-null terminated strings work,\nthis doesn’t guarantee that the code you’re interfacing with\nunderstands it as well as you do.\nConsequently, you’d be best off putting the extra null terminator\nat the end if you are generating a double-null-terminated string,\njust in case the code you are calling expects the extra null\nterminator (even though it technically isn’t necessary).\nExample:\nThe ANSI version of\n\nCreateProcess<\/code><\/a> locates the end of the environment block by\nlooking for two consecutive NULL bytes instead of looking for\nthe empty string terminator.<\/p>\n","protected":false},"excerpt":{"rendered":"One of the data formats peculiar to Windows is the double-null-terminated string. If you have a bunch of strings and you want to build one of these elusive double-null-terminated strings out of it, it’s no big deal. H e l l o \\0 w o r l d \\0 \\0 But what about the edge […]<\/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-16443","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":" One of the data formats peculiar to Windows is the double-null-terminated string. If you have a bunch of strings and you want to build one of these elusive double-null-terminated strings out of it, it’s no big deal. H e l l o \\0 w o r l d \\0 \\0 But what about the edge […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/16443","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=16443"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/16443\/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=16443"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=16443"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=16443"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}} | | | | | | | | | |