{"id":13343,"date":"2010-07-23T07:00:00","date_gmt":"2010-07-23T07:00:00","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/oldnewthing\/2010\/07\/23\/if-im-not-supposed-to-call-isbadxxxptr-how-can-i-check-if-a-pointer-is-bad\/"},"modified":"2010-07-23T07:00:00","modified_gmt":"2010-07-23T07:00:00","slug":"if-im-not-supposed-to-call-isbadxxxptr-how-can-i-check-if-a-pointer-is-bad","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20100723-00\/?p=13343","title":{"rendered":"If I'm not supposed to call IsBadXxxPtr, how can I check if a pointer is bad?"},"content":{"rendered":"

\nSome time ago, I opined that\n\nIs­Bad­Xxx­Ptr<\/code>\nshould really be called Crash­Program­Randomly<\/code><\/a>\nand you really should just let the program crash if somebody passes\nyou a bad pointer.\nIt is common to put pointer validation code at the start of functions\nfor debugging purposes\n(as long as you don’t make logic decisions based on whether the pointer\nis valid).\nBut if you can’t use Is­Bad­Xxx­Ptr<\/code>,\nhow can you validate\nthe pointer?\n<\/p>\n

\nWell, to validate a write pointer, write to it.\nTo validate a read pointer, read from it.\nIf the pointer is invalid, you’ll crash, and at a predictable\nlocation, before the function has gotten halfway through its\nprocessing (making post-mortem debugging more difficult).\nHere are the functions I used:\n<\/p>\n

\n\/\/ Make sure to disable compiler optimizations in these functions\n\/\/ so the code won't be removed by the optimizer.\nvoid DebugValidateWritePtr(void *p, size_t cb)\n{\n memcpy(p, p, cb);\n}\nvoid DebugValidateReadPtr(void *p, size_t cb)\n{\n memcmp(p, p, cb);\n}\n<\/pre>\n
\nTo verify that a buffer can be written to, we write to it\nby copying it to itself.\nSimilarly, to verify that a buffer can be read, we read from it\nby comparing it to itself.\nThe result of the operation is not important; we are interested\nin the side-effect of the memory access itself.\n<\/p>\n
\nNote that the Debug­Validate­Write­Ptr<\/code> function\nis not thread-safe: If another thread modifies the buffer while\nwe are copying it to itself, the write may be lost.\nBut code that does this violates one of the\n\nground rules for programming<\/a> (specifically the parameter\nstability requirements).\nOf course, if your function has specific behavior requirements\nbeyond the ground rules, then that helper function may not\nwork for you.\nI’m just putting it out there as a courtesy.<\/p>\n","protected":false},"excerpt":{"rendered":"
Some time ago, I opined that Is­Bad­Xxx­Ptr should really be called Crash­Program­Randomly and you really should just let the program crash if somebody passes you a bad pointer. It is common to put pointer validation code at the start of functions for debugging purposes (as long as you don’t make logic decisions based on whether […]<\/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-13343","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
Some time ago, I opined that Is­Bad­Xxx­Ptr should really be called Crash­Program­Randomly and you really should just let the program crash if somebody passes you a bad pointer. It is common to put pointer validation code at the start of functions for debugging purposes (as long as you don’t make logic decisions based on whether […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/13343","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=13343"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/13343\/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=13343"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=13343"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=13343"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}