{"id":8343,"date":"2012-02-09T07:00:00","date_gmt":"2012-02-09T07:00:00","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/oldnewthing\/2012\/02\/09\/what-is-the-effect-of-memory-mapped-file-access-on-getlasterror\/"},"modified":"2012-02-09T07:00:00","modified_gmt":"2012-02-09T07:00:00","slug":"what-is-the-effect-of-memory-mapped-file-access-on-getlasterror","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20120209-00\/?p=8343","title":{"rendered":"What is the effect of memory-mapped file access on GetLastError()?"},"content":{"rendered":"

\nA customer was using memory-mapped files and was looking for\ninformation as to whether access to the memory-mapped data\nmodifies the value returned by\nGet­Last­Error<\/code>.\nA member of the kernel team replied,\n“No, memory-mapped I\/O does not ever change the value returned by\nGet­Last­Error<\/code>.”\n<\/p>\n

\nThat answer is simultaneously correct and wrong,\na case of\n\nlooking at the world through kernel-colored glasses<\/a>.\n<\/p>\n

\nWhile it’s true that the kernel does not ever change the value\nreturned by\nGet­Last­Error<\/code>,\nit’s also the case that\nyou<\/i> might change it.\n<\/p>\n

\nIf you set up an exception handler, then your\nexception handler might perform operations that affect\nthe last-error code, and those changes will be visible\nafter the exception handler returns.\n(This applies to all exception handlers and filters,\nnot just ones related to memory-mapped files.)\n<\/p>\n

\nIf you intend to return\nEXCEPTION_CONTINUE_EXECUTION<\/code>\nbecause you handled the exception,\nthen you probably should make sure to leave the last-error\ncode the way you found it.\nOtherwise, the code that you interrupted and then resumed will have had its\nlast-error code changed asynchronously.\nYou just\n\nsabotaged it from above<\/a>.\n<\/p>\n

\n\/\/ Code in italics is wrong\nLONG ExceptionFilter(LPEXCEPTION_POINTERS ExceptionPointers)\n{\n if (IsAnExceptionICanRepair(ExceptionPointers)) {\n   RepairException(ExceptionPointers);\n   \/\/ fixed up error; continuing\n   return EXCEPTION_CONTINUE_EXECUTION;\n }\n if (IsAnExceptionICanHandle(ExceptionPointers)) {\n  \/\/ We cannot repair it, but we can handle it.\n  return EXCEPTION_EXECUTE_HANDLER;\n }\n \/\/ Not our exception. Keep looking.\n return EXCEPTION_CONTINUE_SEARCH;\n}<\/i>\n<\/pre>\n
\nIf the Is­An­Exception­I­Can­Repair<\/code> function\nor\nRepair­Exception<\/code> function\ndoes anything that affects the last-error code,\nthen when the exception filter is executed for a repairable\nexception, the last-error code is magically changed without the\nmainline code’s knowledge.\nAll the mainline code did was execute stuff normally, and somehow\nduring a memory access or a floating point operation or some other\nseemingly-harmless action, the last-error code spontaneously changed!\n<\/p>\n
\nIf you are going to continue execution at the point the exception was\nraised, then you need to “put things back the way you found them”\n(except of course for the part where you repair the exception itself).\n<\/p>\n
\nLONG ExceptionFilter(LPEXCEPTION_POINTERS ExceptionPointers)\n{\n PreserveLastError<\/font><\/a> preserveLastError;<\/font>\n if (IsAnExceptionICanRepair(ExceptionPointers)) {\n   RepairException(ExceptionPointers);\n   \/\/ fixed up error; continuing\n   return EXCEPTION_CONTINUE_EXECUTION;\n }\n if (IsAnExceptionICanHandle(ExceptionPointers)) {\n  \/\/ We cannot repair it, but we can handle it.\n  return EXCEPTION_EXECUTE_HANDLER;\n }\n \/\/ Not our exception. Keep looking.\n return EXCEPTION_CONTINUE_SEARCH;\n}\n<\/pre>\n
\nExercise<\/b>:\nWhy isn’t it important to restore the last error code if\nyou return EXCEPTION_EXECUTE_HANDLER<\/code>?\n<\/p>\n
\nExercise<\/b>:\nIs it important to restore the last error code if you return\nEXCEPTION_CONTINUE_SEARCH<\/code>?<\/p>\n","protected":false},"excerpt":{"rendered":"
A customer was using memory-mapped files and was looking for information as to whether access to the memory-mapped data modifies the value returned by Get­Last­Error. A member of the kernel team replied, “No, memory-mapped I\/O does not ever change the value returned by Get­Last­Error.” That answer is simultaneously correct and wrong, a case of looking […]<\/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-8343","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
A customer was using memory-mapped files and was looking for information as to whether access to the memory-mapped data modifies the value returned by Get­Last­Error. A member of the kernel team replied, “No, memory-mapped I\/O does not ever change the value returned by Get­Last­Error.” That answer is simultaneously correct and wrong, a case of looking […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/8343","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=8343"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/8343\/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=8343"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=8343"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=8343"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}