\nif somebody tries to delete it<\/A>.\n<\/P>\n\n(Rats I fell into the trap of trying to motivate a Little Program.)\n<\/P>\n
\nOkay, enough motivation. Here’s the program:\n<\/P>\n
\n#include <windows.h>\n#include <winioctl.h>\n#include <stdio.h><\/p>\nOVERLAPPED g_o;<\/p>\n
REQUEST_OPLOCK_INPUT_BUFFER g_inputBuffer = {\n REQUEST_OPLOCK_CURRENT_VERSION,\n sizeof(g_inputBuffer),\n OPLOCK_LEVEL_CACHE_READ | OPLOCK_LEVEL_CACHE_HANDLE,\n REQUEST_OPLOCK_INPUT_FLAG_REQUEST,\n};<\/p>\n
REQUEST_OPLOCK_OUTPUT_BUFFER g_outputBuffer = {\n REQUEST_OPLOCK_CURRENT_VERSION,\n sizeof(g_outputBuffer),\n};<\/p>\n
int __cdecl wmain(int argc, wchar_t **argv)\n{\n g_o.hEvent = CreateEvent(nullptr, FALSE, FALSE, nullptr);<\/p>\n
HANDLE hFile = CreateFileW(argv[1], GENERIC_READ,\n FILE_SHARE_READ, nullptr, OPEN_EXISTING,\n FILE_FLAG_OVERLAPPED, nullptr);\n if (hFile == INVALID_HANDLE_VALUE) {\n return 0;\n }<\/p>\n
DeviceIoControl(hFile, FSCTL_REQUEST_OPLOCK,\n &g_inputBuffer, sizeof(g_inputBuffer),\n &g_outputBuffer, sizeof(g_outputBuffer),\n nullptr, &g_o);\n if (GetLastError() != ERROR_IO_PENDING) {\n \/\/ oplock failed\n return 0;\n }<\/p>\n
DWORD dwBytes;\n if (!GetOverlappedResult(hFile, &g_o, &dwBytes, TRUE)) {\n \/\/ oplock failed\n return 0;\n }<\/p>\n
printf(“Cleaning up because somebody wants the file…\\n”);\n Sleep(1000); \/\/ pretend this takes some time<\/p>\n
printf(“Closing file handle\\n”);\n CloseHandle(hFile);<\/p>\n
CloseHandle(g_o.hEvent);<\/p>\n
return 0;\n}\n<\/PRE>\n
\nRun this program with the name of an existing file\non the command line,\nsay\nscratch x.txt<\/CODE>.\nThe program will wait.\n<\/P>\n\nIn another command window, run the command\ntype x.txt<\/CODE>.\nThe program keeps waiting.\n<\/P>\n\nNext, run the command\necho hello > x.txt<\/CODE>.\nNow things get interesting.\n<\/P>\n\nWhen the command prompt opens x.txt<\/CODE> for writing,\nthe DeviceIoControl<\/CODE> call completes.\nAt this point we print the Cleaning up…<\/TT> message.\n<\/P>\n\nTo simulate the program taking a little while to clean up,\nwe sleep for one second.\nObserve that the command prompt\nhas not yet returned<\/I>.\nInstead of immediately failing the request to open for writing\nwith a sharing violation,\nthe kernel puts the open request on hold to give our program\ntime to clean up and close our handle.\n<\/P>\n
\nFinally, our simulated clean-up is complete, and we close\nthe handle.\nAt this point, the kernel allows the command processor to proceed\nand open the file for writing so it can write hello<\/TT>\ninto it.\n<\/P>\n
\nThat’s the basics of opportunistic locks,\nbut your program will almost certainly not be structured this way.\nYou will probably not wait synchronously on the overlapped I\/O\nbut rather have the completion queued up to a completion function,\nan I\/O completion port,\nor have a thread pool task listen on the event handle.\nWhen you do that, remember that you need to keep\nthe OVERLAPPED<\/CODE> structure as well as the\nREQUEST_OPLOCK_INPUT_BUFFER<\/CODE>\nand\nREQUEST_OPLOCK_OUTUT_BUFFER<\/CODE>\nstructures valid until the I\/O completes.\n<\/P>\n\n(You may find the\nCancelIo<\/CODE> function handy to try to accelerate\nthe clean-up of the file handle and any other actions that\nare dependent upon it.)\n<\/P>\n\nYou can read more about\n\nopportunistic locks on MSDN<\/A>.\nNote that there are limitations on explicitly-managed\nopportunistic locks;\nfor example, they don’t work across the network.\n<\/P><\/p>\n","protected":false},"excerpt":{"rendered":"Opportunistic locks allow you to be notified when somebody else tries to access a file you have open. This is usually done if you want to use a file provided nobody else wants it. For example, you might be a search indexer that wants to extract information from a file, but if somebody opens the […]<\/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-4663","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
Opportunistic locks allow you to be notified when somebody else tries to access a file you have open. This is usually done if you want to use a file provided nobody else wants it. For example, you might be a search indexer that wants to extract information from a file, but if somebody opens the […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/4663","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=4663"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/4663\/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=4663"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=4663"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=4663"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}