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authorFilipe David Borba Manana <[email protected]>2012-10-23 17:38:01 +0100
committerFilipe David Borba Manana <[email protected]>2013-01-17 11:25:53 +0000
commit0e02f488971b32ff9ab88a3f0cb144fe5db161b2 (patch)
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parent812f666ea3f9034b78a12dc025366c7c31d87c3c (diff)
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Use share flags for all file operations on Windows
Some file operations provided by the Erlang file module didn't open the target file with all the file share flags. This made some concurrent file operations against the same file fail on Windows, while on other platforms such as GNU/Linux or Mac OS X they succeed. The operations will fail only if they're performed concurrently by different threads (async IO threads or scheduler threads). For example, one Erlang process does a file:delete/1 call while another Erlang process is doing a filelib:file_size/1 call. This made the former process get an eacces error from the file:delete/1 call. On GNU/Linux or Mac OS X the call would succeed. Another example is if one Erlang process attempts to open a file for reading while another one is in the middle of a file:read_file_info/1 call (after it opened the file and before it closed the file). It's easy to verify that if a file is not open with all the share flags, it's impossible for other threads (even if they belong to the same OS process) to open the file while the file is not closed by the first thread. The following test program shows this: #include <windows.h> #include <iostream> // Must be an existing file //#define SHARE_FLAGS (FILE_SHARE_READ) static DWORD WINAPI MyThreadFunction(LPVOID lpParam); static char *lastError(); int main(int argc, char *argv[]) { DWORD threadId; HANDLE threadHandle, hFile; hFile = CreateFile(FILENAME, GENERIC_READ, SHARE_FLAGS, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL); if (hFile == INVALID_HANDLE_VALUE) { std::cerr << "File open error from main: " << lastError() << std::endl; return 1; } std::cout << "Main thread opened file successfully" << std::endl; threadHandle = CreateThread(NULL, 0, MyThreadFunction, NULL, 0, &threadId); if (threadHandle == INVALID_HANDLE_VALUE) { std::cerr << "Thread create error from main: " << lastError() << std::endl; return 1; } WaitForSingleObject(threadHandle, INFINITE); CloseHandle(threadHandle); CloseHandle(hFile); return 0; } static DWORD WINAPI MyThreadFunction( LPVOID lpParam ) { HANDLE hFile; hFile = CreateFile(FILENAME, GENERIC_READ, SHARE_FLAGS, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL); if (hFile == INVALID_HANDLE_VALUE) { std::cerr << "File open error from second thread: " << lastError() << std::endl; return 1; } std::cout << "Second thread opened file successfully" << std::endl; CloseHandle(hFile); return 0; } static char *lastError() { static char *buf = NULL; DWORD dw = GetLastError(); if (buf != NULL) { LocalFree((LPTSTR) &buf); } FormatMessage(FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS, NULL, dw, MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT), (LPTSTR) &buf, 0, NULL); return buf; } Rnning this program with SHARE_FLAGS set to 0 (as efile_fileinfo() does for e.g.), shows that the second thread is unable to open the file: C:\cygwin\home\fdmanana\tmp>touch foo.bar C:\cygwin\home\fdmanana\tmp>threads_fopen_test.exe Main thread opened file successfully File open error from second thread: The process cannot access the file because it is being used by another process. Changing the program's SHARE_FLAGS to FILE_SHARE_READ, shows that both threads are able to open the file: C:\cygwin\home\fdmanana\tmp>touch foo.bar C:\cygwin\home\fdmanana\tmp>threads_test.exe Main thread opened file successfully Second thread opened file successfully Same logic applies to opening files for writing or deleting and renaming files while they're open by some other thread that didn't specify the flags FILE_SHARE_WRITE and FILE_SHARE_DELETE.
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