Accessing a single file with multiple threads

Yes, it’s possible for a program to open the same file multiple times from different threads. You’ll want to avoid reading from the file at the same time you’re writing to it, though. You can use TMultiReadExclusiveWriteSynchronizer to control access to the entire file. It’s less serialized than, say, a critical section. For more granular control, take a look at LockFileEx to control access to specific regions of the file as you need them. When writing, request an exclusive lock; when reading, a shared lock.

As for the code you posted, specifying File_Share_Write in the initial sharing flags means that all subsequent open operations must also share the file for writing. Quoting from the documentation:

If this flag is not specified, but the file or device has been opened for write access or has a file mapping with write access, the function fails.

Your second open request was saying that it did not want anybody else to be allowed to write to the file while that handle remained open. Since there was already another handle open that did allow writing, the second request could not be fulfilled. GetLastError should have returned 32, which is Error_Sharing_Violation, exactly what the documentation says should happen.

Specifying File_Flag_Delete_On_Close means all subsequent open requests need to share the file for deletion. The documentation again:

Subsequent open requests for the file fail, unless the FILE_SHARE_DELETE share mode is specified.

Then, since the second open request shares the file for deletion, all other open handles must have also shared it for deletion. The documentation:

If there are existing open handles to a file, the call fails unless they were all opened with the FILE_SHARE_DELETE share mode.

The bottom line is that either everybody shares alike or nobody shares at all.

FFileSystem := CreateFile(PChar(FFileName),
  Generic_Read or Generic_Write
  File_Share_Read or File_Share_Write or File_Share_Delete,
  nil,
  Create_Always,
  File_Attribute_Normal or File_Flag_Random_Access
    or File_Attribute_Temporary or File_Flag_Delete_On_Close,
  0);

FFileSystem2 := CreateFile(PChar(FFileName),
  Generic_Read,
  File_Share_Read or File_Share_Write or File_Share_Delete,
  nil,
  Open_Existing,
  File_Attribute_Normal or File_Flag_Random_Access
    or File_Attribute_Temporary or File_Flag_Delete_On_Close,
  0);

In other words, all the parameters are the same except for the fifth one.

These rules apply to two attempts to open on the same thread as well as attempts from different threads.