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3.2.4.5.1 Compatibility Mode

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Compatibility Mode (also referred to as "MS-DOS Compatibility Mode") provides the client with exclusive access to an opened file.

  • A file opened in compatibility mode can be opened (also in compatibility mode) any number of times for any combination of reading and writing (subject to the user's permissions) by any UID and PID on the same SMB connection.

  • If one client has the file open for writing in compatibility mode, the file MUST NOT be opened in any way by any other client.

  • As an exception, if the filename has an extension of .EXE, .DLL, .SYM, or .COM (is executable), other clients are permitted to open the file for reading regardless of read/write open modes of other compatibility mode opens. The SMB_FLAGS2_READ_IF_EXECUTE bit (also known as the SMB_FLAGS2_PAGING_IO bit) MUST be set in the open request.

  • If the first client has the file open only for reading in compatibility mode, other clients can open the file for reading in compatibility mode.

  • Once one or more clients have the file open for reading in compatibility mode, other clients MUST NOT open the file in any mode other than compatibility mode.

  • If any client has the file open for reading in compatibility mode, then other clients MUST NOT open the file for writing.

Because Compatibility Mode provides the client with exclusive access, it is incompatible with other open modes that provide shared access to the file. If the file is opened with sharing enabled, a subsequent Compatibility Mode open from the same client or any other client MUST return STATUS_SHARING_VIOLATION (ERRDOS/ERRbadshare).

The other file exclusion modes (Deny read/write, Deny write, Deny read, Deny nothing) provide exclusion at the file level. A file opened in any "Deny" mode MAY only be opened again for the accesses allowed by the Deny mode (subject to the user's permissions).