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File System Support for Contexts

  • Article

To support file contexts (if applicable), stream contexts, and file object (stream handle) contexts, a file system must use the FSRTL_ADVANCED_FCB_HEADER structure. All Microsoft Windows file systems use this structure, and all third-party file system developers are strongly encouraged to do so as well. For more information, see FsRtlSetupAdvancedHeader and FSRTL_ADVANCED_FCB_HEADER.

The NTFS and FAT file systems do not support file, stream, or file object contexts on paging files, in the pre-create or post-close path, or for IRP_MJ_NETWORK_QUERY_OPEN operations.

A minifilter driver can determine whether a file system supports stream contexts and file object contexts for a given file object by calling FltSupportsStreamContexts and FltSupportsStreamHandleContexts, respectively.

File contexts are available on Windows Vista and later.

For file systems (such as FAT) that support only a single data stream per file, file contexts are equivalent to stream contexts. Such file systems usually support stream contexts but do not support file contexts. Instead, the filter manager provides this support, using the file system's existing support for stream contexts. For minifilter driver instances attached to these file systems, FltSupportsFileContexts returns FALSE, while FltSupportsFileContextsEx returns TRUE (when a valid non-NULL value is passed for the Instance parameter).

If a context type is not supported on a file, the minifilter cannot attach a context of that type to that file.

To support file contexts, a file system must:

  • Embed a FileContextSupportPointer member of type PVOID in its file context structure, usually the file context block (FCB). The file system must initialize this member to NULL.

  • Use FsRtlSetupAdvancedHeaderEx (instead of FsRtlSetupAdvancedHeader) to initialize its stream context structure, passing a valid pointer to the FileContextSupportPointer member (embedded in the corresponding file context structure) for the FileContextSupportPointer parameter. For more information, see FsRtlSetupAdvancedHeaderEx and FSRTL_ADVANCED_FCB_HEADER.

  • Call FsRtlTeardownPerFileContexts to free all file context structures that filter and minifilter drivers have associated with a file when the file system deletes its file context structure for the file.