FltCreateMailslotFile function (fltkernel.h)
Minifilter drivers call FltCreateMailslotFile to create a new pipe or open an existing mailslot.
Syntax
NTSTATUS FLTAPI FltCreateMailslotFile(
[in] PFLT_FILTER Filter,
[in, optional] PFLT_INSTANCE Instance,
[out] PHANDLE FileHandle,
[out, optional] PFILE_OBJECT *FileObject,
[in] ULONG DesiredAccess,
[in] POBJECT_ATTRIBUTES ObjectAttributes,
[out] PIO_STATUS_BLOCK IoStatusBlock,
[in] ULONG CreateOptions,
[in] ULONG MailslotQuota,
[in] ULONG MaximumMessageSize,
[in] PLARGE_INTEGER ReadTimeout,
[in, optional] PIO_DRIVER_CREATE_CONTEXT DriverContext
);
Parameters
[in] Filter
An opaque filter pointer for the caller.
[in, optional] Instance
An opaque instance pointer for the minifilter driver instance that the create request is to be sent to. The instance must be attached to the volume for the mailslot file system. This parameter is optional and can be NULL. If this parameter is NULL, the request is sent to the device object at the top of the file system driver stack for the volume. If it is non-NULL, the request is sent only to minifilter driver instances that are attached below the specified instance.
[out] FileHandle
A pointer to a caller-allocated variable that receives the file handle if the call to FltCreateMailslotFile is successful.
[out, optional] FileObject
A pointer to a caller-allocated variable that receives the file object pointer if the call to FltCreateMailslotFile is successful. This parameter is optional and can be NULL.
[in] DesiredAccess
A bitmask of flags that specify the type of access that the caller requires to the file or directory. The set of system-defined DesiredAccess flags determines the following specific access rights for file objects.
| DesiredAccess Flag | Meaning |
|---|---|
| FILE_READ_DATA | Data can be read from the named mailslot. |
| FILE_READ_ATTRIBUTES | FileAttributes flags can be read. For additional information, see the table of valid flag values in the FileAttributes parameter of FltCreateFileEx2. |
| READ_CONTROL | The access control list ACL and ownership information associated with the mailslot can be read. |
| FILE_WRITE_DATA | Data can be written to the mailslot. |
| FILE_WRITE_ATTRIBUTES | FileAttributes flags can be written. |
| FILE_APPEND_DATA | Data can be appended to the mailslot. |
| WRITE_DAC | The discretionary access control list DACL associated with the mailslot can be written. |
| WRITE_OWNER | Ownership information associated with the mailslot can be written. |
| ACCESS_SYSTEM_SECURITY | The caller will have write access to the mailslot's SACL. |
| SYNCHRONIZE | The caller can synchronize the completion of an I/O operation by waiting for the returned FileHandle to be set to the Signaled state. This flag must be set if the CreateOptions FILE_SYNCHRONOUS_IO_ALERT or FILE_SYNCHRONOUS_IO_NONALERT flag is set. |
Alternatively, for any file object that does not represent a directory, you can specify one or more of the following generic ACCESS_MASK flags. (The STANDARD_RIGHTS_XXX flags are predefined system values that are used to enforce security on system objects.) You can also combine these generic flags with additional flags from the preceding table.
| DesiredAccess to File Values | Maps to DesiredAccess Flags |
|---|---|
| GENERIC_READ | STANDARD_RIGHTS_READ, FILE_READ_DATA, and SYNCHRONIZE. |
| GENERIC_WRITE | STANDARD_RIGHTS_WRITE, FILE_WRITE_DATA, FILE_APPEND_DATA, and SYNCHRONIZE. |
[in] ObjectAttributes
Pointer to an opaque OBJECT_ATTRIBUTES structure that is already initialized with InitializeObjectAttributes. If the caller is running in the system process context, this parameter can be NULL. Otherwise, the caller must set the OBJ_KERNEL_HANDLE attribute in the call to InitializeObjectAttributes. Members of this structure for a file object are listed in the following table.
| Member | Value |
|---|---|
| ULONG Length | Number of bytes of data that are contained in the structure pointed to by ObjectAttributes. This value must be at least sizeof(OBJECT_ATTRIBUTES). |
| PUNICODE_STRING ObjectName | Pointer to a UNICODE_STRING structure that contains the name of the mailslot to be created or opened. This name must be a fully qualified file specification or the name of a device object unless it is the name of a file relative to the directory specified by RootDirectory. For example, "\Device\Mailslot\myslot" or "??\mailslot\myslot" could both be valid file specifications. (Note: "??" replaces "\DosDevices" as the name of the Win32 object namespace. "\DosDevices" still works, but "??" is translated faster by the object manager. |
| HANDLE RootDirectory | Optional handle to a directory, obtained by a preceding call to FltCreateFileEx2. If this value is NULL, the ObjectName member must be a fully qualified file specification that includes the full path to the target mailslot. If this value is non-NULL, the ObjectName member specifies a mailslot name that is relative to this directory. |
| PSECURITY_DESCRIPTOR SecurityDescriptor | Optional SECURITY_DESCRIPTOR to be applied to a mailslot. ACLs specified by such a security descriptor are only applied to the mailslot when it is created. If the value is NULL when a mailslot is created, the ACL placed on the mailslot is dependent on the mailslot file system and may allow a client with any access to create an instance. |
| ULONG Attributes | A set of flags that controls the file object attributes. If the caller is running in the system process context, this parameter can be zero. Otherwise, the caller must set the OBJ_KERNEL_HANDLE flag. The caller can also optionally set the OBJ_CASE_INSENSITIVE flag, which indicates that name-lookup code should ignore the case of ObjectName rather than performing an exact-match search. |
[out] IoStatusBlock
Pointer to an IO_STATUS_BLOCK structure that receives the final completion status and information about the requested operation. On return from FltCreateMailslotFile, the Information member of the variable contains one of the following values:
- FILE_CREATED
- FILE_OPENED
[in] CreateOptions
The options to be applied when creating or opening the mailslot, as a compatible combination of the following flags.
| Flags | Meaning |
|---|---|
| FILE_WRITE_THROUGH | System services, file systems, and drivers that write data to the mailslot must actually transfer the data into the mailslot before any requested write operation is considered complete. This flag is automatically set if the CreateOptions flag FILE_NO_INTERMEDIATE_BUFFERING is set. |
| FILE_SYNCHRONOUS_IO_ALERT | All operations on the mailslot are performed synchronously. Any wait on behalf of the caller is subject to premature termination from alerts. This flag also causes the I/O system to maintain the mailslot position context. If this flag is set, the DesiredAccess SYNCHRONIZE flag also must be set so that the I/O Manager uses the file object as a synchronization object. |
| FILE_SYNCHRONOUS_IO_NONALERT | All operations on the mailslot are performed synchronously. Waits in the system to synchronize I/O queuing and completion are not subject to alerts. This flag also causes the I/O system to maintain the file position context. If this flag is set, the DesiredAccess SYNCHRONIZE flag also must be set so that the I/O Manager uses the file object as a synchronization object. |
[in] MailslotQuota
The size, in bytes, of the buffer for writes to the mailslot.
[in] MaximumMessageSize
The maximum size, in bytes, of a message to write to the mailslot. A message of any size is specified by the value 0.
[in] ReadTimeout
The time a read operation waits for a message to be available in the mailslot. The default timeout is expressed in 100-nanosecond increments as a negative integer. For example, 250 milliseconds is specified as –10*1000*250. Additionally, the following values have special meanings.
| Value | Meaning |
|---|---|
| 0 | Returns immediately if no message is present. |
| -1 | Waits forever for a message. |
[in, optional] DriverContext
Optional pointer to an IO_DRIVER_CREATE_CONTEXT structure already initialized by IoInitializeDriverCreateContext.
Return value
FltCreateMailslotFile returns STATUS_SUCCESS or an appropriate NTSTATUS value such as one of the following.
| Return code | Description |
|---|---|
| STATUS_FLT_DELETING_OBJECT | The filter or instance specified in the Filter or Instance parameters is being torn down. This status code can be received if the open request crosses a volume mount point and the Instance parameter is non-NULL. This is an error code. |
| STATUS_OBJECT_PATH_SYNTAX_BAD | The ObjectAttributes parameter did not contain a RootDirectory member, but the ObjectName member in the OBJECT_ATTRIBUTES structure was an empty string or did not contain an OBJECT_NAME_PATH_SEPARATOR character. This error code indicates incorrect syntax for the object path. |
Remarks
The FltCreateMailslotFile function allows minifilter drivers to create or open mailslot instances. This is useful for creating virtual mailslots or for creating a mailslot group that distributes to several other mailslots.
The Instance parameter is either NULL or is previously set by attaching to the mailslot volume. A volume pointer is obtained by passing "\Device\Mailslot" as the volume name to FltGetVolumeFromName.
To specify an extra create parameter (ECP) as part of a create operation, initialize the ExtraCreateParameter member of the IO_DRIVER_CREATE_CONTEXT structure with the FltAllocateExtraCreateParameterList routine. If ECPs are used, they must be allocated, initialized, and freed using their associated support routines. Upon returning from the call of FltCreateMailslotFile, the ECP list is unchanged and may be passed to additional calls of FltCreateMailslotFile for other create operations. The ECP list structure is not automatically deallocated. The caller of FltCreateMailslotFile must deallocate this structure by calling the FltFreeExtraCreateParameterList routine.
If Instance is not NULL, the create request from FltCreateMailslotFile is sent only to the instances attached below the specified minifilter driver instance and to the mailslot file system. The specified instance and the instances attached above it do not receive the create request. If no instance is specified, the request goes to the top of the stack and is received by all instances and the mailslot file system.
Requirements
| Requirement | Value |
|---|---|
| Minimum supported client | Available in Windows 8. |
| Target Platform | Universal |
| Header | fltkernel.h (include FltKernel.h) |
| Library | Fltmgr.lib |
| IRQL | PASSIVE_LEVEL |
See also
FltAllocateExtraCreateParameterList