File Interface and Practical Implementation Issues

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File Interface and Practical Implementation Issues
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File Interface

• Modeled on the UNIX interface
• uuid(3)
• interface file
• exception io_error
• exception invalid_attr
• exception invalid_arguments
• exception eof
• exception not_supported
• exception invalid_handle
• exception no_mem
• exception generic_error

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Open and Close

• // type and constants for file opening
• typedef long fopen_attr_t
• const fopen_attr_t OPEN_READ 1
• const fopen_attr_t OPEN_WRITE 2
• const fopen_attr_t OPEN_APPEND 4
• void open(in handle_t handle, in fopen_attr_t
  attr, out file_id_t filehandle)
• raises (invalid_handle, no_mem, invalid_attr,
  io_error)
• void close(in file_id_t filehandle)
• raises(invalid_handle, io_error)

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Seek and Tell

• // type and constants for seeking
• typedef long seek_mode_t
• const seek_mode_t SEEK_BEGIN 1
• const seek_mode_t SEEK_END 2
• const seek_mode_t SEEK_REL 3
• const long seek_pos_end -1
• void seek(in file_id_t filehandle, in long
  to_pos,
• in seek_mode_t mode)
• raises(invalid_handle, eof, io_error,
  invalid_arguments)
• void tell(in file_id_t filehandle, out long
  cur_pos)
• raises(invalid_handle)

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File Attributes

• // type and constants for file attributes
• typedef long attr_type_t
• const attr_type_t attr_size 1
• const attr_type_t attr_time 2
• void get_attr(in file_id_t filehandle, in
  attr_type_t which,
• out buffer_t buffer)
• raises(invalid_handle, io_error,
  invalid_arguments, not_supported)
• void set_attr(in file_id_t filehandle, in
  attr_type_t which,
• in buffer_t buffer)
• raises(invalid_handle, io_error,
  invalid_arguments, not_supported)

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Object Attributes

• Should the interface to objects be generalized to
  have attributes?
• We currently can get the type only!!
• Can then use more specific interface get object
  specific attributes.

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Read Write

• void read(in file_id_t filehandle, in long
  length,
• prealloc out buffer_t buffer)
• raises(invalid_handle, eof, io_error,
  invalid_arguments)
• void write(in file_id_t filehandle,
• in buffer_t buffer, out long result)
• raises(invalid_handle, io_error,
  invalid_arguments)

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Buffer_t ???

• In IDL
• typedef sequenceltchargt buffer_t
• In C
• typedef struct
• CORBA_unsigned_long _maximum
• CORBA_unsigned_long _length
• CORBA_char _buffer
• buffer_t

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Background to prealloc

• Normally for out sequence parameters CORBA
  allocates storage for the parameter in the stub
• Stub calls CORBA_alloc()
• After receiving the, user must call CORBA_free()
• Source of a memory leak for the unwary
• Make is difficult to implement a read with UNIX
  semantics
• Cant directly specify the receive buffer

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prealloc

• Specifies that the client should allocate the
  buffer explicitly and pass it to the stub
• Allows specification of a particular buffer
• Can implement read semantics
• Can explicitly control buffer usage

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For a prealloc buffer_t

• typedef struct
• // maximum size of buffer to receive in
• CORBA_unsigned_long _maximum
• // actual length of data received
• CORBA_unsigned_long _length
• // pointer to start of the buffer
• CORBA_char _buffer
• buffer_t

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Misc

• void truncate(in file_id_t filehandle)
• raises(invalid_handle, io_error)
• void eof(in file_id_t filehandle, out long end)
• raises(invalid_handle)
• // end 0 not end of file
• // end !0 end of file

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Service Type and Idl

• We need a type to represent services
• Example the logserver
• Given a new type, we need a method to manipulate
  it
• The naming interface can find, identify, create,
  and delete service objects.
• Service interface needed to initialize the
  service object

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Service IDL

• include "types.idl"
• uuid(4)
• interface service
• exception generic_error
• void register(in handle_t service_object, in
  server_t service_thread)

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How to get started

• Modify the Root Task
• Implement parts of the naming interface
• Implement the service interface
• Modify the test task
• Use the interfaces in servers to test them.
• Example test client available
• Implement a Read-only file server task

Test Client
Read-only File Server
Name Server
Simple Pager
Log Server
Sigma0
L4 Micro kernel
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Notes on the build environment

• Run make in the ./sdios directory
• The following are valid targets
• Distclean removes al intermediate files
  including IDL generated header files and
  dependency files
• Clean removes .o files
• Idl build the idl header files and installs them
  in include/services/
• Install builds everything and copies the
  binaries to the boot directory
• You should only need to run IDL4 manually to
  generate server templates.

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Build notes

• To avoid problems
• Delete the old nameservice.idl and run make
  distclean
• Link the root_task and test_client at higher
  addresses
• Edit Makefile in src/root_task etc
• Change LINK_ADDR to something sensible
• Make the stacks bigger in x86-crt0.S
• 64K should be sufficient

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Things to do in the root task

• File name conventions
• / is root of file system
• /services contains services
• /services/log logserver
• /services/name name server
• /tasks task server
• /modules read-only file server

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Restrict address space access

• Modify the pager to do memory bounds checking
• restrict access to paged tasks to a range of
  memory suitable
• Can get this from ELF header etc.

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Modify log_service thread

• Need to use new naming and service interface
• Include services-client and naming-client header
• Mkobject a service log in /services
• Register me as log service thread using service
  interface

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Problem!!

• Name service must implement both naming and
  service interface
• Using multiple inheritance to contruct a root
  interface in server
• Transparent to client
• Client uses previous naming and service
  interfaces as before

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Multiple inheritance

• Construct a root interface
• include "naming.idl"
• include "service.idl"
• interface root name, service
• Generate the root template used to implement the
  root service thread
• New templates are needed if interfaces change
  their inheritance
• Copy it to another file name

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• Rename nameserver.c into .old
• See make file to see why
• Fix the includes
• Server should include root-server.h
• In the template
• Can remove templates related to name_server() and
  service_server(), and the associated vtables
• Implement only root
• Use interface numbers provided

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In template

• Fix the malloc calls
• Replace with
• Int root_server_buffsROOT_STRBUF_SIZEMAX_STR_BU
  FF_SIZE
• for (w0 0w0 lt ROOT_STRBUF_SIZEw0)
• buffer.strw0.rcv_addr
  root_server_buffsw00
• buffer.strw0.rcv_size
  MAX_STR_BUFF_SIZE4
• Fix the includes
• include ltl4/l4.hgt
• include ltdebug.hgt
• include ltidl4.hgt

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This to do in name server

• Create a services directory
• Remember the root dir is defined as object 1
• Have following constant in naming headers
• name_root_dir_handle

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N_pts are awkward to use

• How many parts are in
• ////abc///cde/f///g
• Define
• The first / is part of a name (if present)
• Remaining /s divide the name into parts
• Remain /s not considered parts of the name
  itself
• Example has 5 parts

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To get basic system up

• Without the file server, you need to implement
• Name_resolve
• Name_mkobject (for the service type)
• service_register

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With the file server

• Need to derive a fileserver interface
• Inherit from naming and file
• Need to implement a mount point
• Mount server at /modules
• Need to implement client resolve that handles
  mount points
• Write a library routine

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Tasks and VM to come