Write multisession/growing/modification info to the Tutorial.
Vreixo Formoso
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Vreixo Formoso <metalpain2002@yahoo.es>,
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Mario Danic <mario.danic@gmail.com>,
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Thomas Schmitt <scdbackup@gmx.net>
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Copyright (C) 2007 Vreixo Formoso, Mario Danic, Thomas Schmitt
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Copyright (C) 2007-2008 Vreixo Formoso, Mario Danic, Thomas Schmitt
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This program is free software; you can redistribute it and/or modify
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170
doc/Tutorial
170
doc/Tutorial
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@ -20,7 +20,10 @@ Contents:
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2.1 Image tree manipulation
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2.2 Set the write options
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2.3 Obtaining a burn_source
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3. Image growing and multisession
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3. Image growing and modification
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3.1 Growing vs Modification
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3.2 Image import
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3.3 Generating a new image
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4. Bootable images
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5. Advanced features
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@ -337,3 +340,168 @@ You can get the state of the buffer in any moment, with the function:
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You can also cancel the writer thread at any time, with the cancel() function
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of the burn_source.
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-------------------------------------------------------------------------------
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3. Image growing and modification
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-------------------------------------------------------------------------------
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-------------------------------------------------------------------------------
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3.1 Growing vs Modification
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Libisofs is not restricted only to create new images. It can also be used to
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modify existing images. It supports two kind of image modifications, that we
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have called image growing and image modification:
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Image modification consists in generating a new image, based on the contents
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of an existing image. In this mode, libisofs takes an image, the users modifies
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its contents (adding new files, removing files, changing their names...), and
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finally libisofs generates a completely new image.
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On the other side, image growing is similar, with the difference that the new
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image is dependent on the other, i.e., it refers to files of the other image.
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Thus, it can't be mounted without the old image. The purpose of this kind of
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images is to increment or add files to a multisession disc. The new image only
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contains the new files. Old files are just references to the old image blocks.
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The advantage of the growing approach is that the generated image is smaller,
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as only the new files are written. This mode is suitable when you just want to
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add some files to a very big image, or when dealing with write-once media, such
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as CD-R. Both the time and space needed for the modification is much less than
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with normal image modify.
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The main problem of growing is that the new image needs to be recorded together
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with the old image, in order to be mountable. The total size of the image
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(old + new) is bigger (even much bigger) than a completely new image. So, if
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you plan to distribute an image on Internet, or burn it to a disc, generate a
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completely new image is usually a better alternative.
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To be able to mount a grown image, the OS needs to now you have appended new
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data to the original image. In multisession media (such as CD-R), the new data
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is appended as a new session, so the OS can identify this and mount the image
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propertly. However, when dealing with non-multisession media (such as DVD+RW)
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or plain .iso files, the new data is just appended at the end of the old image,
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and the OS has no way to know that the appended data is in fact a "new
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session". The method introduced by Andy Polyakov in growisofs can be used in
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those cases. It consists in overwrite the volume descriptors of the old image
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with a new ones that refer to the newly appended contents.
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-------------------------------------------------------------------------------
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3.2 Image import
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The first thing you need to do in order to modify or grow an image is to import
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it, with the function:
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iso_image_import()
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It takes several arguments.
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First, the image context, an IsoImage previously obtained with iso_image_new().
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In most cases you will want to use an empty image. However, if you have already
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added files to the image, they will be removed and replaced with the contents
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of the image being imported.
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The second parameter is an IsoDataSource instance. It abstracts the original
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image, and it is used by libisofs to access its contents. You are free to
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implement your own data source to access image contents. However, libisofs has
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a simple implementation suitable for reading images on the local filesystem,
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that can be used for import both .iso files and inserted media, via the block
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device and POSIX functions. You can get it with
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iso_data_source_new_from_file()
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The third parameter of iso_image_import() is a pointer to an IsoReadOpts
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struct. It holds the options for image reading. You get it with:
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iso_read_opts_new()
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and after calling iso_image_import() you should free it with
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iso_read_opts_free()
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Some options are related to select what extensions to read. Default options
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are suitable for most users.
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iso_read_opts_set_no_rockridge()
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iso_read_opts_set_no_joliet()
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iso_read_opts_set_no_iso1999()
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iso_read_opts_set_preferjoliet()
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If RockRidge extensions are not present, many files attributes can't be
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obtained. In those cases libisofs uses default values. You have options to
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configure what default values to use.
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iso_read_opts_set_default_uid()
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iso_read_opts_set_default_gid()
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iso_read_opts_set_default_permissions()
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If the original image has been created in another system with a different
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charset, you may want to use:
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iso_read_opts_set_input_charset()
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to specify the encoding of the file names on image.
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Finally, to import multisession images, you should tell libisofs that it must
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read the last session. For that, you must set the block where the last session
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starts:
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iso_read_opts_set_start_block()
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The last parameter for iso_image_import(), optional, is a pointer that will
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be filled with a library-allocated IsoReadImageFeatures, that lets you access
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some information about the image: size, extensions used,...
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[TODO: explain that iso_image_import uses dir rec options]
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-------------------------------------------------------------------------------
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3.3 Generating a new image
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After importing the image, the old image tree gets loaded. You are free to
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make any changes to it: add new files, remove files, change names or
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attributes... Refer to "2.1 Image tree manipulation" for details.
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When it is ready, you can now create the new image. The process is the same as
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explained in "2.2 Set the write options" and "2.3 Obtaining a burn_source".
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However, there are some write options that should be taken into account.
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First of all, you must select whether you want to grow or modify the image
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(read "3.1 Growing vs Modification" for details). You must call
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iso_write_opts_set_appendable()
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An appendable image leads to image growing, and a non-appendable image leads
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to a completelly new image (modification). An appendable image will be appended
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after the old image (in a new session, for example). Thus, in those cases, the
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first block of the image is not 0. You should set the correct lba of the first
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block with:
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iso_write_opts_set_ms_block()
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That is usually the "Next Writable Address" on a multisession media, and a
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value slightly greater than the old image size on .iso files or DVD+RW media.
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You can obtain the old image size with the iso_read_image_features_get_size()
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function.
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In this last case (i.e., on a non multisession media), you will need to
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overwrite the volume descriptors of the old image with the new ones. To do
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this you need:
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- Allocate a buffer of at least 64 KiBs.
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- Initialize it with the first 64 KiBs of the original image.
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- Pass the buffer to libisofs with the iso_write_opts_set_overwrite_buf()
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option.
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- After appending the new image, you have to overwrite the first 64 KiBs of
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the original image with the new content of the buffer.
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-------------------------------------------------------------------------------
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4. Bootable images
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-------------------------------------------------------------------------------
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-------------------------------------------------------------------------------
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5. Advanced features
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-------------------------------------------------------------------------------
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