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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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Vreixo Formoso
15 years ago