Reading.md

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+Reading of optical media is for now restricted to reading of data sectors,
+which is done by call `burn_read_data()` of [libburn](Libburn),
+reading of audio CD sectors by call `burn_read_audio()`,
+and reading of CD-TEXT from audio CD by `burn_disc_get_leadin_text()`.
+
+This is fully sufficient for DVD media. On CD, a sector not only may host
+2048 bytes of payload but stores more information which can be accessed
+by special SCSI commands. An example are Audio sectors of CD-DA discs which
+store payload in all their 2352 bytes. Other sector formats are known withi
+payload between that maximum and 2048.
+
+Data reading has several use cases in libburnia:
+
+Demo program
+[telltoc](https://dev.lovelyhq.com/libburnia/libburn/raw/HEAD/test/telltoc.c)
+is able to read CD TAO tracks up to the last data sectors which are normally
+inaccessible through the Linux block devices. This is due to the fact
+that a TAO track announces a size which includes two non-data sectors
+at the end of the track. Some drives correct this, some do not. Whether a track
+was recorded by TAO is hardly to tell. In a SAO track, all announced sectors
+are data sectors. telltoc solves this dilemma by reading the end sectors
+of a track one-by-one.
+
+Application program [cdrskin](Cdrskin) emulates ISO multi-session on DVD+RW
+by reading the first 64 kB of the ISO 9660 image from media, patching some
+address fields and writing them back.
+(Learned from Andy Polyakov's `growisofs`.)
+Further, `cdrskin` is able to copy audio tracks and CD-TEXT from CD to
+hard disk.
+
+Library [libisoburn](Libisoburn) creates an interface object for library
+[libisofs](Libisofs) which can read data blocks from libburn drives.
+This is used for detecting an ISO 9660 image and for loading its
+directory tree and management data. `libisoburn` itself performs the same
+patching of DVD+RW and similar media as can be done by `cdrskin`.
+
+Application program [xorriso](Xorriso) uses data reading capabilities for
+comparing the content of disk files with the content of ISO image files
+and for copying files from ISO image to disk.
+
+On several Linux systems, the block device driver needs reloading of thei
+drive tray in order to make available freshly written data via its
+buffered i/o. The libburn read function does not need such a reload.
+