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README revision 1.3 
      1 $NetBSD: README,v 1.3 2004/12/20 20:51:42 jmc Exp $
      2 
      3 makefs - build a file system image from a directory tree
      4 
      5 NOTES:
      6 
      7     *   This tool uses modified local copies of source found in other
      8 	parts of the tree.  This is intentional.
      9 
     10     *	makefs is a work in progress, and subject to change.
     11 
     12 
     13 user overview:
     14 --------------
     15 
     16 makefs creates a file system image from a given directory tree.
     17 the following file system types can be built:
     18 	ffs	BSD fast file system
     19 
     20 Support for the following file systems maybe be added in the future
     21 	ext2fs	Linux EXT2 file system
     22 	fat	MS-DOS `FAT' file system (FAT12, FAT16, FAT32)
     23 	cd9660	ISO 9660 file system
     24 
     25 Various file system independent parameters and contraints can be
     26 specified, such as:
     27 	- minimum file system size (in KB)
     28 	- maximum file system size (in KB)
     29 	- free inodes
     30 	- free blocks (in KB)
     31 	- mtree(8) specification file containing permissions and ownership
     32 	  to use in image, overridding the settings in the directory tree
     33 	- file containing list of files to specifically exclude or include
     34 	- fnmatch(3) pattern of filenames to exclude or include
     35 	- endianness of target file system
     36 
     37 File system specific parameters can be given as well, with a command
     38 line option such as "-o fsspeccific-options,comma-separated".
     39 For example, ffs would allow tuning of:
     40 	- block & fragment size
     41 	- cylinder groups
     42 	- number of blocks per inode
     43 	- minimum free space
     44 
     45 Other file systems might have controls on how to "munge" file names to
     46 fit within the constraints of the target file system.
     47 
     48 Exit codes:
     49 	0	all ok
     50 	1	fatal error
     51 	2	some files couldn't be added during image creation
     52 		(bad perms, missing file, etc). image will continue
     53 		to be made
     54 
     55 
     56 Implementation overview:
     57 ------------------------
     58 
     59 The implementation must allow for easy addition of extra file systems
     60 with minimal changes to the file system independent sections.
     61 
     62 The main program will:
     63 	- parse the options, including calling fs-specific routines to
     64 	  validate fs-specific options
     65 	- walk the tree, building up a data structure which represents
     66 	  the tree to stuff into the image. The structure will
     67 	  probably be a similar tree to what mtree(8) uses internally;
     68 	  a linked list of entries per directory with a child pointer
     69 	  to children of directories. ".." won't be stored in the list;
     70 	  the fs-specific tree walker should add this if required by the fs. 
     71 	  this builder have the smarts to handle hard links correctly.
     72 	- (optionally) Change the permissions in the tree according to
     73 	  the mtree(8) specfile
     74 	- Call an fs-specific routine to build the image based on the
     75 	  data structures.
     76 
     77 Each fs-specific module should have the following external interfaces:
     78 
     79     prepare_options	optional file system specific defaults that need to be
     80 			setup before parsing fs-specific options.
     81 
     82     parse_options	parse the string for fs-specific options, feeding
     83 			errors back to the user as appropriate
     84 
     85     cleanup_options	optional file system specific data that need to be
     86 			cleaned up when done with this filesystem.
     87 
     88     make_fs		take the data structures representing the
     89 			directory tree and fs parameters,
     90 			validate that the parameters are valid
     91 			(e.g, the requested image will be large enough), 
     92 			create the image, and
     93 			populate the image
     94 
     95 prepare_options and cleanup_options are optional and can be NULL.
     96 
     97 NOTE: All file system specific options are referenced via the fs_specific
     98 pointer from the fsinfo_t strucutre. It is up to the filesystem to allocate
     99 and free any data needed for this via the prepare and cleanup callbacks.
    100 
    101 Each fs-specific module will need to add it's routines to the dispatch array
    102 in makefs.c and add prototypes for these to makefs.h
    103 
    104 All other implementation details should not need to change any of the
    105 generic code.
    106 
    107 ffs implementation
    108 ------------------
    109 
    110 In the ffs case, we can leverage off sbin/newfs/mkfs.c to actually build
    111 the image. When building and populating the image, the implementation
    112 can be greatly simplified if some assumptions are made:
    113 	- the total required size (in blocks and inodes) is determined
    114 	  as part of the validation phase
    115 	- a "file" (including a directory) has a known size, so
    116 	  support for growing a file is not necessary
    117 
    118 Two underlying primitives are provided:
    119 	make_inode	create an inode, returning the inode number
    120 
    121 	write_file	write file (from memory if DIR, file descriptor
    122 			if FILE or SYMLINK), referencing given inode.
    123 			it is smart enough to know if a short symlink
    124 			can be stuffed into the inode, etc.
    125 
    126 When creating a directory, the directory entries in the previously
    127 built tree data structure is scanned and built in memory so it can
    128 be written entirely as a single write_file() operation.
    129