tar.tex

unix 下tar 执行程序的源代码

        char    linkflag;
        char    linkname[NAMSIZ];
        char    magic[8];
        char    uname[TUNMLEN];
        char    gname[TGNMLEN];
        char    devmajor[8];
        char    devminor[8];
	/* these following fields were added by JF for gnu */
	/* and are NOT standard */
	char	atime[12];
	char	ctime[12];
	char	offset[12];
	char	longnames[4];
	/* the next three fields were added by JK to deal with
 	   shrinking down sparse files */
	struct	sparse sp[SPARSE_IN_HDR];
	char	isextended;
	char	ending_blanks[12];	/* number of nulls at the
					   end of the file, if any */
    @} header;

    struct extended_header @{
	struct sparse sp[21];
	char isextended;
    @} ext_hdr;

@};

/* The checksum field is filled with this while the checksum is computed. */
#define    CHKBLANKS    "        "        /* 8 blanks, no null */

/* The magic field is filled with this if uname and gname are valid. */
#define    TMAGIC    "ustar  "        /* 7 chars and a null */

/* The magic field is filled with this if this is a GNU format dump entry */
#define    GNUMAGIC  "GNUtar "        /* 7 chars and a null */

/* The linkflag defines the type of file */
#define  LF_OLDNORMAL '\0'       /* Normal disk file, Unix compatible */
#define  LF_NORMAL    '0'        /* Normal disk file */
#define  LF_LINK      '1'        /* Link to previously dumped file */
#define  LF_SYMLINK   '2'        /* Symbolic link */
#define  LF_CHR       '3'        /* Character special file */
#define  LF_BLK       '4'        /* Block special file */
#define  LF_DIR       '5'        /* Directory */
#define  LF_FIFO      '6'        /* FIFO special file */
#define  LF_CONTIG    '7'        /* Contiguous file */

/* Further link types which were defined later. */
#define LF_DUMPDIR	'D'		/* This is a dir entry that contains
					   the names of files that were in
					   the dir at the time the dump
					   was made */
#define LF_MULTIVOL	'M'		/* This is the continuation
					   of a file that began on another
					   volume */
#define LF_SPARSE	'S'		/* This is for sparse files */
#define LF_VOLHDR	'V'		/* This file is a tape/volume header */
					/* Ignore it on extraction */

/* Bits used in the mode field - values in octal */
#define  TSUID    04000        /* Set UID on execution */
#define  TSGID    02000        /* Set GID on execution */
#define  TSVTX    01000        /* Save text (sticky bit) */

/* File permissions */
#define  TUREAD   00400        /* read by owner */
#define  TUWRITE  00200        /* write by owner */
#define  TUEXEC   00100        /* execute/search by owner */
#define  TGREAD   00040        /* read by group */
#define  TGWRITE  00020        /* write by group */
#define  TGEXEC   00010        /* execute/search by group */
#define  TOREAD   00004        /* read by other */
#define  TOWRITE  00002        /* write by other */
#define  TOEXEC   00001        /* execute/search by other */
@end example

All characters in header records are represented by using 8-bit
characters in the local variant of ASCII.  Each field within the
structure is contiguous; that is, there is no padding used within
the structure.  Each character on the archive medium is stored
contiguously.

Bytes representing the contents of files (after the header record of
each file) are not translated in any way and are not constrained to
represent characters in any character set.  The @code{tar} format
does not distinguish text files from binary files, and no
translation of file contents is performed.

The @code{name}, @code{linkname}, @code{magic}, @code{uname}, and
@code{gname} are null-terminated character strings.  All other
fileds are zero-filled octal numbers in ASCII.  Each numeric field
of width @var{w} contains @var{w} minus 2 digits, a space, and a null,
except @code{size}, and @code{mtime}, which do not contain the
trailing null.

The @code{name} field is the pathname of the file, with directory
names (if any) preceding the file name, separated by slashes.

The @code{mode} field provides nine bits specifying file permissions
and three bits to specify the Set UID, Set GID, and Save Text
(``stick'') modes.  Values for these bits are defined above.  When
special permissions are required to create a file with a given mode,
and the user restoring files from the archive does not hold such
permissions, the mode bit(s) specifying those special permissions
are ignored.  Modes which are not supported by the operating system
restoring files from the archive will be ignored.  Unsupported modes
should be faked up when creating or updating an archive; e.g. the
group permission could be copied from the @code{other} permission.

The @code{uid} and @code{gid} fields are the numeric user and group
ID of the file owners, respectively.  If the operating system does
not support numeric user or group IDs, these fields should be
ignored.

The @code{size} field is the size of the file in bytes; linked files
are archived with this field specified as zero.
@xref{Extraction Options}; in particular the @samp{-G} option.@refill

The @code{mtime} field is the modification time of the file at the
time it was archived.  It is the ASCII representation of the octal
value of the last time the file was modified, represented as an
integer number of seconds since January 1, 1970, 00:00 Coordinated
Universal Time.

The @code{chksum} field is the ASCII representation of the octal
value of the simple sum of all bytes in the header record.  Each
8-bit byte in the header is added to an unsigned integer,
initialized to zero, the precision of which shall be no less than
seventeen bits.  When calculating the checksum, the @code{chksum}
field is treated as if it were all blanks.

The @code{typeflag} field specifies the type of file archived.  If a
particular implementation does not recognize or permit the specified
type, the file will be extracted as if it were a regular file.  As
this action occurs, @code{tar} issues a warning to the standard
error.

The @code{atime} and @code{ctime} fields are used in making incremental
backups; they store, respectively, the particular file's access time and
last inode-change time.

The @code{offset} is used by the -M option, when making a multi-volume
archive.  The offset is number of bytes into the file that we need to
restart at to continue the file on the next tape, i.e., where we store
the location that a continued file is continued at.

The @code{longnames} field of the header belongs with something that is
not yet implemented in tar, and is therefore empty.

The following fields were added to deal with sparse files.  A file is
@dfn{sparse} if it takes in unallocated blocks which end up being
represented as zeros, i.e., no useful data.  A test to see if a file is
sparse is to look at the number blocks allocated for it versus the number of
characters in the file; if there are fewer blocks allocated for the file
than would normally be allocated for a file of that size, then the file is
sparse.  This is the method tar uses to detect a sparse file, and once such
a file is detected, it is treated differently from non-sparse files.

Sparse files are often dbm files, or other database-type files which have
data at some points and emptiness in the greater part of the file.  Such
files can appear to be very large when an @code{ls -l} is done on them,
when in truth, there may be a very small amount of important data
contained in the file.  It is thus undesirable to have tar think that it
must back up this entire file, as great quantities of room are wasted on
empty blocks, which can lead to running out of room on a tape far
earlier than is necessary.  Thus, sparse files are dealt with so that
these empty blocks are not written to the tape.  Instead, what is
written to the tape is a description, of sorts, of the sparse file: where
the holes are, how big the holes are, and how much data is found at the
end of the hole.  This way, the file takes up potentially far less room
on the tape, and when the file is extracted later on, it will look
exactly the way it looked beforehand.  The following is a description of
the fields used to handle a sparse file:

The @code{sp} is an array of @code{struct sparse}.  Each @code{struct
sparse} contains two 12-character strings which represent an offset into
the file and a number of bytes to be written at that offset.  The offset
is absolute, and not relative to the offset in preceding array element.

The header can hold four of these @code{struct sparse} at the moment; if
more are needed, they are not stored in the header.

The @code{isextended} flag is set when an @code{extended_header} is
needed to deal with a file.  Note that this means that this flag can only
be set when dealing with a sparse file, and it is only set in the event
that the description of the file will not fit in the alloted room for
sparse structures in the header.  In other words, an extended_header is
needed.

The @code{extended_header} structure is used for sparse files which need
more sparse structures than can fit in the header.  The header can fit
4 such structures; if more are needed, the flag @code{isextended} gets set
and the next record is an @code{extended_header}.

Each @code{extended_header} structure contains an array of 21 sparse
structures, along with a similar @code{isextended} flag that the header
had.  There can be an indeterminate number of such @code{extended_header}s
to describe a sparse file.

@table @code
@item LF_NORMAL
@itemx LF_OLDNORMAL
These flags represent a regular file.  In order to be compatible with
older versions of @code{tar}, a @code{typeflag} value of
@code{LF_OLDNORMAL} should be silently recognized as a regular
file.  New archives should be created using @code{LF_NORMAL}.  Also,
for backward compatibility, @code{tar} treats a regular file whose
name ends with a slash as a directory.

@item LF_LINK
This flag represents a file linked to another file, of any type,
previously archived.  Such files are identified in Unix by each file
having the same device and inode number.  The linked-to
name is specified in the @code{linkname} field with a trailing null.

@item LF_SYMLINK
This represents a symbolic link to another file.  The linked-to
name is specified in the @code{linkname} field with a trailing null.

@item LF_CHR
@itemx LF_BLK
These represent character special files and block special files
respectively.  In this case the @code{devmajor} and @code{devminor}
fields will contain the major and minor device numbers
respectively.  Operating systems may map the device specifications
to their own local specification, or may ignore the entry.

@item LF_DIR
This flag specifies a directory or sub-directory.  The directory name in
the @code{name} field should end with a slash.  On systems where
disk allocation is performed on a directory basis, the @code{size}
field will contain the maximum number of bytes (which may be rounded
to the nearest disk block allocation unit) which the directory may
hold.  A @code{size} field of zero indicates no such limiting.
Systems which do not support limiting in this manner should ignore
the @code{size} field.

@item LF_FIFO
This specifies a FIFO special file.  Note that the archiving of a
FIFO file archives the existence of this file and not its contents.

@item LF_CONTIG
This specifies a contiguous file, which is the same as a normal
file except that, in operating systems which support it,
all its space is allocated contiguously on the disk.  Operating
systems which do not allow contiguous allocation should silently treat
this type as a normal file.

@item 'A' @dots{}
@itemx 'Z'
These are reserved for custom implementations.  Some of these are
used in the GNU modified format, as described below.
@end table

Other values are reserved for specification in future revisions of
the P1003 standard, and should not be used by any @code{tar} program.

The @code{magic} field indicates that this archive was output in the
P1003 archive format.  If this field contains @code{TMAGIC}, the
@code{uname} and @code{gname} fields will contain the ASCII
representation of the owner and group of the file respectively.  If
found, the user and group ID represented by these names will be used
rather than the values within the @code{uid} and @code{gid} fields.

@section GNU Extensions to the Archive Format
The GNU format uses additional file types to describe new types of
files in an archive.  These are listed below.

@table @code
@item LF_DUMPDIR
@itemx 'D'
This represents a directory and a list of files created by the
@samp{-G} option.  The @code{size} field gives the total size of the
associated list of files.  Each filename is preceded by either a @code{'Y'}
(the file should be in this archive) or an @code{'N'} (The file is a
directory, or is not stored in the archive).  Each filename is
terminated by a null.  There is an additional null after the last
filename.

@item LF_MULTIVOL
@itemx 'M'
This represents a file continued from another volume of a
multi-volume archive created with the @samp{-M} option.  The original
type of the file is not given here.  The @code{size} field gives the
maximum size of this piece of the file (assuming the volume does not
end before the file is written out).  The @code{offset} field gives
the offset from the beginning of the file where this part of the
file begins.  Thus @code{size} plus @code{offset} should equal the
original size of the file.

@item LF_SPARSE
@itemx 'S' 
This flag indicates that we are dealing with a sparse file.  Note that
archiving a sparse file requires special operations to find holes in the
file, which mark the positions of these holes, along with the number of
bytes of data to be found after the hole.

@item LF_VOLHDR
@itemx 'V'
This file type is used to mark the volume header that was given with
the @samp{-V} option when the archive was created.  The @code{name}
field contains the @code{name} given after the @samp{-V} option.
The @code{size} field is zero.  Only the first file in each volume
of an archive should have this type.

@end table

You may have trouble reading a GNU format archive on a non-GNU system if
the options @samp{-G}, @samp{-M}, @samp{-S}, or @samp{-V} were used when
writing the archive.  In general, if tar does not use the GNU-added
fields of the header, other versions of tar should be able to read the
archive.  Otherwise, the tar program will give an error, the most likely
one being a checksum error.

@unnumbered Concept Index
@printindex cp
@setchapternewpage odd
@contents
@bye