tar只是一个归档文件，并不进行压缩。
struct tar_header
{
char name[100];
char mode[8];
char uid[8];
char gid[8];
char size[12];
char mtime[12];
char chksum[8];
char typeflag;
char linkname[100];
char magic[6];
char version[2];
char uname[32];
char gname[32];
char devmajor[8];
char devminor[8];
char prefix[155];
char padding[12];
};
以上是tar中保存文件信息的数据结构，其后跟着的就是文件的内容。
size为文件大小的八进制字节表示，例如文件大小为90个字节，那么这里就是八进制的90，即为132。
其中，文件大小，修改时间，checksum都是存储的对应的八进制字符串，字符串最后一个字符为空格字符。
checksum的计算方法为出去checksum字段其他所有的512-8共504个字节的ascii码相加的值再加上256(checksum当作八个空格，即8*0x20）
文件内容以512字节为一个block进行分割，最后一个block不足部分以0补齐。
两个文件的tar包首先存放第一个文件的tar头结构，然后存储文件内容，接着存储第二个文件的tar头结构，然后存储文件内容。
所有文件都存储完了以后，最后存放一个全零的tar结构
所有的tar文件大小应该都是512的倍数，一个空文件打包后为512*3字节，包括一个tar结构头，一个全零的block存储文件内容，一个全零的tar结构。
tar 格式详细说明
the unix tar program is an archiver program which stores files in a single
archive without compression.
offset count type description
@section the standard format
a @dfn{tar tape} or file contains a series of records. each record
contains @code{recordsize} bytes. although this format may be
thought of as being on magnetic tape, other media are often used.
each file archived is represented by a header record which describes
the file, followed by zero or more records which give the contents
of the file. at the end of the archive file there may be a record
filled with binary zeros as an end-of-file marker. a reasonable
system should write a record of zeros at the end, but must not
assume that such a record exists when reading an archive.
the records may be @dfn{blocked} for physical i/o operations. each
block of @var{n} records (where @var{n} is set by the @samp{-b}
option to @code{tar}) is written with a single @code{write()}
operation. on magnetic tapes, the result of such a write is a
single tape record. when writing an archive, the last block of
records should be written at the full size, with records after the
zero record containing all zeroes. when reading an archive, a
reasonable system should properly handle an archive whose last block
is shorter than the rest, or which contains garbage records after a
zero record.
the header record is defined in c as follows:
@example
/*
* standard archive format - standard tar - ustar
*/
#define recordsize 512
#define namsiz 100
#define tunmlen 32
#define tgnmlen 32
union record @{
char charptr[recordsize];
struct header @{
char name[namsiz];
char mode[8];
char uid[8];
char gid[8];
char size[12];
char mtime[12];
char chksum[8];
char linkflag;
char linkname[namsiz];
char magic[8];
char uname[tunmlen];
char gname[tgnmlen];
char devmajor[8];
char devminor[8];
@} header;
@};
/* 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 may be defined later. */
/* 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.
@table @code
@item lf_normal
@itemx lf_oldnormal
these 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 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 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_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
extension:
occurences:
programs:
reference:
see also:
validation:
offset count type description
0000h 256 byte other header info ?
0100h 6 char id='ustar',0
extension:tar
occurences:pc, unix
programs:tar