This document describes the format of POSIX-defined archives read and written
by the pax utility using the
A pax archive, in either format, consists of one or more
blocks, which are used to represent member files.
Each block is 512 bytes long; the
Each member file consists of a header block (as described later in this page) followed by 0 or more blocks containing the file contents. The end of the archive is indicated by two blocks filled with binary zeros. Unused space in the header is left as binary zeros.
The header information in a block is stored in a printable ASCII form, so that pax archives are easily ported to different environments. If the contents of the files on the archive are all ASCII, the entire archive is ASCII.
Headers have the format shown in Table 1.
|Field Width||Field Name||Meaning|
|100||name||name of file|
|8||uid||owner user ID|
|8||gid||owner group ID|
|12||size||length of file in bytes|
|12||mtime||modify time of file|
|8||chksum||checksum for header|
|1||typeflag||type of file|
|100||linkname||name of linked file|
|32||uname||owner user name|
|32||gname||owner group name|
|8||devmajor||device major number|
|8||devminor||device minor number|
|155||prefix||prefix for file name|
Table 1: pax Header Block
This information is compatible with that returned by the UNIX
The name field contains the name of the archived file. The value of the prefix field, if non-null, is prefixed to the name field to allow names longer then 100 characters. For compatibility with older tar commands, the PTC MKS Toolkit version of pax leaves prefix null unless the file name exceeds 100 characters.
The size field is 0 if the header describes a link.
The chksum field is a checksum of all the bytes in the header, assuming that the chksum field itself is all blanks.
The typeflag field is a compatible extension of the link field of the older TAR format. Table 2 shows the values that are recognized.
|Type Flag||File Type|
|0 or null||Regular file|
|1||Link to another file already archived|
|3||Character special device|
|4||Block special device|
|6||FIFO special file|
|x||Represents extended header records for the following file|
|A-Z||Available for custom usage|
Table 2: Type Flag Values for pax Format Files
In pax format, the uname and gname fields contain the name of the owner and group of the file respectively.
To overcome certain limitations in the USTAR format, the pax features an extended header that contain informatin normally not allowed in a USTAR header block. This information inclues fields which require a character encoding not described in the ISO/IEC 646: 1991 standard, fields representing file attributes not covered in the USTAR heder, and fields whose length or format do not meet the requirements of the USTAR header. Values in the extended header add attributes to the following file or override values in the following header block, as described below.
The pax extended header contains one more records in the following format:
keyword can be any of the following:
is the file access time for the following file.
is the name of the character set used to encode the data in the following file.
is a comment string which is ignored by pax.
is the group name that owns the following file.
is the path name of a link to another file. This overrides the linkname field in the following header block. If the typeflag field in the following header block is 1, the link is a hard link. If typeflag is 2, it is a symbolic link.
is the file modification time of the follwing file. This overrides the mtime field in the following header block.
is the path name of the folloiwng file. This overrides the name and prefix fields in the following header block.
is the size of the file in octets. This overrides the size field in the following header block.
is the user name that owns the following file.
When the value field has a length of zero, any header block field or previous extended header record of the same name is deleted.
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