cpio File Format

Purpose

Describes the copy in/out (cpio) archive file.

Description

The cpio utility backs up and recovers files. The files are saved on the backup medium in the cpio format.

When the cpio command is used with the -c flag, the header for the cpio structure reads as follows:

sscanf(Chdr,"%6ho%6ho%6ho%6ho%6ho%6ho%6ho%6ho%11lo%6ho%11lo%s",
&Hdr.h_magic, &Hdr.h_dev, &Hdr.h_ino, &Hdr.h_mode,
&Hdr.h_uid, &Hdr.h_gid, &Hdr.h_nlink, &Hdr.h_rdev,
&Longtime, &Hdr.h_namesize, &Longfile, &Hdr.h_name);

Longtime and Longfile are equivalent to Hdr.h_mtime and Hdr.h_filesize, respectively. The contents of each file, and other items describing the file, are recorded in an element of the array of structures with varying lengths.

Note: Files saved with the -c flag must be restored with the -c flag.

When the -c flag of the cpio command is not used, the header structure contains the following fields:

Item	Description
`h_magic`	Contains the constant octal 070707 (or 0x71c7).
`h_dev`	Device that contains a directory entry for this file.
`h_ino`	I-node number that identifies the input file to the file system.
`h_mode`	Mode of the input file, as defined in the mode.h file.
`h_uid`	User ID of the owner of the input file.
`h_gid`	Group ID of the owner of the input file.

For remote files, these fields contain the ID after reverse translation:

Item	Description
`h_nlink`	Number of links that are connected to the input file.
`h_rdev`	ID of the remote device from which the input file is taken.
`h_mtime`	Time when data was last modified. For remote files, this field contains the time at the server. This time can be changed by the creat, fclearf, truncate, mknod, openx, pipe, utime, or writex subroutine.
`h_namesize`	Length of the path name, including the terminating null byte.
`h_filesize`	Length of the file in bytes. This is the length of the data section that follows the header structure.
`h_name`	Null-terminated path name. The length of the path name, including the null byte, is indicated by the n variable, where n equals `((h_namesize % 2) + h_namesize)`. That is, the n variable is equal to the `h_namesize` field if the `h_namesize` field is even. If the `h_namesize` field is odd, the n variable is equal to the `h_namesize` field + 1.

The last record of the archive always contains the name TRAILER!!!. Special files, directories, and the trailer are recorded with the h_filesize field equal to 0.