initramfs buffer format

Al Viro, H. Peter Anvin

Last revision: 2002-01-13

Starting with kernel 2.5.x, the old "initial ramdisk" protocol is getting {replaced/complemented} with the new "initial ramfs" (initramfs) protocol. The initramfs contents is passed using the same memory buffer protocol used by the initrd protocol, but the contents is different. The initramfs buffer contains an archive which is expanded into a ramfs filesystem; this document details the format of the initramfs buffer format.

The initramfs buffer format is based around the "newc" or "crc" CPIO formats, and can be created with the cpio(1) utility. The cpio archive can be compressed using gzip(1). One valid version of an initramfs buffer is thus a single .cpio.gz file.

The full format of the initramfs buffer is defined by the following grammar, where:

*       is used to indicate "0 or more occurrences of"
(|)     indicates alternatives
+       indicates concatenation
GZIP()  indicates the gzip(1) of the operand
ALGN(n) means padding with null bytes to an n-byte boundary

initramfs  := ("\0" | cpio_archive | cpio_gzip_archive)*

cpio_gzip_archive := GZIP(cpio_archive)

cpio_archive := cpio_file* + (<nothing> | cpio_trailer)

cpio_file := ALGN(4) + cpio_header + filename + "\0" + ALGN(4) + data

cpio_trailer := ALGN(4) + cpio_header + "TRAILER!!!\0" + ALGN(4)

In human terms, the initramfs buffer contains a collection of compressed and/or uncompressed cpio archives (in the "newc" or "crc" formats); arbitrary amounts zero bytes (for padding) can be added between members.

The cpio "TRAILER!!!" entry (cpio end-of-archive) is optional, but is not ignored; see "handling of hard links" below.

The structure of the cpio_header is as follows (all fields contain hexadecimal ASCII numbers fully padded with '0' on the left to the full width of the field, for example, the integer 4780 is represented by the ASCII string "000012ac"):

Field name

Field size



6 bytes

The string "070701" or "070702"


8 bytes

File inode number


8 bytes

File mode and permissions


8 bytes

File uid


8 bytes

File gid


8 bytes

Number of links


8 bytes

Modification time


8 bytes

Size of data field


8 bytes

Major part of file device number


8 bytes

Minor part of file device number


8 bytes

Major part of device node reference


8 bytes

Minor part of device node reference


8 bytes

Length of filename, including final 0


8 bytes

Checksum of data field if c_magic is 070702; otherwise zero

The c_mode field matches the contents of st_mode returned by stat(2) on Linux, and encodes the file type and file permissions.

The c_filesize should be zero for any file which is not a regular file or symlink.

The c_chksum field contains a simple 32-bit unsigned sum of all the bytes in the data field. cpio(1) refers to this as "crc", which is clearly incorrect (a cyclic redundancy check is a different and significantly stronger integrity check), however, this is the algorithm used.

If the filename is "TRAILER!!!" this is actually an end-of-archive marker; the c_filesize for an end-of-archive marker must be zero.