: Embedded Linux Primer: A Practical, Real-World Approach

9.6. cramfs

9.6. cramfs

From the README file in the cramfs project, the goal of cramfs is to "cram a file system into a small ROM." The cramfs file system is very useful for embedded systems that contain a small ROM or FLASH memory that holds static data and programs. Borrowing again from the cramfs README file, "cramfs is designed to be simple and small, and compress things well."

The cramfs file system is read only. It is created with a command line utility called mkcramfs. If you don't have it on your development workstation, you can download it from the link at the end of this chapter. As with JFFS2, mkcramfs builds a file system image from a directory specified on the command line. Listing 9-10 details the procedure for building a cramfs image. We use the same file system structure from Listing 9-8 that we used to build the JFFS2 image.

Listing 9-10. mkcramfs Command Example

# mkcramfs
usage: mkcramfs [-h] [-v] [-b blksize] [-e edition] [-i file] [-n name]
dirname outfile
-h print this help
-E make all warnings errors (non-zero exit status)
-b blksize use this blocksize, must equal page size
-e edition set edition number (part of fsid)
-i file insert a file image into the filesystem (requires >= 2.4.0)
-n name set name of cramfs filesystem
-p pad by 512 bytes for boot code
-s sort directory entries (old option, ignored)
-v be more verbose
-z make explicit holes (requires >= 2.3.39)
dirname root of the directory tree to be compressed
outfile output file
# mkcramfs . ../cramfs.image
warning: gids truncated to 8 bits (this may be a security concern)
# ls -l ../cramfs.image
-rw-rw-r-- 1 chris chris 1019904 Sep 19 18:06 ../cramfs.image

The mkcramfs command was initially issued without any command line parameters to reproduce the usage message. Because there is no man page for this utility, this is the best way to understand its usage. We subsequently issued the command specifying the current directory, ., as the source of the files for the cramfs file system, and a file called cramfs.image as the destination. Finally, we listed the file just created, and we see a new file called cramfs.image.

Note that if your kernel is configured with cramfs support, you can mount this file system image on your Linux development workstation and examine its contents. Of course, because it is a read-only file system, you cannot modify it. Listing 9-11 demonstrates mounting the cramfs file system on a mount point called /mnt/flash.

Listing 9-11. Examining the cramfs File System

# mount -o loop cramfs.image /mnt/flash
# ls -l /mnt/flash
total 6
drwxr-xr-x 1 root root 704 Dec 31 1969 bin
drwxr-xr-x 1 root root 0 Dec 31 1969 dev
drwxr-xr-x 1 root root 416 Dec 31 1969 etc
drwxr-xr-x 1 root root 0 Dec 31 1969 home
drwxr-xr-x 1 root root 172 Dec 31 1969 lib
drwxr-xr-x 1 root root 0 Dec 31 1969 proc
drws------ 1 root root 0 Dec 31 1969 root
drwxr-xr-x 1 root root 272 Dec 31 1969 sbin
drwxrwxrwt 1 root root 0 Dec 31 1969 tmp
drwxr-xr-x 1 root root 124 Dec 31 1969 usr
drwxr-xr-x 1 root root 212 Dec 31 1969 var

You might have noticed the warning message regarding group ID (GID) when the mkcramfs command was executed. The cramfs file system uses very terse metadata to reduce file system size and increase the speed of execution. One of the "features" of the cramfs file system is that it truncates the group ID field to 8 bits. Linux uses 16-bit group ID field. The result is that files created with group IDs greater than 255 are truncated with the warning issued in Listing 9-10.

Although somewhat limited in terms of maximum file sizes, maximum number of files, and so on, the cramfs file system is ideal for boot ROMS, in which read-only operation and fast compression are ideally suited.

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