/dev/zero

Special device files

/dev/zero is a special file in Unix-like operating systems that provides as many null characters (ASCII NUL, 0x00) as are read from it.[1] One of the typical uses is to provide a character stream for initializing data storage.[2]

Function

Read operations from /dev/zero return as many null characters (0x00) as requested in the read operation.

Unlike /dev/null, /dev/zero may be used as a source, not only as a sink for data. All write operations to /dev/zero succeed with no other effects. However, /dev/null is more commonly used for this purpose.

When /dev/zero is memory-mapped, e.g., with mmap, to the virtual address space, it is equivalent to using anonymous memory; i.e. memory not connected to any file.

History

/dev/zero was introduced in 1988 by SunOS-4.0 in order to allow a mappable BSS segment for shared libraries using anonymous memory. In the mid 1990s, HP-UX introduced the mmap() flag MAP_ANONYMOUS that maps anonymous memory directly without a need to open /dev/zero. Since the late 1990s, MAP_ANONYMOUS is supported by most UNIX versions, removing the original purpose of /dev/zero.

Examples

The dd Unix utility program reads octet streams from a source to a destination, possibly performing data conversions in the process. Destroying existing data on a file system partition (low-level formatting):

dd if=/dev/zero of=/dev/<destination partition>

Creating a 1 MiB file, called foobar, filled with zeroes:[3]

dd if=/dev/zero of=foobar count=1024 bs=1024

Note: The block size value can be given in SI (decimal) values, e.g. in GB, MB, etc. To create a 1 GB file one would simply type:

dd if=/dev/zero of=foobar count=1 bs=1GB

Note: Instead of creating a real file with only zero bytes, many file systems also support the creation of sparse files which returns zeros upon reading but use less actual space.[4]

See also

References

  1. ↑ Mitchell, Mark; Oldham, Jeffrey; Samuel, Alex (2001), "6.5.2 /dev/zero", Advanced Linux Programming, Sams Publishing, p. 136, ISBN 9780735710436
  2. ↑ Love, Robert (2007), "Mapping /dev/zero", Linux System Programming: Talking Directly to the Kernel and C Library, O'Reilly Media, Inc., pp. 259–260, ISBN 9780596009588
  3. ↑ Optimizing NFS Performance: Tuning and Troubleshooting NFS on HP-UX Systems, Dave (2002). Olker. Prentice Hall Professional. pp. 26–7. ISBN 9780130428165. Retrieved 16 July 2014.
  4. ↑ Sparse file
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