/dev/zero

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Not to be confused with /dev/null.

/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.[3] HP-UX 8.x introduced the MAP_ANONYMOUS flag for mmap(), which maps anonymous memory directly without a need to open /dev/zero.[4] Since the late 1990s, MAP_ANONYMOUS[5] or MAP_ANON are supported by most UNIX versions, removing the original purpose of /dev/zero.[6]

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 null characters:[7] 

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.[8]

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. ^ ""C" run-time program bootstrap from SunOS, contributed to CSRG for inclusion in 4.4BSD". TUHS.
  4. ^ "HP-UX 8.0.7 install media".
  5. ^ Beal, Chris. "So what the heck is anonymous memory". Oracle Blog.
  6. ^ "MAP_ANON description in mmap(2)". NetBSD.
  7. ^ Optimizing NFS Performance: Tuning and Troubleshooting NFS on HP-UX Systems, Dave (2002). Olker. Prentice Hall Professional. pp. 26–7. ISBN 9780130428165. Retrieved 2014-07-16.
  8. ^ Sparse file
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