/dev/zero: Difference between revisions

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Read operations from {{mono|/dev/zero}} return as many [[null character]]s (0x00) as requested in the read operation.
Read operations from {{mono|/dev/zero}} return as many [[null character]]s (0x00) as requested in the read operation.


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


When {{mono|/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.
When {{mono|/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==
==History==
{{mono|/dev/zero}} was introduced in 1988 by SunOS-4.0 in order to allow a mappable BSS segment for shared libraries using anonymous memory.<ref>{{cite web |title="C" run-time program bootstrap from SunOS, contributed to CSRG for inclusion in 4.4BSD |url=https://minnie.tuhs.org/cgi-bin/utree.pl?file=4.4BSD/usr/src/contrib/sun.sharedlib/lib/csu/m68k/crt0.s |website=TUHS}}</ref> HP-UX 8.x introduced the MAP_ANONYMOUS flag for mmap(), which maps anonymous memory directly without a need to open {{mono|/dev/zero}}.<ref>{{cite web |title=HP-UX 8.0.7 install media |date=22 July 1992 |url=https://archive.org/details/hp-ux8.07forhp9000s7xx}}</ref> Since the late 1990s, MAP_ANONYMOUS<ref>{{cite web |last1=Beal |first1=Chris |title=So what the heck is anonymous memory |url=https://blogs.oracle.com/cwb/so-what-the-heck-is-anonymous-memory |website=Oracle Blog |access-date=2019-09-09  |archive-date=2021-04-15  |archive-url=https://web.archive.org/web/20210415103111/https://blogs.oracle.com/cwb/so-what-the-heck-is-anonymous-memory |url-status=dead }}</ref> or MAP_ANON are supported by most UNIX versions, removing the original purpose of {{mono|/dev/zero}}.<ref>{{cite web |title=MAP_ANON description in mmap(2) |url=https://netbsd.gw.com/cgi-bin/man-cgi?mmap |website=NetBSD |access-date=2019-09-09  |archive-date=2019-11-25  |archive-url=https://web.archive.org/web/20191125152633/https://netbsd.gw.com/cgi-bin/man-cgi?mmap |url-status=dead }}</ref>
{{mono|/dev/zero}} was introduced in 1988 in SunOS-4.0 to allow a mappable BSS segment for shared libraries using anonymous memory.<ref>{{cite web |title="C" run-time program bootstrap from SunOS, contributed to CSRG for inclusion in 4.4BSD |url=https://minnie.tuhs.org/cgi-bin/utree.pl?file=4.4BSD/usr/src/contrib/sun.sharedlib/lib/csu/m68k/crt0.s |website=TUHS}}</ref> HP-UX 8.x introduced the MAP_ANONYMOUS flag for mmap(), which maps anonymous memory directly without a need to open {{mono|/dev/zero}}.<ref>{{cite web |title=HP-UX 8.0.7 install media |date=22 July 1992 |url=https://archive.org/details/hp-ux8.07forhp9000s7xx}}</ref> Since the late 1990s, MAP_ANONYMOUS<ref>{{cite web |last1=Beal |first1=Chris |title=So what the heck is anonymous memory |url=https://blogs.oracle.com/cwb/so-what-the-heck-is-anonymous-memory |website=Oracle Blog |access-date=2019-09-09  |archive-date=2021-04-15  |archive-url=https://web.archive.org/web/20210415103111/https://blogs.oracle.com/cwb/so-what-the-heck-is-anonymous-memory |url-status=dead }}</ref> or MAP_ANON are supported by most UNIX versions, removing the original purpose of {{mono|/dev/zero}}.<ref>{{cite web |title=MAP_ANON description in mmap(2) |url=https://netbsd.gw.com/cgi-bin/man-cgi?mmap |website=NetBSD |access-date=2019-09-09  |archive-date=2019-11-25  |archive-url=https://web.archive.org/web/20191125152633/https://netbsd.gw.com/cgi-bin/man-cgi?mmap |url-status=dead }}</ref>


==Examples==
==Examples==
[[Data erasure|Erasing]] a file system partition or drive:
[[Data erasure|Erasing]] a file system partition or drive:


  cp /dev/zero /dev/<destination drive or partition>
  cp /dev/zero /dev/<destination drive or partition>


(Note that this does not perform a secure erasure, may not destroy the data at all, and may take significantly more time than required – for this purpose, domain-specific tooling like ''blkdiscard'' may be preferred for devices that support [[Trim (computing)|TRIM]].)
However, this may not perform a secure erasure, may not destroy the data at all, and may take significantly more time than required–for this purpose.
 
Creating a 1 [[mebibyte|MiB]] file, called ''[[foobar]]'', filled with null characters:
 
head -c $(( 1024 * 1024 )) /dev/zero > foobar
 
Instead of creating a file really filled with only zero bytes, many file systems also support the creation of [[sparse file]]s, which return zeros upon reading but use less actual space. The classic way of doing this (without the domain-specific ''truncate'' utility) would be, to create a 1 [[gibibyte|GiB]] file:


dd {{abbr|bs|block size}}=1 seek=$(( 1024 * 1024 * 1024 - 1 )) count=1 < /dev/zero > foobar
Creating a {{val|1|ul=MiB}} file, called ''[[foobar]]'', filled with null characters:


which seeks to position ''seek''·''bs''=1GiB−1 in the output and copies ''count''·''bs''=1 byte from /dev/zero, thus making the file contain only one data block.
{{sxhl|2=bash|head -c $(( 1024 * 1024 )) /dev/zero >foobar}}


==See also==
==See also==

Latest revision as of 16:10, 29 December 2025

Script error: No such module "Distinguish". Template:Short description Template:Use dmy dates <templatestyles src="Mono/styles.css" />/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 <templatestyles src="Mono/styles.css" />/dev/zero return as many null characters (0x00) as requested in the read operation.

All write operations to <templatestyles src="Mono/styles.css" />/dev/zero succeed with no other effects. However, <templatestyles src="Mono/styles.css" />/dev/null is more commonly used for this purpose.

When <templatestyles src="Mono/styles.css" />/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

<templatestyles src="Mono/styles.css" />/dev/zero was introduced in 1988 in SunOS-4.0 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 <templatestyles src="Mono/styles.css" />/dev/zero.[4] Since the late 1990s, MAP_ANONYMOUS[5] or MAP_ANON are supported by most UNIX versions, removing the original purpose of <templatestyles src="Mono/styles.css" />/dev/zero.[6]

Examples

Erasing a file system partition or drive: 

cp /dev/zero /dev/<destination drive or partition>

However, this may not perform a secure erasure, may not destroy the data at all, and may take significantly more time than required–for this purpose.

Creating a Script error: No such module "val". file, called foobar, filled with null characters:

Template:Sxhl

See also

References

<templatestyles src="Reflist/styles.css" />

  1. ↑ Script error: No such module "citation/CS1".
  2. ↑ Script error: No such module "citation/CS1".
  3. ↑ Script error: No such module "citation/CS1".
  4. ↑ Script error: No such module "citation/CS1".
  5. ↑ Script error: No such module "citation/CS1".
  6. ↑ Script error: No such module "citation/CS1".

Script error: No such module "Check for unknown parameters".

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