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{{Short description|Basic memory management system calls used in Unix}}
{{redirect|brk|the NYSE stock symbol BRK|Berkshire Hathaway| the software interrupt BRK| Interrupts in 65xx processors}}
{{lowercase title}}
{{cleanup- rewrite|date=February 2012}}▼
 
'''{{mono|brk}}''' and '''{{mono|sbrk}}''' are basic [[memory management]] [[system call]]s used in [[Unix]] and [[Unix-like]] operating systems to control the amount of memory allocated to the [[dataheap segment]] of the [[Processprocess (computing)|process]].<ref>{{cite web|url=https://www.gnu.org/software/libc/manual/html_node/Memory-Concepts.html|title=Process Memory Concepts|publisher=Free Software Foundation|accessdate=9 October 2015}}</ref> These callsfunctions are typically madecalled from a higher-level memory management library function such as {{mono|[[malloc]]}}. In the original Unix system, {{mono|brk}} and {{mono|sbrk}} were the only ways in which applications could acquire additional dataheap space; later versions allowed this to also be done using the {{mono|[[mmap]]}} call.<ref name=bsdmalloc>{{cite web|title=A new malloc(3) for OpenBSD|url=http://www.openbsd.org/papers/eurobsdcon2009/otto-malloc.pdf|accessdate=13 June 2018}}</ref><ref>{{cite web |title=POSIX Memory Management |url=http://blog.pr4tt.com/2016/02/01/posix-memory-management/ |website=blog.pr4tt.com}}</ref>▼
▲{{cleanup-rewrite|date=February 2012}}
{{Technical|date=August 2009}}
 
▲'''brk''' and '''sbrk''' are basic [[memory management]] [[system call]]s used in [[Unix]] and [[Unix-like]] operating systems to control the amount of memory allocated to the [[data segment]] of the [[Process (computing)|process]]. These calls are typically made from a higher-level memory management library such as [[malloc]]. In the original Unix system, brk and sbrk were the only ways in which applications could acquire additional data space; later versions allowed this to also be done using the [[mmap]] call.
 
==Description==
The brk and sbrk calls [[dynamic memory allocation|dynamically change]] the amount of space allocated for the dataheap segment of the calling process. The change is made by resetting the [[program break]] of the process, which determines the maximum space that can be allocated. The program break is the address of the first location beyond the current end of the data region. The amount of available space increases as the break value increases. The available space is initialized to a value of zero, unless the break is lowered and then increased, as it may reuse the same pages in some unspecified way. The break value can be automatically rounded up to a size appropriate for the [[memory management]] architecture.<ref name=OpenGrpStdSIH>{{cite book|title=X/Open CAE Specification, System Interfaces and Headers |date=September 1994|issue=4|edition=2|publisher=X/Open Company Ltd., U.K.|page=64|url=http://pubs.opengroup.org/onlinepubs/9695969499/toc.pdf|accessdate=9 October 2015}}</ref>
 
{{mono|sbrk}} and {{mono|brk}} were considered legacy even by 1997 standards ([[Single UNIX Specification]] v2 or POSIX.1-1998).<ref name=SUSv2>{{cite web |title=brk, sbrk - change space allocation (LEGACY) |url=https://pubs.opengroup.org/onlinepubs/7908799/xsh/brk.html |website=The Single UNIX ® Specification, Version 2 |accessdate=30 November 2019 |date=1997}}</ref> They were removed in POSIX.1-2001.<ref>{{man|2|brk|Linux}}</ref>
 
==Function signatures and behavior==
<syntaxhighlight lang="c">
<pre>
#include <unistd.h>
 
int brk(void * end_data_segment);
 
void *sbrk(intptr_t increment);
</syntaxhighlight>
</pre>
The{{mono|sbrk}} brkis subroutineused to setsadjust the program break value by adding a possibly negative size, while {{mono|brk}} is used to set the break value ofto the ''end_data_segment''value of a pointer. Set {{var|increment}} parameter andto changeszero to fetch the amountcurrent value of availablethe spaceprogram accordinglybreak.
 
Upon successful completion, the {{mono|brk}} subroutine returns a value of 0, and the {{mono|sbrk}} subroutine returns the prior value of the program break (if the available space is increased thethen returnthis prior value also points to the start of the new area). If either subroutine is unsuccessful, a value of {{num|−1}}<!-- this is a running text in Wikipedia, not a C code, so avoid ASCII surrogates please --1> is returned and the {{mono|[[Errnoerrno.h|errno]]}} [[global variable]] is set to indicate the error.<ref name=SUSv2/>▼
The sbrk subroutine adds to the program break value the number of bytes contained in the ''increment'' parameter and changes the amount of available space accordingly. The ''increment'' parameter can be a [[negative number]], in which case the amount of available space is decreased.
 
Not every Unix-like system entertains the concept of having the user control the data segment. The [[Mac OS X]] implementation of {{mono|sbrk}} is an emulation and has a maximum allocation of 4 megabytes. On first call an area exactly this large is allocated to hold the simulated segment. When this limit is reached, −1 is returned and the {{mono|errno}} is set to {{mono|ENOMEM}}. {{mono|brk}} always errors.<ref>{{Cite web|url=https://opensource.apple.com/source/Libc/Libc-1272.250.1/emulated/brk.c.auto.html|title=BRK.c}}</ref>
▲Upon successful completion, the brk subroutine returns a value of 0, and the sbrk subroutine returns the prior value of the program break (if the available space is increased the return value points to the start of the new area). If either subroutine is unsuccessful, a value of -1 is returned and the [[Errno.h|errno]] [[global variable]] is set to indicate the error.
 
The current [[Mac OS X]] implementation of sbrk is an emulation, and has a maximum allocation of 4 Megabytes.<ref>http://www.opensource.apple.com/source/Libc/Libc-763.12/emulated/brk.c</ref> When this limit is reached, -1 is returned and the errno is not set.
 
==Error codes==
The error ''{{mono|ENOMEM''}} is set and the allocated space remains unchanged if one or more of the following are true:
* The requested change allocates more space than is allowed by a system-imposed maximum.
* The requested change sets the break value to a value [[inequality (mathematics)|greater than or equal to]] the start address of any attached [[Shared memory (interprocess communication)|shared memory]] segment.
 
== See also ==
 
* [[Exec (operating systemcomputing)]]
* {{section link|Memory address|Address space in application programming}}
* [[Shared memory]]
 
==References==
 
* [http://linux.die.net/man/2/sbrk sbrk &ndash; Linux man page]
{{Reflist}}
 
[[Category:Memory management software]]
[[Category:UnixOperating processsystem and task management-related softwareAPIs]]
 
 
{{Unix-stub}}
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