| <html lang="en"> |
| <head> |
| <title>Resizing the Data Segment - The GNU C Library</title> |
| <meta http-equiv="Content-Type" content="text/html"> |
| <meta name="description" content="The GNU C Library"> |
| <meta name="generator" content="makeinfo 4.13"> |
| <link title="Top" rel="start" href="index.html#Top"> |
| <link rel="up" href="Memory.html#Memory" title="Memory"> |
| <link rel="prev" href="Locking-Pages.html#Locking-Pages" title="Locking Pages"> |
| <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> |
| <!-- |
| This file documents the GNU C library. |
| |
| This is Edition 0.12, last updated 2007-10-27, |
| of `The GNU C Library Reference Manual', for version |
| 2.8 (Sourcery G++ Lite 2011.03-41). |
| |
| Copyright (C) 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2001, 2002, |
| 2003, 2007, 2008, 2010 Free Software Foundation, Inc. |
| |
| Permission is granted to copy, distribute and/or modify this document |
| under the terms of the GNU Free Documentation License, Version 1.3 or |
| any later version published by the Free Software Foundation; with the |
| Invariant Sections being ``Free Software Needs Free Documentation'' |
| and ``GNU Lesser General Public License'', the Front-Cover texts being |
| ``A GNU Manual'', and with the Back-Cover Texts as in (a) below. A |
| copy of the license is included in the section entitled "GNU Free |
| Documentation License". |
| |
| (a) The FSF's Back-Cover Text is: ``You have the freedom to |
| copy and modify this GNU manual. Buying copies from the FSF |
| supports it in developing GNU and promoting software freedom.''--> |
| <meta http-equiv="Content-Style-Type" content="text/css"> |
| <style type="text/css"><!-- |
| pre.display { font-family:inherit } |
| pre.format { font-family:inherit } |
| pre.smalldisplay { font-family:inherit; font-size:smaller } |
| pre.smallformat { font-family:inherit; font-size:smaller } |
| pre.smallexample { font-size:smaller } |
| pre.smalllisp { font-size:smaller } |
| span.sc { font-variant:small-caps } |
| span.roman { font-family:serif; font-weight:normal; } |
| span.sansserif { font-family:sans-serif; font-weight:normal; } |
| --></style> |
| <link rel="stylesheet" type="text/css" href="../cs.css"> |
| </head> |
| <body> |
| <div class="node"> |
| <a name="Resizing-the-Data-Segment"></a> |
| <p> |
| Previous: <a rel="previous" accesskey="p" href="Locking-Pages.html#Locking-Pages">Locking Pages</a>, |
| Up: <a rel="up" accesskey="u" href="Memory.html#Memory">Memory</a> |
| <hr> |
| </div> |
| |
| <h3 class="section">3.3 Resizing the Data Segment</h3> |
| |
| <p>The symbols in this section are declared in <samp><span class="file">unistd.h</span></samp>. |
| |
| <p>You will not normally use the functions in this section, because the |
| functions described in <a href="Memory-Allocation.html#Memory-Allocation">Memory Allocation</a> are easier to use. Those |
| are interfaces to a GNU C Library memory allocator that uses the |
| functions below itself. The functions below are simple interfaces to |
| system calls. |
| |
| <!-- unistd.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Function: int <b>brk</b> (<var>void *addr</var>)<var><a name="index-brk-350"></a></var><br> |
| <blockquote> |
| <p><code>brk</code> sets the high end of the calling process' data segment to |
| <var>addr</var>. |
| |
| <p>The address of the end of a segment is defined to be the address of the |
| last byte in the segment plus 1. |
| |
| <p>The function has no effect if <var>addr</var> is lower than the low end of |
| the data segment. (This is considered success, by the way). |
| |
| <p>The function fails if it would cause the data segment to overlap another |
| segment or exceed the process' data storage limit (see <a href="Limits-on-Resources.html#Limits-on-Resources">Limits on Resources</a>). |
| |
| <p>The function is named for a common historical case where data storage |
| and the stack are in the same segment. Data storage allocation grows |
| upward from the bottom of the segment while the stack grows downward |
| toward it from the top of the segment and the curtain between them is |
| called the <dfn>break</dfn>. |
| |
| <p>The return value is zero on success. On failure, the return value is |
| <code>-1</code> and <code>errno</code> is set accordingly. The following <code>errno</code> |
| values are specific to this function: |
| |
| <dl> |
| <dt><code>ENOMEM</code><dd>The request would cause the data segment to overlap another segment or |
| exceed the process' data storage limit. |
| </dl> |
| |
| <!-- The Brk system call in Linux (as opposed to the GNU C Library function) --> |
| <!-- is considerably different. It always returns the new end of the data --> |
| <!-- segment, whether it succeeds or fails. The GNU C library Brk determines --> |
| <!-- it's a failure if and only if the system call returns an address less --> |
| <!-- than the address requested. --> |
| </blockquote></div> |
| |
| <!-- unistd.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Function: void <b>*sbrk</b> (<var>ptrdiff_t delta</var>)<var><a name="index-g_t_002asbrk-351"></a></var><br> |
| <blockquote><p>This function is the same as <code>brk</code> except that you specify the new |
| end of the data segment as an offset <var>delta</var> from the current end |
| and on success the return value is the address of the resulting end of |
| the data segment instead of zero. |
| |
| <p>This means you can use ‘<samp><span class="samp">sbrk(0)</span></samp>’ to find out what the current end |
| of the data segment is. |
| |
| </blockquote></div> |
| |
| </body></html> |
| |
