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<title>sbrk</title>
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<h2 align=center>sbrk</h2>
<h4 align=center>OS/161 Reference Manual</h4>

<h3>Name</h3>
sbrk - set process break (allocate memory)

<h3>Library</h3>
Standard C Library (libc, -lc)

<h3>Synopsis</h3>
#include &lt;unistd.h&gt;<br>
<br>
void *<br>
sbrk(intptr_t <em>amount</em>);

<h3>Description</h3>

The "break" is the end address of a process's heap region. The sbrk
call adjusts the "break" by the amount <em>amount</em>. It returns the
old "break". Thus, to determine the current "break", call sbrk(0).
<p>

The heap region is initially empty, so at process startup, the
beginning of the heap region is the same as the end and may thus be
retrieved using sbrk(0).
<p>

In OS/161, the initial "break" must be page-aligned, and sbrk only
need support values of <em>amount</em> that result in page-aligned
"break" addresses. Other values of <em>amount</em> may be rejected.
(This may simplify the implementation. On the other hand, you may
choose to support unaligned values anyway, as that may simplify your
malloc code.)
<p>

Traditionally, the initial "break" is specifically defined to be the
end of the BSS (uninitialized data) region, and any <em>amount</em>,
page-aligned or not, may legally be used with sbrk.
<p>

Ordinarily, user-level code should call
<A HREF=../libc/malloc.html>malloc</A> for memory allocation. The
sbrk interface is intended only to be the back-end interface for
malloc. Mixing calls to malloc and sbrk will likely confuse malloc and
produces undefined behavior.
<p>

While one can lower the "break" by passing negative values of
<em>amount</em>, one may not set the end of the heap to an address
lower than the beginning of the heap. Attempts to do so must be
rejected. 
<p>

<h3>Return Values</h3>

On success, sbrk returns the previous value of the "break".  On error,
((void *)-1) is returned, and <A HREF=errno.html>errno</A> is set
according to the error encountered.

<h3>Errors</h3>

The following error codes should be returned under the conditions
given. Other error codes may be returned for other errors not
mentioned here.

<blockquote><table width=90%>
<td width=10%>&nbsp;</td><td>&nbsp;</td></tr>
<tr><td>ENOMEM</td>	<td>Sufficient virtual memory to satisfy the
				request was not available, or the
				process has reached the limit of the
				memory it is allowed to allocate.</td></tr>
<tr><td>EINVAL</td>	<td>The request would move the "break" below
				its initial value.</td></tr>
</table></blockquote>

<h3>Restrictions</h3>

While you can return pages that happen to be at the end of the heap to
the system, there is no way to use the sbrk interface to return unused
pages in the middle of the heap. If you wish to do this, you will need
to design a new or supplemental interface.

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