145 lines
4.9 KiB
HTML
145 lines
4.9 KiB
HTML
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<html>
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<head>
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<title>sbrk</title>
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<link rel="stylesheet" type="text/css" media="all" href="../man.css">
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</head>
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<body bgcolor=#ffffff>
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<h2 align=center>sbrk</h2>
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<h4 align=center>OS/161 Reference Manual</h4>
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<h3>Name</h3>
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<p>
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sbrk - set process break (allocate memory)
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</p>
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<h3>Library</h3>
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<p>
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Standard C Library (libc, -lc)
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</p>
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<h3>Synopsis</h3>
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<p>
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<tt>#include <unistd.h></tt><br>
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<br>
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<tt>void *</tt><br>
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<tt>sbrk(intptr_t </tt><em>amount</em><tt>);</tt>
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</p>
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<h3>Description</h3>
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<p>
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The "break" is the end address of a process's heap region. The
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<tt>sbrk</tt> call adjusts the "break" by the amount <em>amount</em>.
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It returns the old "break". Thus, to determine the current "break",
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call <tt>sbrk(0)</tt>.
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</p>
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<p>
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The heap region is initially empty, so at process startup, the
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beginning of the heap region is the same as the end and may thus be
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retrieved using sbrk(0).
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</p>
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<p>
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In OS/161, the initial "break" must be page-aligned, and <tt>sbrk</tt>
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only need support values of <em>amount</em> that result in
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page-aligned "break" addresses. Other values of <em>amount</em> may be
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rejected. This may simplify the implementation. You may place the
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heap wherever you like in a process's address space (though obviously
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not on top of something else) and it need not appear at the same
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location in every process.
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</p>
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<p>
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Traditionally, the initial "break" is specifically defined to be the
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end of the BSS (uninitialized data) region, and traditionally any
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<em>amount</em>, page-aligned or not, may legally be used with
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<tt>sbrk</tt>.
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</p>
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<p>
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Ordinarily, user-level code should call
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<A HREF=../libc/malloc.html>malloc</A> for memory allocation. The
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<tt>sbrk</tt> interface is intended only to be the back-end interface
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for <tt>malloc</tt>. Mixing calls to <tt>malloc</tt> and <tt>sbrk</tt>
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will likely confuse <tt>malloc</tt> and produces undefined behavior.
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</p>
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<p>
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While one can lower the "break" by passing negative values of
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<em>amount</em>, one may not set the end of the heap to an address
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lower than the beginning of the heap. Attempts to do so must be
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rejected.
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</p>
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<p>
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The call (like all system calls) should be atomic. In this case, that
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means that if you have a multithreaded process, simultaneous calls to
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<tt>sbrk</tt> from different threads should not interfere with each
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other and should update the "break" state atomically.
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</p>
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<h3>Return Values</h3>
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<p>
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On success, <tt>sbrk</tt> returns the previous value of the "break".
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On error, ((void *)-1) is returned, and <A HREF=errno.html>errno</A>
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is set according to the error encountered.
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</p>
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<h3>Errors</h3>
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<p>
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The following error codes should be returned under the conditions
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given. Other error codes may be returned for other cases not
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mentioned here.
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<table width=90%>
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<tr><td width=5% rowspan=2> </td>
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<td with=10% valign=top>ENOMEM</td>
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<td>Sufficient virtual memory to satisfy the
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request was not available, or the
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process has reached the limit of the
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memory it is allowed to allocate.</td></tr>
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<tr><td valign=top>EINVAL</td>
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<td>The request would move the "break" below
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its initial value.</td></tr>
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</table>
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</p>
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<h3>Restrictions</h3>
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<p>
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While you can return pages that happen to be at the end of the heap to
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the system, there is no way to use the <tt>sbrk</tt> interface to
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return unused pages in the middle of the heap. If you wish to do this,
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you will need to design a new or supplemental interface.
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</p>
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</body>
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</html>
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