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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml"><head><title>Single Thread Example</title><meta name="generator" content="DocBook XSL-NS Stylesheets V1.76.1"/><meta name="keywords" content=" ISO C++ , allocator "/><meta name="keywords" content=" ISO C++ , library "/><meta name="keywords" content=" ISO C++ , runtime , library "/><link rel="home" href="../index.html" title="The GNU C++ Library"/><link rel="up" href="mt_allocator.html" title="Chapter 20. The mt_allocator"/><link rel="prev" href="bk01pt03ch20s03.html" title="Implementation"/><link rel="next" href="bk01pt03ch20s05.html" title="Multiple Thread Example"/></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Single Thread Example</th></tr><tr><td align="left"><a accesskey="p" href="bk01pt03ch20s03.html">Prev</a> </td><th width="60%" align="center">Chapter 20. The mt_allocator</th><td align="right"> <a accesskey="n" href="bk01pt03ch20s05.html">Next</a></td></tr></table><hr/></div><div class="section" title="Single Thread Example"><div class="titlepage"><div><div><h2 class="title"><a id="allocator.mt.example_single"/>Single Thread Example</h2></div></div></div><p>
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Let's start by describing how the data on a freelist is laid out in memory.
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This is the first two blocks in freelist for thread id 3 in bin 3 (8 bytes):
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</p><pre class="programlisting">
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+----------------+
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| next* ---------|--+ (_S_bin[ 3 ].first[ 3 ] points here)
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+----------------+ |
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| thread_id = 3 | |
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+----------------+ |
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| DATA | | (A pointer to here is what is returned to the
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| | | the application when needed)
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+----------------+ |
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+----------------+ |
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| next* |<-+ (If next == NULL it's the last one on the list)
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+----------------+
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| thread_id = 3 |
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+----------------+
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| DATA |
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+----------------+
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</pre><p>
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With this in mind we simplify things a bit for a while and say that there is
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only one thread (a ST application). In this case all operations are made to
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what is referred to as the global pool - thread id 0 (No thread may be
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assigned this id since they span from 1 to _S_max_threads in a MT application).
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</p><p>
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When the application requests memory (calling allocate()) we first look at the
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requested size and if this is > _S_max_bytes we call new() directly and return.
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</p><p>
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If the requested size is within limits we start by finding out from which
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bin we should serve this request by looking in _S_binmap.
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</p><p>
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A quick look at _S_bin[ bin ].first[ 0 ] tells us if there are any blocks of
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this size on the freelist (0). If this is not NULL - fine, just remove the
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block that _S_bin[ bin ].first[ 0 ] points to from the list,
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update _S_bin[ bin ].first[ 0 ] and return a pointer to that blocks data.
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</p><p>
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If the freelist is empty (the pointer is NULL) we must get memory from the
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system and build us a freelist within this memory. All requests for new memory
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is made in chunks of _S_chunk_size. Knowing the size of a block_record and
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the bytes that this bin stores we then calculate how many blocks we can create
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within this chunk, build the list, remove the first block, update the pointer
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(_S_bin[ bin ].first[ 0 ]) and return a pointer to that blocks data.
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</p><p>
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Deallocation is equally simple; the pointer is casted back to a block_record
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pointer, lookup which bin to use based on the size, add the block to the front
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of the global freelist and update the pointer as needed
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(_S_bin[ bin ].first[ 0 ]).
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</p><p>
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The decision to add deallocated blocks to the front of the freelist was made
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after a set of performance measurements that showed that this is roughly 10%
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faster than maintaining a set of "last pointers" as well.
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</p></div><div class="navfooter"><hr/><table width="100%" summary="Navigation footer"><tr><td align="left"><a accesskey="p" href="bk01pt03ch20s03.html">Prev</a> </td><td align="center"><a accesskey="u" href="mt_allocator.html">Up</a></td><td align="right"> <a accesskey="n" href="bk01pt03ch20s05.html">Next</a></td></tr><tr><td align="left" valign="top">Implementation </td><td align="center"><a accesskey="h" href="../index.html">Home</a></td><td align="right" valign="top"> Multiple Thread Example</td></tr></table></div></body></html>
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