Edit File: buffer.html
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <title>Buffer Objects — Python v2.6.6 documentation</title> <link rel="stylesheet" href="../_static/default.css" type="text/css" /> <link rel="stylesheet" href="../_static/pygments.css" type="text/css" /> <script type="text/javascript"> var DOCUMENTATION_OPTIONS = { URL_ROOT: '../', VERSION: '2.6.6', COLLAPSE_MODINDEX: false, FILE_SUFFIX: '.html', HAS_SOURCE: true }; </script> <script type="text/javascript" src="../_static/jquery.js"></script> <script type="text/javascript" src="../_static/doctools.js"></script> <link rel="search" type="application/opensearchdescription+xml" title="Search within Python v2.6.6 documentation" href="../_static/opensearch.xml"/> <link rel="author" title="About these documents" href="../about.html" /> <link rel="copyright" title="Copyright" href="../copyright.html" /> <link rel="top" title="Python v2.6.6 documentation" href="../index.html" /> <link rel="up" title="Concrete Objects Layer" href="concrete.html" /> <link rel="next" title="Tuple Objects" href="tuple.html" /> <link rel="prev" title="Unicode Objects and Codecs" href="unicode.html" /> <link rel="shortcut icon" type="image/png" href="../_static/py.png" /> </head> <body> <div class="related"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" accesskey="I">index</a></li> <li class="right" > <a href="../modindex.html" title="Global Module Index" accesskey="M">modules</a> |</li> <li class="right" > <a href="tuple.html" title="Tuple Objects" accesskey="N">next</a> |</li> <li class="right" > <a href="unicode.html" title="Unicode Objects and Codecs" accesskey="P">previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="../index.html">Python v2.6.6 documentation</a> »</li> <li><a href="index.html" >Python/C API Reference Manual</a> »</li> <li><a href="concrete.html" accesskey="U">Concrete Objects Layer</a> »</li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body"> <div class="section" id="buffer-objects"> <span id="bufferobjects"></span><h1>Buffer Objects<a class="headerlink" href="#buffer-objects" title="Permalink to this headline">¶</a></h1> <p id="index-0">Python objects implemented in C can export a group of functions called the “buffer interface.” These functions can be used by an object to expose its data in a raw, byte-oriented format. Clients of the object can use the buffer interface to access the object data directly, without needing to copy it first.</p> <p>Two examples of objects that support the buffer interface are strings and arrays. The string object exposes the character contents in the buffer interface’s byte-oriented form. An array can also expose its contents, but it should be noted that array elements may be multi-byte values.</p> <p>An example user of the buffer interface is the file object’s <tt class="xref docutils literal"><span class="pre">write()</span></tt> method. Any object that can export a series of bytes through the buffer interface can be written to a file. There are a number of format codes to <a title="PyArg_ParseTuple" class="reference external" href="arg.html#PyArg_ParseTuple"><tt class="xref docutils literal"><span class="pre">PyArg_ParseTuple()</span></tt></a> that operate against an object’s buffer interface, returning data from the target object.</p> <p>Starting from version 1.6, Python has been providing Python-level buffer objects and a C-level buffer API so that any built-in or used-defined type can expose its characteristics. Both, however, have been deprecated because of various shortcomings, and have been officially removed in Python 3.0 in favour of a new C-level buffer API and a new Python-level object named <tt class="xref docutils literal"><span class="pre">memoryview</span></tt>.</p> <p>The new buffer API has been backported to Python 2.6, and the <tt class="xref docutils literal"><span class="pre">memoryview</span></tt> object has been backported to Python 2.7. It is strongly advised to use them rather than the old APIs, unless you are blocked from doing so for compatibility reasons.</p> <div class="section" id="the-new-style-py-buffer-struct"> <h2>The new-style Py_buffer struct<a class="headerlink" href="#the-new-style-py-buffer-struct" title="Permalink to this headline">¶</a></h2> <dl class="ctype"> <dt id="Py_buffer"> <tt class="descname">Py_buffer</tt><a class="headerlink" href="#Py_buffer" title="Permalink to this definition">¶</a></dt> <dd><dl class="cmember"> <dt id="buf"> void *<tt class="descname">buf</tt><a class="headerlink" href="#buf" title="Permalink to this definition">¶</a></dt> <dd>A pointer to the start of the memory for the object.</dd></dl> <dl class="cmember"> <dt> Py_ssize_t <tt class="descname">len</tt></dt> <dd>The total length of the memory in bytes.</dd></dl> <dl class="cmember"> <dt id="readonly"> int <tt class="descname">readonly</tt><a class="headerlink" href="#readonly" title="Permalink to this definition">¶</a></dt> <dd>An indicator of whether the buffer is read only.</dd></dl> <dl class="cmember"> <dt> const char *<tt class="descname">format</tt></dt> <dd>A <em>NULL</em> terminated string in <a title="Interpret strings as packed binary data." class="reference external" href="../library/struct.html#module-struct"><tt class="xref docutils literal"><span class="pre">struct</span></tt></a> module style syntax giving the contents of the elements available through the buffer. If this is <em>NULL</em>, <tt class="docutils literal"><span class="pre">"B"</span></tt> (unsigned bytes) is assumed.</dd></dl> <dl class="cmember"> <dt id="ndim"> int <tt class="descname">ndim</tt><a class="headerlink" href="#ndim" title="Permalink to this definition">¶</a></dt> <dd>The number of dimensions the memory represents as a multi-dimensional array. If it is 0, <a title="strides" class="reference internal" href="#strides"><tt class="xref docutils literal"><span class="pre">strides</span></tt></a> and <a title="suboffsets" class="reference internal" href="#suboffsets"><tt class="xref docutils literal"><span class="pre">suboffsets</span></tt></a> must be <em>NULL</em>.</dd></dl> <dl class="cmember"> <dt id="shape"> Py_ssize_t *<tt class="descname">shape</tt><a class="headerlink" href="#shape" title="Permalink to this definition">¶</a></dt> <dd>An array of <tt class="xref docutils literal"><span class="pre">Py_ssize_t</span></tt>s the length of <a title="ndim" class="reference internal" href="#ndim"><tt class="xref docutils literal"><span class="pre">ndim</span></tt></a> giving the shape of the memory as a multi-dimensional array. Note that <tt class="docutils literal"><span class="pre">((*shape)[0]</span> <span class="pre">*</span> <span class="pre">...</span> <span class="pre">*</span> <span class="pre">(*shape)[ndims-1])*itemsize</span></tt> should be equal to <tt class="xref docutils literal"><span class="pre">len</span></tt>.</dd></dl> <dl class="cmember"> <dt id="strides"> Py_ssize_t *<tt class="descname">strides</tt><a class="headerlink" href="#strides" title="Permalink to this definition">¶</a></dt> <dd>An array of <tt class="xref docutils literal"><span class="pre">Py_ssize_t</span></tt>s the length of <a title="ndim" class="reference internal" href="#ndim"><tt class="xref docutils literal"><span class="pre">ndim</span></tt></a> giving the number of bytes to skip to get to a new element in each dimension.</dd></dl> <dl class="cmember"> <dt id="suboffsets"> Py_ssize_t *<tt class="descname">suboffsets</tt><a class="headerlink" href="#suboffsets" title="Permalink to this definition">¶</a></dt> <dd><p>An array of <tt class="xref docutils literal"><span class="pre">Py_ssize_t</span></tt>s the length of <a title="ndim" class="reference internal" href="#ndim"><tt class="xref docutils literal"><span class="pre">ndim</span></tt></a>. If these suboffset numbers are greater than or equal to 0, then the value stored along the indicated dimension is a pointer and the suboffset value dictates how many bytes to add to the pointer after de-referencing. A suboffset value that it negative indicates that no de-referencing should occur (striding in a contiguous memory block).</p> <p>Here is a function that returns a pointer to the element in an N-D array pointed to by an N-dimesional index when there are both non-NULL strides and suboffsets:</p> <div class="highlight-c"><div class="highlight"><pre><span class="kt">void</span> <span class="o">*</span><span class="nf">get_item_pointer</span><span class="p">(</span><span class="kt">int</span> <span class="n">ndim</span><span class="p">,</span> <span class="kt">void</span> <span class="o">*</span><span class="n">buf</span><span class="p">,</span> <span class="n">Py_ssize_t</span> <span class="o">*</span><span class="n">strides</span><span class="p">,</span> <span class="n">Py_ssize_t</span> <span class="o">*</span><span class="n">suboffsets</span><span class="p">,</span> <span class="n">Py_ssize_t</span> <span class="o">*</span><span class="n">indices</span><span class="p">)</span> <span class="p">{</span> <span class="kt">char</span> <span class="o">*</span><span class="n">pointer</span> <span class="o">=</span> <span class="p">(</span><span class="kt">char</span><span class="o">*</span><span class="p">)</span><span class="n">buf</span><span class="p">;</span> <span class="kt">int</span> <span class="n">i</span><span class="p">;</span> <span class="k">for</span> <span class="p">(</span><span class="n">i</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span> <span class="n">i</span> <span class="o"><</span> <span class="n">ndim</span><span class="p">;</span> <span class="n">i</span><span class="o">++</span><span class="p">)</span> <span class="p">{</span> <span class="n">pointer</span> <span class="o">+=</span> <span class="n">strides</span><span class="p">[</span><span class="n">i</span><span class="p">]</span> <span class="o">*</span> <span class="n">indices</span><span class="p">[</span><span class="n">i</span><span class="p">];</span> <span class="k">if</span> <span class="p">(</span><span class="n">suboffsets</span><span class="p">[</span><span class="n">i</span><span class="p">]</span> <span class="o">>=</span><span class="mi">0</span> <span class="p">)</span> <span class="p">{</span> <span class="n">pointer</span> <span class="o">=</span> <span class="o">*</span><span class="p">((</span><span class="kt">char</span><span class="o">**</span><span class="p">)</span><span class="n">pointer</span><span class="p">)</span> <span class="o">+</span> <span class="n">suboffsets</span><span class="p">[</span><span class="n">i</span><span class="p">];</span> <span class="p">}</span> <span class="p">}</span> <span class="k">return</span> <span class="p">(</span><span class="kt">void</span><span class="o">*</span><span class="p">)</span><span class="n">pointer</span><span class="p">;</span> <span class="p">}</span> </pre></div> </div> </dd></dl> <dl class="cmember"> <dt id="itemsize"> Py_ssize_t <tt class="descname">itemsize</tt><a class="headerlink" href="#itemsize" title="Permalink to this definition">¶</a></dt> <dd>This is a storage for the itemsize (in bytes) of each element of the shared memory. It is technically un-necessary as it can be obtained using <a title="PyBuffer_SizeFromFormat" class="reference internal" href="#PyBuffer_SizeFromFormat"><tt class="xref docutils literal"><span class="pre">PyBuffer_SizeFromFormat()</span></tt></a>, however an exporter may know this information without parsing the format string and it is necessary to know the itemsize for proper interpretation of striding. Therefore, storing it is more convenient and faster.</dd></dl> <dl class="cmember"> <dt id="internal"> void *<tt class="descname">internal</tt><a class="headerlink" href="#internal" title="Permalink to this definition">¶</a></dt> <dd>This is for use internally by the exporting object. For example, this might be re-cast as an integer by the exporter and used to store flags about whether or not the shape, strides, and suboffsets arrays must be freed when the buffer is released. The consumer should never alter this value.</dd></dl> </dd></dl> </div> <div class="section" id="buffer-related-functions"> <h2>Buffer related functions<a class="headerlink" href="#buffer-related-functions" title="Permalink to this headline">¶</a></h2> <dl class="cfunction"> <dt id="PyObject_CheckBuffer"> int <tt class="descname">PyObject_CheckBuffer</tt><big>(</big><a title="PyObject" class="reference external" href="structures.html#PyObject">PyObject</a><em> *obj</em><big>)</big><a class="headerlink" href="#PyObject_CheckBuffer" title="Permalink to this definition">¶</a></dt> <dd>Return 1 if <em>obj</em> supports the buffer interface otherwise 0.</dd></dl> <dl class="cfunction"> <dt id="PyObject_GetBuffer"> int <tt class="descname">PyObject_GetBuffer</tt><big>(</big><a title="PyObject" class="reference external" href="structures.html#PyObject">PyObject</a><em> *obj</em>, <a title="Py_buffer" class="reference internal" href="#Py_buffer">Py_buffer</a><em> *view</em>, int<em> flags</em><big>)</big><a class="headerlink" href="#PyObject_GetBuffer" title="Permalink to this definition">¶</a></dt> <dd><p>Export <em>obj</em> into a <a title="Py_buffer" class="reference internal" href="#Py_buffer"><tt class="xref docutils literal"><span class="pre">Py_buffer</span></tt></a>, <em>view</em>. These arguments must never be <em>NULL</em>. The <em>flags</em> argument is a bit field indicating what kind of buffer the caller is prepared to deal with and therefore what kind of buffer the exporter is allowed to return. The buffer interface allows for complicated memory sharing possibilities, but some caller may not be able to handle all the complexity but may want to see if the exporter will let them take a simpler view to its memory.</p> <p>Some exporters may not be able to share memory in every possible way and may need to raise errors to signal to some consumers that something is just not possible. These errors should be a <tt class="xref docutils literal"><span class="pre">BufferError</span></tt> unless there is another error that is actually causing the problem. The exporter can use flags information to simplify how much of the <a title="Py_buffer" class="reference internal" href="#Py_buffer"><tt class="xref docutils literal"><span class="pre">Py_buffer</span></tt></a> structure is filled in with non-default values and/or raise an error if the object can’t support a simpler view of its memory.</p> <p>0 is returned on success and -1 on error.</p> <p>The following table gives possible values to the <em>flags</em> arguments.</p> <table border="1" class="docutils"> <colgroup> <col width="37%" /> <col width="63%" /> </colgroup> <thead valign="bottom"> <tr><th class="head">Flag</th> <th class="head">Description</th> </tr> </thead> <tbody valign="top"> <tr><td><tt class="xref docutils literal"><span class="pre">PyBUF_SIMPLE</span></tt></td> <td>This is the default flag state. The returned buffer may or may not have writable memory. The format of the data will be assumed to be unsigned bytes. This is a “stand-alone” flag constant. It never needs to be ‘|’d to the others. The exporter will raise an error if it cannot provide such a contiguous buffer of bytes.</td> </tr> <tr><td><tt class="xref docutils literal"><span class="pre">PyBUF_WRITABLE</span></tt></td> <td>The returned buffer must be writable. If it is not writable, then raise an error.</td> </tr> <tr><td><tt class="xref docutils literal"><span class="pre">PyBUF_STRIDES</span></tt></td> <td>This implies <tt class="xref docutils literal"><span class="pre">PyBUF_ND</span></tt>. The returned buffer must provide strides information (i.e. the strides cannot be NULL). This would be used when the consumer can handle strided, discontiguous arrays. Handling strides automatically assumes you can handle shape. The exporter can raise an error if a strided representation of the data is not possible (i.e. without the suboffsets).</td> </tr> <tr><td><tt class="xref docutils literal"><span class="pre">PyBUF_ND</span></tt></td> <td>The returned buffer must provide shape information. The memory will be assumed C-style contiguous (last dimension varies the fastest). The exporter may raise an error if it cannot provide this kind of contiguous buffer. If this is not given then shape will be <em>NULL</em>.</td> </tr> <tr><td><tt class="xref docutils literal"><span class="pre">PyBUF_C_CONTIGUOUS</span></tt> <tt class="xref docutils literal"><span class="pre">PyBUF_F_CONTIGUOUS</span></tt> <tt class="xref docutils literal"><span class="pre">PyBUF_ANY_CONTIGUOUS</span></tt></td> <td>These flags indicate that the contiguity returned buffer must be respectively, C-contiguous (last dimension varies the fastest), Fortran contiguous (first dimension varies the fastest) or either one. All of these flags imply <tt class="xref docutils literal"><span class="pre">PyBUF_STRIDES</span></tt> and guarantee that the strides buffer info structure will be filled in correctly.</td> </tr> <tr><td><tt class="xref docutils literal"><span class="pre">PyBUF_INDIRECT</span></tt></td> <td>This flag indicates the returned buffer must have suboffsets information (which can be NULL if no suboffsets are needed). This can be used when the consumer can handle indirect array referencing implied by these suboffsets. This implies <tt class="xref docutils literal"><span class="pre">PyBUF_STRIDES</span></tt>.</td> </tr> <tr><td><tt class="xref docutils literal"><span class="pre">PyBUF_FORMAT</span></tt></td> <td>The returned buffer must have true format information if this flag is provided. This would be used when the consumer is going to be checking for what ‘kind’ of data is actually stored. An exporter should always be able to provide this information if requested. If format is not explicitly requested then the format must be returned as <em>NULL</em> (which means <tt class="docutils literal"><span class="pre">'B'</span></tt>, or unsigned bytes)</td> </tr> <tr><td><tt class="xref docutils literal"><span class="pre">PyBUF_STRIDED</span></tt></td> <td>This is equivalent to <tt class="docutils literal"><span class="pre">(PyBUF_STRIDES</span> <span class="pre">|</span> <span class="pre">PyBUF_WRITABLE)</span></tt>.</td> </tr> <tr><td><tt class="xref docutils literal"><span class="pre">PyBUF_STRIDED_RO</span></tt></td> <td>This is equivalent to <tt class="docutils literal"><span class="pre">(PyBUF_STRIDES)</span></tt>.</td> </tr> <tr><td><tt class="xref docutils literal"><span class="pre">PyBUF_RECORDS</span></tt></td> <td>This is equivalent to <tt class="docutils literal"><span class="pre">(PyBUF_STRIDES</span> <span class="pre">|</span> <span class="pre">PyBUF_FORMAT</span> <span class="pre">|</span> <span class="pre">PyBUF_WRITABLE)</span></tt>.</td> </tr> <tr><td><tt class="xref docutils literal"><span class="pre">PyBUF_RECORDS_RO</span></tt></td> <td>This is equivalent to <tt class="docutils literal"><span class="pre">(PyBUF_STRIDES</span> <span class="pre">|</span> <span class="pre">PyBUF_FORMAT)</span></tt>.</td> </tr> <tr><td><tt class="xref docutils literal"><span class="pre">PyBUF_FULL</span></tt></td> <td>This is equivalent to <tt class="docutils literal"><span class="pre">(PyBUF_INDIRECT</span> <span class="pre">|</span> <span class="pre">PyBUF_FORMAT</span> <span class="pre">|</span> <span class="pre">PyBUF_WRITABLE)</span></tt>.</td> </tr> <tr><td><tt class="xref docutils literal"><span class="pre">PyBUF_FULL_RO</span></tt></td> <td>This is equivalent to <tt class="docutils literal"><span class="pre">(PyBUF_INDIRECT</span> <span class="pre">|</span> <span class="pre">PyBUF_FORMAT)</span></tt>.</td> </tr> <tr><td><tt class="xref docutils literal"><span class="pre">PyBUF_CONTIG</span></tt></td> <td>This is equivalent to <tt class="docutils literal"><span class="pre">(PyBUF_ND</span> <span class="pre">|</span> <span class="pre">PyBUF_WRITABLE)</span></tt>.</td> </tr> <tr><td><tt class="xref docutils literal"><span class="pre">PyBUF_CONTIG_RO</span></tt></td> <td>This is equivalent to <tt class="docutils literal"><span class="pre">(PyBUF_ND)</span></tt>.</td> </tr> </tbody> </table> </dd></dl> <dl class="cfunction"> <dt id="PyBuffer_Release"> void <tt class="descname">PyBuffer_Release</tt><big>(</big><a title="Py_buffer" class="reference internal" href="#Py_buffer">Py_buffer</a><em> *view</em><big>)</big><a class="headerlink" href="#PyBuffer_Release" title="Permalink to this definition">¶</a></dt> <dd>Release the buffer <em>view</em>. This should be called when the buffer is no longer being used as it may free memory from it.</dd></dl> <dl class="cfunction"> <dt id="PyBuffer_SizeFromFormat"> Py_ssize_t <tt class="descname">PyBuffer_SizeFromFormat</tt><big>(</big>const char<em> *</em><big>)</big><a class="headerlink" href="#PyBuffer_SizeFromFormat" title="Permalink to this definition">¶</a></dt> <dd>Return the implied <tt class="xref docutils literal"><span class="pre">~Py_buffer.itemsize</span></tt> from the struct-stype <tt class="xref docutils literal"><span class="pre">~Py_buffer.format</span></tt>.</dd></dl> <dl class="cfunction"> <dt id="PyObject_CopyToObject"> int <tt class="descname">PyObject_CopyToObject</tt><big>(</big><a title="PyObject" class="reference external" href="structures.html#PyObject">PyObject</a><em> *obj</em>, void<em> *buf</em>, Py_ssize_t<em> len</em>, char<em> fortran</em><big>)</big><a class="headerlink" href="#PyObject_CopyToObject" title="Permalink to this definition">¶</a></dt> <dd>Copy <em>len</em> bytes of data pointed to by the contiguous chunk of memory pointed to by <em>buf</em> into the buffer exported by obj. The buffer must of course be writable. Return 0 on success and return -1 and raise an error on failure. If the object does not have a writable buffer, then an error is raised. If <em>fortran</em> is <tt class="docutils literal"><span class="pre">'F'</span></tt>, then if the object is multi-dimensional, then the data will be copied into the array in Fortran-style (first dimension varies the fastest). If <em>fortran</em> is <tt class="docutils literal"><span class="pre">'C'</span></tt>, then the data will be copied into the array in C-style (last dimension varies the fastest). If <em>fortran</em> is <tt class="docutils literal"><span class="pre">'A'</span></tt>, then it does not matter and the copy will be made in whatever way is more efficient.</dd></dl> <dl class="cfunction"> <dt id="PyBuffer_IsContiguous"> int <tt class="descname">PyBuffer_IsContiguous</tt><big>(</big><a title="Py_buffer" class="reference internal" href="#Py_buffer">Py_buffer</a><em> *view</em>, char<em> fortran</em><big>)</big><a class="headerlink" href="#PyBuffer_IsContiguous" title="Permalink to this definition">¶</a></dt> <dd>Return 1 if the memory defined by the <em>view</em> is C-style (<em>fortran</em> is <tt class="docutils literal"><span class="pre">'C'</span></tt>) or Fortran-style (<em>fortran</em> is <tt class="docutils literal"><span class="pre">'F'</span></tt>) contiguous or either one (<em>fortran</em> is <tt class="docutils literal"><span class="pre">'A'</span></tt>). Return 0 otherwise.</dd></dl> <dl class="cfunction"> <dt id="PyBuffer_FillContiguousStrides"> void <tt class="descname">PyBuffer_FillContiguousStrides</tt><big>(</big>int<em> ndim</em>, Py_ssize_t<em> *shape</em>, Py_ssize_t<em> *strides</em>, Py_ssize_t<em> itemsize</em>, char<em> fortran</em><big>)</big><a class="headerlink" href="#PyBuffer_FillContiguousStrides" title="Permalink to this definition">¶</a></dt> <dd>Fill the <em>strides</em> array with byte-strides of a contiguous (C-style if <em>fortran</em> is <tt class="docutils literal"><span class="pre">'C'</span></tt> or Fortran-style if <em>fortran</em> is <tt class="docutils literal"><span class="pre">'F'</span></tt> array of the given shape with the given number of bytes per element.</dd></dl> <dl class="cfunction"> <dt id="PyBuffer_FillInfo"> int <tt class="descname">PyBuffer_FillInfo</tt><big>(</big><a title="Py_buffer" class="reference internal" href="#Py_buffer">Py_buffer</a><em> *view</em>, <a title="PyObject" class="reference external" href="structures.html#PyObject">PyObject</a><em> *obj</em>, void<em> *buf</em>, Py_ssize_t<em> len</em>, int<em> readonly</em>, int<em> infoflags</em><big>)</big><a class="headerlink" href="#PyBuffer_FillInfo" title="Permalink to this definition">¶</a></dt> <dd>Fill in a buffer-info structure, <em>view</em>, correctly for an exporter that can only share a contiguous chunk of memory of “unsigned bytes” of the given length. Return 0 on success and -1 (with raising an error) on error.</dd></dl> </div> <div class="section" id="old-style-buffer-objects"> <h2>Old-style buffer objects<a class="headerlink" href="#old-style-buffer-objects" title="Permalink to this headline">¶</a></h2> <p id="index-1">More information on the buffer interface is provided in the section <a class="reference external" href="typeobj.html#buffer-structs"><em>Buffer Object Structures</em></a>, under the description for <a title="PyBufferProcs" class="reference external" href="typeobj.html#PyBufferProcs"><tt class="xref docutils literal"><span class="pre">PyBufferProcs</span></tt></a>.</p> <p>A “buffer object” is defined in the <tt class="docutils literal"><span class="pre">bufferobject.h</span></tt> header (included by <tt class="docutils literal"><span class="pre">Python.h</span></tt>). These objects look very similar to string objects at the Python programming level: they support slicing, indexing, concatenation, and some other standard string operations. However, their data can come from one of two sources: from a block of memory, or from another object which exports the buffer interface.</p> <p>Buffer objects are useful as a way to expose the data from another object’s buffer interface to the Python programmer. They can also be used as a zero-copy slicing mechanism. Using their ability to reference a block of memory, it is possible to expose any data to the Python programmer quite easily. The memory could be a large, constant array in a C extension, it could be a raw block of memory for manipulation before passing to an operating system library, or it could be used to pass around structured data in its native, in-memory format.</p> <dl class="ctype"> <dt id="PyBufferObject"> <tt class="descname">PyBufferObject</tt><a class="headerlink" href="#PyBufferObject" title="Permalink to this definition">¶</a></dt> <dd>This subtype of <a title="PyObject" class="reference external" href="structures.html#PyObject"><tt class="xref docutils literal"><span class="pre">PyObject</span></tt></a> represents a buffer object.</dd></dl> <dl class="cvar"> <dt id="PyBuffer_Type"> <a title="PyTypeObject" class="reference external" href="type.html#PyTypeObject">PyTypeObject</a> <tt class="descname">PyBuffer_Type</tt><a class="headerlink" href="#PyBuffer_Type" title="Permalink to this definition">¶</a></dt> <dd><p id="index-2">The instance of <a title="PyTypeObject" class="reference external" href="type.html#PyTypeObject"><tt class="xref docutils literal"><span class="pre">PyTypeObject</span></tt></a> which represents the Python buffer type; it is the same object as <tt class="docutils literal"><span class="pre">buffer</span></tt> and <tt class="docutils literal"><span class="pre">types.BufferType</span></tt> in the Python layer. .</p> </dd></dl> <dl class="cvar"> <dt id="Py_END_OF_BUFFER"> int <tt class="descname">Py_END_OF_BUFFER</tt><a class="headerlink" href="#Py_END_OF_BUFFER" title="Permalink to this definition">¶</a></dt> <dd>This constant may be passed as the <em>size</em> parameter to <a title="PyBuffer_FromObject" class="reference internal" href="#PyBuffer_FromObject"><tt class="xref docutils literal"><span class="pre">PyBuffer_FromObject()</span></tt></a> or <a title="PyBuffer_FromReadWriteObject" class="reference internal" href="#PyBuffer_FromReadWriteObject"><tt class="xref docutils literal"><span class="pre">PyBuffer_FromReadWriteObject()</span></tt></a>. It indicates that the new <a title="PyBufferObject" class="reference internal" href="#PyBufferObject"><tt class="xref docutils literal"><span class="pre">PyBufferObject</span></tt></a> should refer to <em>base</em> object from the specified <em>offset</em> to the end of its exported buffer. Using this enables the caller to avoid querying the <em>base</em> object for its length.</dd></dl> <dl class="cfunction"> <dt id="PyBuffer_Check"> int <tt class="descname">PyBuffer_Check</tt><big>(</big><a title="PyObject" class="reference external" href="structures.html#PyObject">PyObject</a><em> *p</em><big>)</big><a class="headerlink" href="#PyBuffer_Check" title="Permalink to this definition">¶</a></dt> <dd>Return true if the argument has type <a title="PyBuffer_Type" class="reference internal" href="#PyBuffer_Type"><tt class="xref docutils literal"><span class="pre">PyBuffer_Type</span></tt></a>.</dd></dl> <dl class="cfunction"> <dt id="PyBuffer_FromObject"> <a title="PyObject" class="reference external" href="structures.html#PyObject">PyObject</a>* <tt class="descname">PyBuffer_FromObject</tt><big>(</big><a title="PyObject" class="reference external" href="structures.html#PyObject">PyObject</a><em> *base</em>, Py_ssize_t<em> offset</em>, Py_ssize_t<em> size</em><big>)</big><a class="headerlink" href="#PyBuffer_FromObject" title="Permalink to this definition">¶</a></dt> <dd><em class="refcount">Return value: New reference.</em><p>Return a new read-only buffer object. This raises <a title="exceptions.TypeError" class="reference external" href="../library/exceptions.html#exceptions.TypeError"><tt class="xref docutils literal"><span class="pre">TypeError</span></tt></a> if <em>base</em> doesn’t support the read-only buffer protocol or doesn’t provide exactly one buffer segment, or it raises <a title="exceptions.ValueError" class="reference external" href="../library/exceptions.html#exceptions.ValueError"><tt class="xref docutils literal"><span class="pre">ValueError</span></tt></a> if <em>offset</em> is less than zero. The buffer will hold a reference to the <em>base</em> object, and the buffer’s contents will refer to the <em>base</em> object’s buffer interface, starting as position <em>offset</em> and extending for <em>size</em> bytes. If <em>size</em> is <a title="Py_END_OF_BUFFER" class="reference internal" href="#Py_END_OF_BUFFER"><tt class="xref docutils literal"><span class="pre">Py_END_OF_BUFFER</span></tt></a>, then the new buffer’s contents extend to the length of the <em>base</em> object’s exported buffer data.</p> <p class="versionchanged"> <span class="versionmodified">Changed in version 2.5: </span>This function used an <tt class="xref docutils literal"><span class="pre">int</span></tt> type for <em>offset</em> and <em>size</em>. This might require changes in your code for properly supporting 64-bit systems.</p> </dd></dl> <dl class="cfunction"> <dt id="PyBuffer_FromReadWriteObject"> <a title="PyObject" class="reference external" href="structures.html#PyObject">PyObject</a>* <tt class="descname">PyBuffer_FromReadWriteObject</tt><big>(</big><a title="PyObject" class="reference external" href="structures.html#PyObject">PyObject</a><em> *base</em>, Py_ssize_t<em> offset</em>, Py_ssize_t<em> size</em><big>)</big><a class="headerlink" href="#PyBuffer_FromReadWriteObject" title="Permalink to this definition">¶</a></dt> <dd><em class="refcount">Return value: New reference.</em><p>Return a new writable buffer object. Parameters and exceptions are similar to those for <a title="PyBuffer_FromObject" class="reference internal" href="#PyBuffer_FromObject"><tt class="xref docutils literal"><span class="pre">PyBuffer_FromObject()</span></tt></a>. If the <em>base</em> object does not export the writeable buffer protocol, then <a title="exceptions.TypeError" class="reference external" href="../library/exceptions.html#exceptions.TypeError"><tt class="xref docutils literal"><span class="pre">TypeError</span></tt></a> is raised.</p> <p class="versionchanged"> <span class="versionmodified">Changed in version 2.5: </span>This function used an <tt class="xref docutils literal"><span class="pre">int</span></tt> type for <em>offset</em> and <em>size</em>. This might require changes in your code for properly supporting 64-bit systems.</p> </dd></dl> <dl class="cfunction"> <dt id="PyBuffer_FromMemory"> <a title="PyObject" class="reference external" href="structures.html#PyObject">PyObject</a>* <tt class="descname">PyBuffer_FromMemory</tt><big>(</big>void<em> *ptr</em>, Py_ssize_t<em> size</em><big>)</big><a class="headerlink" href="#PyBuffer_FromMemory" title="Permalink to this definition">¶</a></dt> <dd><em class="refcount">Return value: New reference.</em><p>Return a new read-only buffer object that reads from a specified location in memory, with a specified size. The caller is responsible for ensuring that the memory buffer, passed in as <em>ptr</em>, is not deallocated while the returned buffer object exists. Raises <a title="exceptions.ValueError" class="reference external" href="../library/exceptions.html#exceptions.ValueError"><tt class="xref docutils literal"><span class="pre">ValueError</span></tt></a> if <em>size</em> is less than zero. Note that <a title="Py_END_OF_BUFFER" class="reference internal" href="#Py_END_OF_BUFFER"><tt class="xref docutils literal"><span class="pre">Py_END_OF_BUFFER</span></tt></a> may <em>not</em> be passed for the <em>size</em> parameter; <a title="exceptions.ValueError" class="reference external" href="../library/exceptions.html#exceptions.ValueError"><tt class="xref docutils literal"><span class="pre">ValueError</span></tt></a> will be raised in that case.</p> <p class="versionchanged"> <span class="versionmodified">Changed in version 2.5: </span>This function used an <tt class="xref docutils literal"><span class="pre">int</span></tt> type for <em>size</em>. This might require changes in your code for properly supporting 64-bit systems.</p> </dd></dl> <dl class="cfunction"> <dt id="PyBuffer_FromReadWriteMemory"> <a title="PyObject" class="reference external" href="structures.html#PyObject">PyObject</a>* <tt class="descname">PyBuffer_FromReadWriteMemory</tt><big>(</big>void<em> *ptr</em>, Py_ssize_t<em> size</em><big>)</big><a class="headerlink" href="#PyBuffer_FromReadWriteMemory" title="Permalink to this definition">¶</a></dt> <dd><em class="refcount">Return value: New reference.</em><p>Similar to <a title="PyBuffer_FromMemory" class="reference internal" href="#PyBuffer_FromMemory"><tt class="xref docutils literal"><span class="pre">PyBuffer_FromMemory()</span></tt></a>, but the returned buffer is writable.</p> <p class="versionchanged"> <span class="versionmodified">Changed in version 2.5: </span>This function used an <tt class="xref docutils literal"><span class="pre">int</span></tt> type for <em>size</em>. This might require changes in your code for properly supporting 64-bit systems.</p> </dd></dl> <dl class="cfunction"> <dt id="PyBuffer_New"> <a title="PyObject" class="reference external" href="structures.html#PyObject">PyObject</a>* <tt class="descname">PyBuffer_New</tt><big>(</big>Py_ssize_t<em> size</em><big>)</big><a class="headerlink" href="#PyBuffer_New" title="Permalink to this definition">¶</a></dt> <dd><em class="refcount">Return value: New reference.</em><p>Return a new writable buffer object that maintains its own memory buffer of <em>size</em> bytes. <a title="exceptions.ValueError" class="reference external" href="../library/exceptions.html#exceptions.ValueError"><tt class="xref docutils literal"><span class="pre">ValueError</span></tt></a> is returned if <em>size</em> is not zero or positive. Note that the memory buffer (as returned by <a title="PyObject_AsWriteBuffer" class="reference external" href="objbuffer.html#PyObject_AsWriteBuffer"><tt class="xref docutils literal"><span class="pre">PyObject_AsWriteBuffer()</span></tt></a>) is not specifically aligned.</p> <p class="versionchanged"> <span class="versionmodified">Changed in version 2.5: </span>This function used an <tt class="xref docutils literal"><span class="pre">int</span></tt> type for <em>size</em>. This might require changes in your code for properly supporting 64-bit systems.</p> </dd></dl> </div> </div> </div> </div> </div> <div class="sphinxsidebar"> <div class="sphinxsidebarwrapper"> <h3><a href="../contents.html">Table Of Contents</a></h3> <ul> <li><a class="reference external" href="#">Buffer Objects</a><ul> <li><a class="reference external" href="#the-new-style-py-buffer-struct">The new-style Py_buffer struct</a></li> <li><a class="reference external" href="#buffer-related-functions">Buffer related functions</a></li> <li><a class="reference external" href="#old-style-buffer-objects">Old-style buffer objects</a></li> </ul> </li> </ul> <h4>Previous topic</h4> <p class="topless"><a href="unicode.html" title="previous chapter">Unicode Objects and Codecs</a></p> <h4>Next topic</h4> <p class="topless"><a href="tuple.html" title="next chapter">Tuple Objects</a></p> <h3>This Page</h3> <ul class="this-page-menu"> <li><a href="../bugs.html">Report a Bug</a></li> <li><a href="../_sources/c-api/buffer.txt" rel="nofollow">Show Source</a></li> </ul> <div id="searchbox" style="display: none"> <h3>Quick search</h3> <form class="search" action="../search.html" method="get"> <input type="text" name="q" size="18" /> <input type="submit" value="Go" /> <input type="hidden" name="check_keywords" value="yes" /> <input type="hidden" name="area" value="default" /> </form> <p class="searchtip" style="font-size: 90%"> Enter search terms or a module, class or function name. </p> </div> <script type="text/javascript">$('#searchbox').show(0);</script> </div> </div> <div class="clearer"></div> </div> <div class="related"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" >index</a></li> <li class="right" > <a href="../modindex.html" title="Global Module Index" >modules</a> |</li> <li class="right" > <a href="tuple.html" title="Tuple Objects" >next</a> |</li> <li class="right" > <a href="unicode.html" title="Unicode Objects and Codecs" >previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="../index.html">Python v2.6.6 documentation</a> »</li> <li><a href="index.html" >Python/C API Reference Manual</a> »</li> <li><a href="concrete.html" >Concrete Objects Layer</a> »</li> </ul> </div> <div class="footer"> © <a href="../copyright.html">Copyright</a> 1990-2011, Python Software Foundation. <br /> The Python Software Foundation is a non-profit corporation. <a href="http://www.python.org/psf/donations/">Please donate.</a> <br /> Last updated on Jul 20, 2011. <a href="../bugs.html">Found a bug</a>? <br /> Created using <a href="http://sphinx.pocoo.org/">Sphinx</a> 0.6.6. </div> </body> </html>
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