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<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 “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 bytes and
arrays. The bytes 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 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>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="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 bytes 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-dimensional 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 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="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><big>)</big><a class="headerlink" href="#PyBuffer_Release" title="Permalink to this definition">¶</a></dt>
<dd>Release the buffer <em>view</em> over <em>obj</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>, 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="memoryview-objects">
<h2>MemoryView objects<a class="headerlink" href="#memoryview-objects" title="Permalink to this headline">¶</a></h2>
<p>A memoryview object is an extended buffer object that could replace the buffer
object (but doesn’t have to as that could be kept as a simple 1-d memoryview
object). It, unlike <a title="Py_buffer" class="reference internal" href="#Py_buffer"><tt class="xref docutils literal"><span class="pre">Py_buffer</span></tt></a>, is a Python object (exposed as
<a title="memoryview" class="reference external" href="../library/stdtypes.html#memoryview"><tt class="xref docutils literal"><span class="pre">memoryview</span></tt></a> in <a title="The module that provides the built-in namespace." class="reference external" href="../library/builtins.html#module-builtins"><tt class="xref docutils literal"><span class="pre">builtins</span></tt></a>), so it can be used with Python code.</p>
<dl class="cfunction">
<dt id="PyMemoryView_FromObject">
<a title="PyObject" class="reference external" href="structures.html#PyObject">PyObject</a>* <tt class="descname">PyMemoryView_FromObject</tt><big>(</big><a title="PyObject" class="reference external" href="structures.html#PyObject">PyObject</a><em> *obj</em><big>)</big><a class="headerlink" href="#PyMemoryView_FromObject" title="Permalink to this definition">¶</a></dt>
<dd>Return a memoryview object from an object that defines the buffer interface.</dd></dl>
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<li><a class="reference external" href="">Buffer Objects</a><ul>
<li><a class="reference external" href="#buffer-related-functions">Buffer related functions</a></li>
<li><a class="reference external" href="#memoryview-objects">MemoryView objects</a></li>
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