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<title>Debugging with GDB: Blocks In Python</title>

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<a name="Blocks-In-Python"></a>
<div class="header">
<p>
Next: <a href="Symbols-In-Python.html#Symbols-In-Python" accesskey="n" rel="next">Symbols In Python</a>, Previous: <a href="Frames-In-Python.html#Frames-In-Python" accesskey="p" rel="previous">Frames In Python</a>, Up: <a href="Python-API.html#Python-API" accesskey="u" rel="up">Python API</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="Accessing-blocks-from-Python"></a>
<h4 class="subsubsection">23.2.2.26 Accessing blocks from Python</h4>

<a name="index-blocks-in-python"></a>
<a name="index-gdb_002eBlock"></a>

<p>In <small>GDB</small>, symbols are stored in blocks.  A block corresponds
roughly to a scope in the source code.  Blocks are organized
hierarchically, and are represented individually in Python as a
<code>gdb.Block</code>.  Blocks rely on debugging information being
available.
</p>
<p>A frame has a block.  Please see <a href="Frames-In-Python.html#Frames-In-Python">Frames In Python</a>, for a more
in-depth discussion of frames.
</p>
<p>The outermost block is known as the <em>global block</em>.  The global
block typically holds public global variables and functions.
</p>
<p>The block nested just inside the global block is the <em>static
block</em>.  The static block typically holds file-scoped variables and
functions.
</p>
<p><small>GDB</small> provides a method to get a block&rsquo;s superblock, but there
is currently no way to examine the sub-blocks of a block, or to
iterate over all the blocks in a symbol table (see <a href="Symbol-Tables-In-Python.html#Symbol-Tables-In-Python">Symbol Tables In Python</a>).
</p>
<p>Here is a short example that should help explain blocks:
</p>
<div class="smallexample">
<pre class="smallexample">/* This is in the global block.  */
int global;

/* This is in the static block.  */
static int file_scope;

/* 'function' is in the global block, and 'argument' is
   in a block nested inside of 'function'.  */
int function (int argument)
{
  /* 'local' is in a block inside 'function'.  It may or may
     not be in the same block as 'argument'.  */
  int local;

  {
     /* 'inner' is in a block whose superblock is the one holding
        'local'.  */
     int inner;

     /* If this call is expanded by the compiler, you may see
        a nested block here whose function is 'inline_function'
        and whose superblock is the one holding 'inner'.  */
     inline_function ();
  }
}
</pre></div>

<p>A <code>gdb.Block</code> is iterable.  The iterator returns the symbols
(see <a href="Symbols-In-Python.html#Symbols-In-Python">Symbols In Python</a>) local to the block.  Python programs
should not assume that a specific block object will always contain a
given symbol, since changes in <small>GDB</small> features and
infrastructure may cause symbols move across blocks in a symbol
table.  You can also use Python&rsquo;s <em>dictionary syntax</em> to access
variables in this block, e.g.:
</p>
<div class="smallexample">
<pre class="smallexample">symbol = some_block['variable']  # symbol is of type gdb.Symbol
</pre></div>

<p>The following block-related functions are available in the <code>gdb</code>
module:
</p>
<a name="index-gdb_002eblock_005ffor_005fpc"></a>
<dl>
<dt><a name="index-gdb_002eblock_005ffor_005fpc-1"></a>Function: <strong>gdb.block_for_pc</strong> <em>(pc)</em></dt>
<dd><p>Return the innermost <code>gdb.Block</code> containing the given <var>pc</var>
value.  If the block cannot be found for the <var>pc</var> value specified,
the function will return <code>None</code>.  This is identical to
<code>gdb.current_progspace().block_for_pc(pc)</code> and is included for
historical compatibility.
</p></dd></dl>

<p>A <code>gdb.Block</code> object has the following methods:
</p>
<dl>
<dt><a name="index-Block_002eis_005fvalid"></a>Function: <strong>Block.is_valid</strong> <em>()</em></dt>
<dd><p>Returns <code>True</code> if the <code>gdb.Block</code> object is valid,
<code>False</code> if not.  A block object can become invalid if the block it
refers to doesn&rsquo;t exist anymore in the inferior.  All other
<code>gdb.Block</code> methods will throw an exception if it is invalid at
the time the method is called.  The block&rsquo;s validity is also checked
during iteration over symbols of the block.
</p></dd></dl>

<p>A <code>gdb.Block</code> object has the following attributes:
</p>
<dl>
<dt><a name="index-Block_002estart"></a>Variable: <strong>Block.start</strong></dt>
<dd><p>The start address of the block.  This attribute is not writable.
</p></dd></dl>

<dl>
<dt><a name="index-Block_002eend"></a>Variable: <strong>Block.end</strong></dt>
<dd><p>One past the last address that appears in the block.  This attribute
is not writable.
</p></dd></dl>

<dl>
<dt><a name="index-Block_002efunction"></a>Variable: <strong>Block.function</strong></dt>
<dd><p>The name of the block represented as a <code>gdb.Symbol</code>.  If the
block is not named, then this attribute holds <code>None</code>.  This
attribute is not writable.
</p>
<p>For ordinary function blocks, the superblock is the static block.
However, you should note that it is possible for a function block to
have a superblock that is not the static block &ndash; for instance this
happens for an inlined function.
</p></dd></dl>

<dl>
<dt><a name="index-Block_002esuperblock"></a>Variable: <strong>Block.superblock</strong></dt>
<dd><p>The block containing this block.  If this parent block does not exist,
this attribute holds <code>None</code>.  This attribute is not writable.
</p></dd></dl>

<dl>
<dt><a name="index-Block_002eglobal_005fblock"></a>Variable: <strong>Block.global_block</strong></dt>
<dd><p>The global block associated with this block.  This attribute is not
writable.
</p></dd></dl>

<dl>
<dt><a name="index-Block_002estatic_005fblock"></a>Variable: <strong>Block.static_block</strong></dt>
<dd><p>The static block associated with this block.  This attribute is not
writable.
</p></dd></dl>

<dl>
<dt><a name="index-Block_002eis_005fglobal"></a>Variable: <strong>Block.is_global</strong></dt>
<dd><p><code>True</code> if the <code>gdb.Block</code> object is a global block,
<code>False</code> if not.  This attribute is not
writable.
</p></dd></dl>

<dl>
<dt><a name="index-Block_002eis_005fstatic"></a>Variable: <strong>Block.is_static</strong></dt>
<dd><p><code>True</code> if the <code>gdb.Block</code> object is a static block,
<code>False</code> if not.  This attribute is not writable.
</p></dd></dl>

<hr>
<div class="header">
<p>
Next: <a href="Symbols-In-Python.html#Symbols-In-Python" accesskey="n" rel="next">Symbols In Python</a>, Previous: <a href="Frames-In-Python.html#Frames-In-Python" accesskey="p" rel="previous">Frames In Python</a>, Up: <a href="Python-API.html#Python-API" accesskey="u" rel="up">Python API</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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