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<h4 class="subsection">29.4.2 Writing JIT Debug Info Readers</h4>

<p><a name="index-writing-JIT-debug-info-readers-3112"></a>
As mentioned, a reader is essentially a shared object conforming to a
certain ABI.  This ABI is described in <samp><span class="file">jit-reader.h</span></samp>.

   <p><samp><span class="file">jit-reader.h</span></samp> defines the structures, macros and functions
required to write a reader.  It is installed (along with
<span class="sc">gdb</span>), in <samp><var>includedir</var><span class="file">/gdb</span></samp> where <var>includedir</var> is
the system include directory.

   <p>Readers need to be released under a GPL compatible license.  A reader
can be declared as released under such a license by placing the macro
<code>GDB_DECLARE_GPL_COMPATIBLE_READER</code> in a source file.

   <p>The entry point for readers is the symbol <code>gdb_init_reader</code>,
which is expected to be a function with the prototype

   <p><a name="index-gdb_005finit_005freader-3113"></a>
<pre class="smallexample">     extern struct gdb_reader_funcs *gdb_init_reader (void);
</pre>
   <p><a name="index-g_t_0040code_007bstruct-gdb_005freader_005ffuncs_007d-3114"></a>
<code>struct gdb_reader_funcs</code> contains a set of pointers to callback
functions.  These functions are executed to read the debug info
generated by the JIT compiler (<code>read</code>), to unwind stack frames
(<code>unwind</code>) and to create canonical frame IDs
(<code>get_Frame_id</code>).  It also has a callback that is called when the
reader is being unloaded (<code>destroy</code>).  The struct looks like this

<pre class="smallexample">     struct gdb_reader_funcs
     {
       /* Must be set to GDB_READER_INTERFACE_VERSION.  */
       int reader_version;
     
       /* For use by the reader.  */
       void *priv_data;
     
       gdb_read_debug_info *read;
       gdb_unwind_frame *unwind;
       gdb_get_frame_id *get_frame_id;
       gdb_destroy_reader *destroy;
     };
</pre>
   <p><a name="index-g_t_0040code_007bstruct-gdb_005fsymbol_005fcallbacks_007d-3115"></a><a name="index-g_t_0040code_007bstruct-gdb_005funwind_005fcallbacks_007d-3116"></a>
The callbacks are provided with another set of callbacks by
<span class="sc">gdb</span> to do their job.  For <code>read</code>, these callbacks are
passed in a <code>struct gdb_symbol_callbacks</code> and for <code>unwind</code>
and <code>get_frame_id</code>, in a <code>struct gdb_unwind_callbacks</code>. 
<code>struct gdb_symbol_callbacks</code> has callbacks to create new object
files and new symbol tables inside those object files.  <code>struct
gdb_unwind_callbacks</code> has callbacks to read registers off the current
frame and to write out the values of the registers in the previous
frame.  Both have a callback (<code>target_read</code>) to read bytes off the
target's address space.

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