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<title>Debugging with GDB: Writing JIT Debug Info Readers</title>
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<a name="Writing-JIT-Debug-Info-Readers"></a>
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<div class="header">
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<p>
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Previous: <a href="Using-JIT-Debug-Info-Readers.html#Using-JIT-Debug-Info-Readers" accesskey="p" rel="previous">Using JIT Debug Info Readers</a>, Up: <a href="Custom-Debug-Info.html#Custom-Debug-Info" accesskey="u" rel="up">Custom Debug Info</a> [<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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<hr>
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<a name="Writing-JIT-Debug-Info-Readers-1"></a>
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<h4 class="subsection">29.4.2 Writing JIT Debug Info Readers</h4>
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<a name="index-writing-JIT-debug-info-readers"></a>
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<p>As mentioned, a reader is essentially a shared object conforming to a
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certain ABI. This ABI is described in <samp>jit-reader.h</samp>.
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</p>
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<p><samp>jit-reader.h</samp> defines the structures, macros and functions
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required to write a reader. It is installed (along with
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<small>GDB</small>), in <samp><var>includedir</var>/gdb</samp> where <var>includedir</var> is
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the system include directory.
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</p>
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<p>Readers need to be released under a GPL compatible license. A reader
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can be declared as released under such a license by placing the macro
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<code>GDB_DECLARE_GPL_COMPATIBLE_READER</code> in a source file.
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</p>
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<p>The entry point for readers is the symbol <code>gdb_init_reader</code>,
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which is expected to be a function with the prototype
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</p>
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<a name="index-gdb_005finit_005freader"></a>
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<div class="smallexample">
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<pre class="smallexample">extern struct gdb_reader_funcs *gdb_init_reader (void);
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</pre></div>
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<a name="index-struct-gdb_005freader_005ffuncs"></a>
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<p><code>struct gdb_reader_funcs</code> contains a set of pointers to callback
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functions. These functions are executed to read the debug info
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generated by the JIT compiler (<code>read</code>), to unwind stack frames
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(<code>unwind</code>) and to create canonical frame IDs
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(<code>get_frame_id</code>). It also has a callback that is called when the
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reader is being unloaded (<code>destroy</code>). The struct looks like this
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</p>
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<div class="smallexample">
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<pre class="smallexample">struct gdb_reader_funcs
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{
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/* Must be set to GDB_READER_INTERFACE_VERSION. */
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int reader_version;
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/* For use by the reader. */
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void *priv_data;
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gdb_read_debug_info *read;
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gdb_unwind_frame *unwind;
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gdb_get_frame_id *get_frame_id;
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gdb_destroy_reader *destroy;
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};
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</pre></div>
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<a name="index-struct-gdb_005fsymbol_005fcallbacks"></a>
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<a name="index-struct-gdb_005funwind_005fcallbacks"></a>
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<p>The callbacks are provided with another set of callbacks by
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<small>GDB</small> to do their job. For <code>read</code>, these callbacks are
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passed in a <code>struct gdb_symbol_callbacks</code> and for <code>unwind</code>
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and <code>get_frame_id</code>, in a <code>struct gdb_unwind_callbacks</code>.
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<code>struct gdb_symbol_callbacks</code> has callbacks to create new object
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files and new symbol tables inside those object files. <code>struct
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gdb_unwind_callbacks</code> has callbacks to read registers off the current
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frame and to write out the values of the registers in the previous
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frame. Both have a callback (<code>target_read</code>) to read bytes off the
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target’s address space.
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</p>
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<p>
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Previous: <a href="Using-JIT-Debug-Info-Readers.html#Using-JIT-Debug-Info-Readers" accesskey="p" rel="previous">Using JIT Debug Info Readers</a>, Up: <a href="Custom-Debug-Info.html#Custom-Debug-Info" accesskey="u" rel="up">Custom Debug Info</a> [<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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