29.4.2 Writing JIT Debug Info Readers

As mentioned, a reader is essentially a shared object conforming to a certain ABI. This ABI is described in jit-reader.h.

jit-reader.h defines the structures, macros and functions required to write a reader. It is installed (along with GDB), in includedir/gdb where includedir is the system include directory.

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 GDB_DECLARE_GPL_COMPATIBLE_READER in a source file.

The entry point for readers is the symbol gdb_init_reader, which is expected to be a function with the prototype

extern struct gdb_reader_funcs *gdb_init_reader (void);

struct gdb_reader_funcs contains a set of pointers to callback functions. These functions are executed to read the debug info generated by the JIT compiler (read), to unwind stack frames (unwind) and to create canonical frame IDs (get_Frame_id). It also has a callback that is called when the reader is being unloaded (destroy). The struct looks like this

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;
};

The callbacks are provided with another set of callbacks by GDB to do their job. For read, these callbacks are passed in a struct gdb_symbol_callbacks and for unwind and get_frame_id, in a struct gdb_unwind_callbacks. struct gdb_symbol_callbacks has callbacks to create new object files and new symbol tables inside those object files. struct gdb_unwind_callbacks 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 (target_read) to read bytes off the target’s address space.