bug#30019: 26.0.90; Invalid specification of parent-frame with desktop-read

[Eli Zaretskii]

> From: Aaron Jensen <aaronjensen@gmail.com>
> Date: Tue, 9 Jan 2018 08:21:57 -0800
> 
> Btw, is there a guide to setting a breakpoint in C code for emacs
> source?

You mean, how to use GDB?  You'd need to read the GDB manual.  The
command is "break".  In this case, I'd set a breakpoint at entry to
x_set_parent_frame:

  (gdb) break x_set_parent_frame

and when it breaks, do this:

  (gdb) p f->output_method

If the result is not output_x_window, then FRAME_X_P would yield
zero, because it's definition is

  #define FRAME_X_P(f) ((f)->output_method == output_x_window)

There's some advice on debugging Emacs in etc/DEBUG, but it doesn't
teach you basic GDB.