> I mean when gdb-display-source-buffer will fail to find the source
> window?

I dug into gud.el and gdb-mi.el to see what’s really going on. It’s not very straightforward. Let me present my understanding of the logic below. Please correct me if I’m wrong.

First, “frame” in gud is a cons of file name and line, basically at where gdb stops (file:line).

When gud needs to display the current line at where gdb stops at (“frame”), it tries to display it in a new window if the target buffer is not already shown. `gud-last-frame` is set to the frame we want to display, and we call `gud-display-frame` to display it. `gud-display-frame` calls `gud-display-line` calls `display-buffer` with `(inhibit-same-window . t)`. Then it sets `gud-last-last-frame` to the frame just displayed, and `gdb-last-frame` to nil. So `last-frame` is basically the frame to be displayed, and `last-last-frame` is the last displayed frame.

As for gdb-mi, in `gdb-display-source-buffer`, it first looks for the window displaying `gud-last-last-frame` (last displayed frame, that is). If that doesn’t exist, it looks for `gdb-source-window` and use that. Either way it sets the window found to `gdb-source-window`. And it returns nil if no last-last-frame window nor `gdb-source-window` is found. How can `gdb-source-window` be set? There are two ways, 1) when gdb-mi starts with many-windows or show-main option, it opens a source window and set that to `gdb-source-window`, 2) if `gdb-display-source-window` finds a window displaying `gud-last-last-frame`, that window will be set to `gdb-source-window`. 

Gdb-mi also uses `gud-display-frame`: when I type commands in comint buffer, the result from gdb is handled by `gdb-frame-handler`, which sets `gud-last-frame` and calls `gud-display-frame`.

So, commands like step, next, etc, uses `gud-display-frame`. Except that `gdb-goto-breakpoint` uses gdb-display-source.

So it seems the intension is to display source in new window, not using only one window as I originally thought. I’d like to simplify it, put the decision logic into one place. And then we can add customizability to it.

>> It shouldn't harm to try that.  I'd still make it optional - maybe
>> someone wants to see the other buffers.
>> 
>> I tend to agree with the last bit.
>> 
>> At the very least I want to fix the inconsistency in gdb-mi — goto-breakpoint uses `(or
>> gdb-display-source-buffer display-buffer)` and gud-display-line uses `display-buffer`. We can modify the
>> display logic to allow user customization once gdb-mi is consistent within itself.
> 
> Let's add a defcustom to control this new behavior.


Do you mean options like `only-one-window`, `switch-between-two`, `as-much-as-possible`?

Yuan