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From: Eli Zaretskii <eliz@gnu.org>
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Subject: Re: Debugging printing to stderr prior glyph row values ....
Date: Fri, 24 May 2019 09:45:44 +0300
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To: Keith David Bershatsky <esq@lawlist.com>
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In-reply-to: <m2tvdku3af.wl%esq@lawlist.com> (message from Keith David
	Bershatsky on Thu, 23 May 2019 17:03:36 -0700)
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> Date:  Thu, 23 May 2019 17:03:36 -0700
> From:  Keith David Bershatsky <esq@lawlist.com>
> Cc:  emacs-devel@gnu.org
> 
> I got it to crash ... again.  "pgrowx previous_row" from gdb prints nothing and returns to a new gdb command prompt.

This means the previous glyph row had its 'used' member equal to zero.

> [CAVEAT:  I wanted a crash, so I did not insert a test for the 'used' member being greater than zero.]

And it shows:

> (gdb) print *previous_row
> $3 = {
>   glyphs = {0x11a800000, 0x11a800000, 0x11a803520, 0x11a803520},
>   used = {0, 0, 0, 0},

See? all of the 'used' members are zero.  IOW, there are no glyphs in
that row.