On the latest emacs-26 and master branches, calling the function frame-list-z-order crashes a daemon Emacs via segfault: 1. ./src/emacs -Q --daemon= 2. ./lib-src/emacsclient --socket-name= --create-frame 3. cd src 4. gdb -p 5. Issue 'continue' in gdb 6. Evaluating (frame-list-z-order) in the emacsclient frame gives the following output in gdb: Thread 1 "emacs" received signal SIGSEGV, Segmentation fault. 0x0000000000565112 in x_frame_list_z_order (dpy=0x33018f0, window=227) at xfns.c:5306 5306 if (XFRAME (frame)->output_data.x->parent_desc == children[i] Issuing 'set logging on' followed by 'bt full' in gdb at this point gives the attached backtrace. A quick step-through of x_frame_list_z_order indicates that the members of the struct XFRAME (frame)->output_data are initialised to NULL for the initial daemon frame F1. Some information gathered by report-emacs-bug follows my signature. Please let me know if I can provide any further information. Thanks, -- Basil In GNU Emacs 26.1 (build 2, x86_64-pc-linux-gnu, X toolkit, Xaw3d scroll bars) of 2018-05-06 built on thunk Repository revision: 91a68b5f61db50344c6a5df497f55370d54a7b15 Windowing system distributor 'The X.Org Foundation', version 11.0.11906000 System Description: Debian GNU/Linux testing (buster) Configured using: 'configure 'CC=ccache gcc' 'CFLAGS=-O0 -g3 -ggdb -gdwarf-4 -pipe' --config-cache --prefix=/home/blc/.local --program-suffix=26 --enable-checking=yes,glyphs --enable-check-lisp-object-type --with-mailutils --with-x-toolkit=lucid --with-modules --with-file-notification=yes --with-x' Configured features: XAW3D XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GSETTINGS NOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB TOOLKIT_SCROLL_BARS LUCID X11 MODULES THREADS LIBSYSTEMD LCMS2 Important settings: value of $LANG: en_IE.UTF-8 locale-coding-system: utf-8-unix