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https://developer-old.gnome.org/gtk4/

download file
gtk4-html-3.90.0.tar.gz

$ cp -r gtk4-html-3.90.0/ /usr/share/gtk-doc/html/gtk4

devhelp -> preferences
disable "Gtk+3 Reference Manual"

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$ ./exampleapp
./exampleapp: symbol lookup error: /opt/gtk/lib/x86_64-linux-gnu/libgtk-4.so.1: undefined symbol: pango_fc_font_get_languages

$ ldd exampleapp | grep pango
        libpangocairo-1.0.so.0 => /usr/lib/x86_64-linux-gnu/libpangocairo-1.0.so.0 (0x00007f6f38dc3000)
       	libpango-1.0.so.0 => /usr/lib/x86_64-linux-gnu/libpango-1.0.so.0 (0x00007f6f38d74000)
      	libpangoft2-1.0.so.0 => /usr/lib/x86_64-linux-gnu/libpangoft2-1.0.so.0 (0x00007f6f38d3c000)

using old pango library.
new one is under

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cc $(pkg-config --cflags gtk4) -o example-0 example-0.c $(pkg-config --libs gtk4)

# error
/usr/bin/ld: /opt/gtk/lib/x86_64-linux-gnu/libgtk-4.so: undefined reference to `pango_fc_font_get_languages'
collect2: error: ld returned 1 exit status



# link libpangofc 
cc $(pkg-config --cflags gtk4 pangofc) -o example-0 example-0.c $( pkg-config --libs gtk4 pangofc)