If ~/.fonts.conf contains:
<edit mode="assign_replace" name="spacing">
<int>mono</int>
</edit>
fontconfig crashes:
mfabian@magellan:~$ fc-match sans
Fontconfig error: "~/.fonts.conf", line 46: "mono": not a valid
integer
セグメンテーション違反です (core dumped)
mfabian@magellan:~$
Of course the above is nonsense, “mono” is no valid integer indeed.
But I think nevertheless fontconfig should not crash in that case.
The problem was caused by partially truncated expression trees caused by
parse errors -- typechecking these walked the tree without verifying the
integrity of the structure. Of course, the whole tree will be discarded
shortly after being loaded as it contained an error.