Solution: When compiling uvspec, please make sure that you have all required compilers and tools installed, see Download. For the newer versions of Linux, some of the standard tools are no longer installed by default (as modern users don't want to fiddle around with such ugly things as compilers etc). One particular example is Flex - The Fast Lexical Analyzer. It is usually available for all flavours of Linux, under Mac, and for cygwin, but not necessarily installed by default. You may have to select installation of flex manually. If you have tried compiling uvspec without having flex installed, you may need to remove src/*_lex.c.