The identifier "register" defined in section ยง209 of TEX is a reserved word in C.
It needs to be added to the list of reserved words at the end of section 5.5 of web2w:
SYM_PTR("register")->name="internal_register";
Support
If you want to know how to compile, run, and test ctex, read the corresponding chapter in the book (see links above).
If you have read the book and still have questions, send me an email to ruckert (at) cs.hm.edu
If you want to help, send me immediately an email to ruckert (at) cs.hm.edu.