my function called inside postgres.c

Hi everyone,
I'm working on a project that require me to call my function inside the postegres.c, precisely in exec_simple_query.
I wrote my function in a new file called test.c with the library test.h.
I did: make distclean, ./configure, make, sudo make install
but I've got the error: undecleare reference to my_function...
Where is my error?
What should I do to solve it? Are there extra steps that I don't know?
Thank you!