Files:
demo_fn.h - define the functions to calculate value and derivative; 
	    the function parameters is not used.
demo_fn.c - function definitions
netwon.c - driver function for netwon method

Compile & Link:
gcc -c demo_fn.c
gcc -c netwon.c
gcc -o netwon netwon.o -lgsl -lgslcblas -lm

Run
./netwon