About modify cairo source code

Hi guys,
I have just built cairo and run a small sample with cairo (just draw a
rectangle and fill it with black and output to a png file).
I just want to gain better unstanding of how cairo implements these drawing
verbs. So I modify the cairo_rectangle() and cairo_fill() function in
<cairosrc>/src/cairo.c(just add some printf to print some message) and then
I rebuilt cairo. However, when I recompile my sample code and run the
program, it does not give me the printf message which I just modified. I
kinda wonder why? Isn't the implementation of cairo_?() function defined in
cairo.c? Why doesn't my modification make any difference?

Thanks in advance.

-- 
*Zhaotian Wang*

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