a question on du.c and df.c rather than a statement of a bug

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Hello there,
i am trying to use both the du functionality and the df one in my code in
order to do some memory management.
basically i have generated the object files df.o and du.o from the
corresponding du.c and df.c that i got from the package fileutils-4.1 .

of course, slight tweaking was necessary in order to extract the needed
value (the corresponding main(){} were changed since they would no longer be
used on their own).
then i retrieve the outcome of df from "available" and the outcome of du
from "tot_size". (you should assume that the rest of the source has not been
tampered with)

so i have compiled only and i am now linking directly the du.o and df.o in
my code.
the problem is that when both objects are linked in i get faulty returned
values for both of them. if only one object is present at a time i get the
right result for the object file that was used.

i know you are not a newsgroup. however i could not get an answer from there
and google groups comp.unix.programmer couldn't help me either.
is there a change you can point out to me how to overcome this bit?
Thank you in advance for your help
nass