[fitsbits] Substract sky background

[Boris Basic]

Hi!

I'm new to this newsgroup, so my question may have already been asked here.
My problem is that I need to write a program that must calculate the mean
value of the sky background of a FITS file, and then create a new file with
this mean value substract from all pixel values of the original file.
Unfortunately, I don't have a lot of time to work on this, so I would enjoy
if somebody could give me a subroutine written in C which perform the
requested operation. I already have an old subroutine but it's too old and
gcc doesn't want to work with it.