redir and glob

I want to create a list of files that my project is using. This list will be stored in a file and subsequently will be used by cscope.

Here is what I have so far:

function! BuildFileList()
s:dir_list = ['dir1', 'dir2', 'dir3']
s:output_file = 'cscope.files'
redir! > s:output_file
for dir in s:dir_list
glob(dir.'*.[ch]')
endfor
redir END
endfunction

silent call BuildFileList

I'm getting the following errors when I execute this function:
E190: Cannot open "s:output_file" for writing
E486: Pattern not found: dir."*.[ch]"

Obviously there is something wrong with the way I use redir and glob, but I can't get my finger on it. Does anybody know what am I doing wrong?

Any help is really appreciated.

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