xgettext output nothing

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Hello,

I try to generate the .po file using 'xgettext' on a source file but nothing in 
the output:

$ xgettext hello.c

If I give the output file:

$ xgettext hello.c -o hello.po

The hello.po file is not created.
I read the manual and the official documentation, but I don't understand why I 
got nothing at the output when I use 'xgettext'.

How to generate the .po file?

Thank you.

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The source is available here:
http://ftp.gnu.org/gnu/hello/hello-2.6.tar.xz