mapfile usage

[Alex Reed]

Can someone please explain how 'mapfile' should be used?  I am trying:

cat file.txt | mapfile
for i in ${MAPFILE};do echo $i; done

and I see no output.  I've tried adding the -t option to strip
trailing newlines.  If I use the following command:

mapfile -u file.txt

I get the error:

bash: mapfile: file.txt: invalid file descriptor specification
Exit 1


This is sad...  Any suggestions?