Can't run an executable on an RS 6000

[Robert Lupton the Good]

I have some code (20k lines of C or thereabouts) that I wanted to test
on an RS6000 that I have access to. After some trouble I managed to
compile it with "c89 -Dunix", and it completed without complaint,
producing an executable file. When I tried to run it I got a message
about it not being in XCOFF format -- I am confused. I could compile
hello_world.c in an identical way, "file" thought that the two 
executables were identical but hello_world ran fine. What am I doing wrong?

			Robert