RPG and IFS

I am having trouble writing a program in RPG that checks to see if a file
in the IFS is already open.  The program opens the file with the
O_SHARE_NONE option, so I'm assuming if anything has the file open it will
return me a negative code.  My first problem is I've looked at many
resources, and I cannot seem to find a consistent definition for the
O_SHARE_NONE option.  Second, I can get the program to compile and run,
however, it always returns me the same code.  I've made a couple of
modifications to the program, and I've gotten a -1 return code and a 3
return code.  Does anyone have any experience with the IFS that might be
able to point me in the right direction?  Thanks for any help.

Andy