CONST vs. VALUE on a pointer parameter

[darren@xxxxxxxxx]

I attempted to call the Qp0lGetAttr API to get the creation date of a file
in the IFS.  I used the following prototype.

     D Qp0lGetAttr     PR            10I 0 ExtProc('Qp0lGetAttr')
     D  path                           *   Const
     D  AttArrPtr                      *   Const
     D  BuffPtr                        *   Const
     D  BuffLenSent                  10U 0 Const
     D  BuffLenNeed                  10U 0
     D  BuffLenRet                   10U 0
     D  FollowLink                   10U 0 Value


The above did not work, apparently because I had used Const.  The program
compiles fine both ways, but the API returns a -1 indicating that some kind
of error occurred.  I can toggle the 'path' parameter from Value to Const
and get it to fail, with everything else as shown below.  My reason for
using Const was to document that the variable being pointed to was an Input
only variable per the API's specs.  Can someone in the know, educate me as
to the different between Const and Value in this case?

     D Qp0lGetAttr     PR            10I 0 ExtProc('Qp0lGetAttr')
     D  path                           *   Value
     D  AttArrPtr                      *   Value
     D  BuffPtr                        *   Value
     D  BuffLenSent                  10U 0 Value
     D  BuffLenNeed                  10U 0
     D  BuffLenRet                   10U 0
     D  FollowLink                   10U 0 Value