On 11/01/2006, at 1:45 AM, Peter Grace wrote:

Both explanations do help me with a segue question I was going to ask,
which is what is the proper way to pass data structures back and forth
between C and RPG.  I had a suspicion that struct would come into play,
since in my mind as a C programmer, seeing an RPG DS just looks like a
struct to me!

If you really do want to pass data structures back and forth then you should do that using a bi-directional parameter just as you would in C.

Rather than returning the structure from the C function (via return) just accept the address of the structure into the C function and change it in place.

RPG passes by reference (unless told otherwise) and C passes by value (unless told otherwise). Passing a variable by reference is the same as passing a pointer by value therefore you can satisfy both the RPG view of the world and the C view by suitable prototyping. For example:

C view of the structure:
typedef _Packed struct { int fld1;
                         char fld2[25];
                         int fld3;
                       } foo_t;

RPG IV view of the structure:
D foo           DS                           QUALIFIED INZ
D   fld1                              10I 0
D   fld2                              25
D   fld3                              10I 0

Note the use of _Packed to force C to stop natural alignment. RPG does not align unless told to do so. So either use _Packed on C structs or ALIGN on RPG data structures.

void doFoo( foo_t* foo )
   foo->fld1 = 10;
   foo->fld3 = 42;
memset( foo->fld2, 0x40, sizeof(foo->fld2) ); /* blank pad for caller */
   memcpy( foo->fld2, "SOME TEXT", 9 ); /* Omit null-terminator */

D foo             DS                  QUALIFIED
D  fld1                         10I 0
D  fld2                         25
D  fld3                         10I 0
D doFoo           PR                  EXTPROC('doFoo')
D   foo                               LIKEDS(foo)
C                   CALLP     doFoo( foo )
C     foo.fld1      DSPLY
C     foo.fld2      DSPLY
C     foo.fld3      DSPLY
C                   SETON                                        LR
C                   RETURN                                          

This technique works with any variable type and can make the lives of both types of programmer much happier. However nothing seems to stop C programmers from objecting to fixed-length strings. Yet another of the afflictions created by K&R.

Simon Coulter.
   FlyByNight Software         AS/400 Technical Specialists

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