Re: Question regarding passing variables to ILE-RPG from C -- C400-L


On 10/01/2006, at 8:59 AM, Peter Grace wrote:

I think that this has something to do with the pointer that I'mcreating
doesn't exist outside of the service program?  Is this correct, or am I
just making a big goof?

Yes to both questions actually. The C function is defined as returninga pointer. This pointer will contain the address of the 25 byte array.However this array is declared in automatic storage so it's gone by thetime the pointer is returned. That's problem 1.

Problem 2 is that C expects an int (4 bytes) but the prototype ispassing a 2-byte integer. RPG will widen the value because that's whatC expects but you shouldn't rely on this behaviour.

Problem 3 is that the C function returns a pointer but the RPGprototype says it returns a 25 byte variable. Not the same thing atall.


The correct prototype for the function is:
        D getStr          PR              *   extproc('getStr')
        D                               10I 0 VALUE

But that only makes them map correctly. It doesn't deal with problem 1.

What is the "Right Way" for me to return a char* array (or even a data
structure) via a bound-C service program to a RPG program?  Is there
even a right way to do it?


"Right" depends on what you're trying to accomplish.

A quick-and-dirty solution to Problem 1 is to make the array static butusing static storage is messy and sloppy.


#include <stdio.h>
#include <string.h>
#include <stdlib.h>

char* getStr(int incomingVar);

char* getStr(int incomingVar)
{
  static char foo[25];
  sprintf(foo.data,"%d",incomingVar);
  return foo;
}

You could also return a structure containing the 25 bytes. This wouldmap better to RPG's view of the world but is likely to give Cprogrammers the squits. They start squawking about "no respect for thestack" and other such nonsense.


#include <stdio.h>
#include <string.h>
#include <stdlib.h>

char* getStr(int incomingVar);

char* getStr(int incomingVar)
{
  struct { char data[25];
         } foo;
  sprintf(foo.data,"%d",incomingVar);
  return foo;
}

This still requires that RPG knows it is dealing with null-terminatedstrings. That's not 'normal' but you can treat it as a fixed-lengthstring by doing the work in C to blank out the null byte and ensure therest of the field is blanks too.

Finally you could use the %STR function in RPG to convert thenull-terminated string to something RPG can process as long as thestorage referenced by the pointer is still available.

        C                               EVAL            fooStr = %STR(getstr(1))

I presume this code is simply an example because there is no reason toinvoke C just to convert a number to a character representation. You dothat in RPG in any number of ways.


Regards,
Simon Coulter.
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