The following little program in ILE RPG uses a few of the DSM API's. Browsing quickly through the API reference manual I couldn't see any DSM API's requiring value declarations.

     D f3              c                   x'33'
     D sa_norm         c                   x'20'

     D txt             s            128    inz('Press Enter to Roll, F3 to quit-
     D                                     .')
     D txtlen          s              9b 0 inz(32)
     D err             s              8    inz(x'0000000000000000')
     D aid             s              1
     D lines           s              9b 0 inz(1)
     D wf1             s              1
     D wrtn            s              9b 0

     D clrscr          pr             9b 0 extproc('QsnClrScr')
     D  mode                          1    options(*nopass) const
     D  cmdbuf                        9b 0 options(*nopass) const
     D  env                           9b 0 options(*nopass) const
     D  error                         8    options(*nopass)

     D wrtdta          pr             9b 0 extproc('QsnWrtDta')
     D  data                        128
     D  datalen                       9b 0
     D  fldid                         9b 0 options(*nopass) const
     D  row                           9b 0 options(*nopass) const
     D  col                           9b 0 options(*nopass) const
     D  strmatr                       1    options(*nopass) const
     D  endmatr                       1    options(*nopass) const
     D  strcatr                       1    options(*nopass) const
     D  endcatr                       1    options(*nopass) const
     D  cmdbuf                        9b 0 options(*nopass) const
     D  env                           9b 0 options(*nopass) const
     D  error                         8    options(*nopass)

     D getaid          pr             1    extproc('QsnGetAID')
     D  aid                           1    options(*nopass)
     D  env                           9b 0 options(*nopass) const
     D  error                         8    options(*nopass)

     D rollup          pr             9b 0 extproc('QsnRollUp')
     D  lines                         9b 0                  const
     D  top                           9b 0                  const
     D  bottom                        9b 0                  const
     D  cmdbuf                        9b 0 options(*nopass) const
     D  env                           9b 0 options(*nopass) const
     D  error                         8    options(*nopass)

     C                   eval      wrtn = clrscr('0' : 0 : 0 : err)
     C                   dow       wrtn = 0
     C                   eval      wrtn = wrtdta (txt : txtlen : 0 : 23 : 2 :
     C                             sa_norm : sa_norm : sa_norm : sa_norm :
     C                             0 : 0 : err)
     C                   eval      wf1 = getaid (aid : 0 : err)
     C                   if        aid = f3
     C                   leave
     C                   endif
     C                   eval      wrtn = rollup (lines : 1 : 24 : 0 : 0 : err)
     C                   enddo

     C                   eval      *inlr = *on

Remember to bind this to serviceprogram QSNAPI

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You're clearly bringing up an interesting issue. Not being a C expert, I can't say much one way or another except that I've used numerous APIs with no noticeable problem. I've yet to run into an API that I can't call successfuly in ILE RPG in fact. (Unfortunately, I have yet to get into DSM.) The IFS APIs in 'OS/400 UNIX-Type APIs' are examples where prototyping for the VALUE keyword seems to work just fine. By just looking at the documentation, I don't see any significant difference between the ways the DSM APIs and the IFS APIs are defined. So, my first question is: How can you tell by looking at the documentation that the DSM APIs will react to parameter definitions differently than the IFS APIs, for example? Are IFS APIs *not* expecting parameters to be passed 'by value'?

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Unless a program (including an API) is expecting to receive a parameter passed by value, this will not work. C library functions receive parameters by value. However, most other APIs receive parameters by reference. Maybe you're referring to passing by read-only reference (i. e., specifying the Const keyword in an RPG IV prototype). Mike Cravitz NEWS/400 Technical Editor

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I have created ILE RPG prototypes for the APIs that I use. When you create the prototype you can specify that the parameter is passed by value, then just use the CALLP opcode. Works quite well.

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I will give you an example. The Dynamic Screen Manager APIs tend to have their parameter lists set up to pass by value rather than by reference. Here (off the top of my head) is one specific example. The API QsnCrtCmdBuf is passed 5 parameters as follows,

p1    Initial buffer size    int   or   10I in rpg
p2    increment size       int    ditto
p3    max size               int    ditto
p4    address of buffer handle    long *
p5    error data structure            char *

this differs say from the dataqueue send APIs which expect all of their parameters to be pointers. The above API, because pointers are 16 bytes long on the AS/400, decides to pass the variable contents instead, presumably for performance reasons. Now If you read the AS/400 ILE RPG manuals carefully you spot a pattern. The PARM statement explicitly states that parameters are passed by reference. Hence you can never receive a parameter into a data structure because it is already allocated in storage elsewhere. Using procedure calls you may be forgiven for thinking that parameters are passed differently, but this is not the case. Parameters can be passed by reference, value and as const. Pass by reference is exactly the same as using parm, and the parameters must match exact size ad type for obvious reasons. Pass by value allows you to use different field sizes and types. This is because the parameter is copied into the storage of the procedure. Pass as const is an interesting one. It is supposed to represent a read only variable, but if you read the manuals carefully you see that you have to make sure the procedure does not change the variable. Why? well obviously because it is really being passed by reference, and changing it would change the original storage, even if it is a constant !! So this leaves me believing that it is not possible to call a lot of the system APIs, because as I said earlier some of them expect the variable contents to be passed on the stack (if the AS/400 has a concept of a stack) instead of the reference to the variable. Another give-away is the fact that functions like QsnCrtCmdBuf returns a result field as well, and there is no support for this in RPG without procedures. Have you written much in C ? If you have then the above can be explained more simply by showing that variables are always passed by address e.g..

callproc(&parm1, &parm2, &parm3);
.
.
.
callproc(int *parm1, char *parm2, packed(7,2) *parm3)
{
.
.

If you have any examples of code you have written to such APIs as these, I would love to see how you coded them.

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I guess I don't quiet understand your problem. I have used MANY system APIs with RPG with no problem whatsoever. Do you have any examples?

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