@Fuerchau: Danke für den Tip!

So tuts:
Code:
      *** externe Prozeduren ***
     D qcapcmd         pr                  extpgm('QCAPCMD')                    Befehl prüfen
     D                              200                                          Command
     D                               10i 0 const                                   Len
     D                               20    const                                 Options
     D                               10i 0 const                                   Len
     D                                8    const                                 Options format
     D                              200                                          Changed Command
     D                               10i 0 const                                   Len avail
     D                               10i 0 const                                   Len returned
     D                              515                                          API-Error
     
      *** Datenstrukturen ***
     D cpop0100        ds                  qualified                            CPOP0100-Format
     D  cmdType                      10i 0 inz(4)                                Comand type
     D  dbcsHandling                  1    inz('0')                              DBCS data handling
     D  promptAction                  1    inz('1')                              Prompt action
     D  cmdSyntax                     1    inz('0')                              Comand syntax
     D  msgRtvKey                     4    inz(*allx'00')                        Messange rtv. Key
     D  ccsid                        10i 0 inz(0)                                CCSID of Command
     D  res1                          5    inz(*allx'00')                        Reserviert

     D apiError        ds                  qualified                            API-Error
     D  bytesProv                    10i 0 inz(%len(apiError))                   Länge erw.
     D  bytesAvail                   10i 0                                       Länge verf.
     D  msgId                         7                                          MsgId
     D  res1                          1                                          reserved
     D  msgData                     500                                          MsgData

      *** Felder ***
     D p_cmd           s            200
     D p_cmdLen        s             10i 0

      /free

       // Anfangswerte setzen
       // ===================
          p_cmd = 'CPYF FROMFILE(&TSYSSLIB/&FILE) ' +
              'TOFILE(&TSYSTLIB/&FILE) FROMMBR(*ALL) TOMBR(*FROMMBR) ' +
              'FROMRCD(1) COMPRESS(*YES)';

       // Befehl prüfen
       // =============
          callp(e) qcapcmd(p_cmd: %len(p_cmd): cpop0100: %len(cpop0100):
              'CPOP0100': p_cmd: %len(p_cmd): p_cmdLen: apiError);

          if apiError.bytesAvail > 0;
              ...