Re: Additional Error Specs from C-Proc. 'system'

Hello Nelson,

You wrote:
>I've probably missed this in previous threads, but what is the value or
>benefit of using the C 'system' over using QCAPCMD in the first place?

Because it is simple and it allows literal strings to be used in an easier
manner.  For example, the following definitions:

D int_t           S             10I 0
D pointer_t       S               *
D rc              S                   LIKE(int_t)

D system          PR                  LIKE(int_t) EXTPROC('system')
 *
 * Required parameter group:
 *                             OS/400 command
D   command                           LIKE(pointer_t) VALUE
D                                     OPTIONS(*STRING)

allow me to code:

C               callp           system("DSPMSG")

or

C               eval            rc = system("DSPMSG")

whereas using QCMDEXC with the following prototype:

 /if defined(LONG_PROC_NAMES)
D QcaRunClCmd     PR
 /else
D QCMDEXC         PR
 /endif
D                                     EXTPGM('QCMDEXC')
 *
 * Required parameter group:
 *                              A Control Language command string
D  clCmd                     32702    CONST OPTIONS(*VARSIZE)
 *                              Length of command string
D  clCmdLen                     15  5 CONST
 *
 * Optional parameter group:
 *                              A DBCS data flag
D  dbcsFlag                      3    CONST OPTIONS(*NOPASS)

requires:

C               callp           QCMDEXC("DSPMSG" : 6)

or

D $CMD          C                       CONST('DSPMSG')

C               callp           QCMDEXC($CMD : %SIZE($CMD))

both of which are slightly uglier than system().

Using QCAPCMD is uglier still but much more powerful since its prototype
looks like:

 /if defined(LONG_PROC_NAMES)
D QcaProcessClCmd...
D                 PR
 /else
D QCAPCMD         PR
 /endif
D                                     EXTPGM('QCAPCMD')
 *
 * Required parameter group:
 *                              Command string to process
D  clCmd                     32702    CONST OPTIONS(*VARSIZE)
 *                              Length of command string
D  cmdLen                       10I 0 CONST
 *                              Options control block
D  optCtlBlk                          LIKE(Qca_PCMD_CPOP0100)
D                                     CONST OPTIONS(*VARSIZE)
 *                              Length of options control block
D  optCtlBlkLen                 10I 0 CONST
 *                              Format of options control block
D  format                        8    CONST
 *                              Changed command string
D  chgClCmd                  32767    OPTIONS(*VARSIZE:*OMIT)
 *                              Length provided for changed command string
D  chgClCmdLenProv...
D                               10I 0 CONST
 *                              Length available for changed command
string
D  chgClCmdLenAvail...
D                               10I 0
 *                              Error code
D  errorCode                  1024    OPTIONS(*VARSIZE)

In all the "ugly" cases the solution is to create a wrapper procedure that
masks the ugliness behind a nice interface.  For example:

C               eval            errCode = runCLcmd("DSPMSG")

where runCLcmd is a nice wrapper procedure that may in turn use either
QCMDEXC or QCAPCMD or even system.

I use system where I expect the command to succeed (e.g., DSPMSG is
unlikely to fail) but I use the other methods when there is a reasonable
chance of the command failing.  However, if you take the effort to create
a decent runCLcmd procedure you get the best of both worlds.

Regards,
Simon Coulter.
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