http://qs321.pair.com?node_id=1167987


in reply to Passing integer pointer in XS?

Change all of those "int *" to "int &" and I suspect it will "just work".

Update: Or, worst case, you might have to change the XS code to something like: 

int
tdSensor(protocol, protocolLen, model, modelLen, id, dataTypes)
        char *  protocol
        int     protocolLen
        char *  model
        int     modelLen
        int &   id
        int &   dataTypes
CODE:
    RETVAL = tdSensor(protocol, protocolLen, model, modelLen, &id, &da
+taTypes);

[download]

- tye        

Replies are listed 'Best First'.
Re^2: Passing integer pointer in XS? (&)
by martin67 (Novice) on Jul 18, 2016 at 20:32 UTC
    Thanks, But it still is a mystery... By using "int & " instead of "int *", the C-code looks alright and the C-function is called with the right parameters. But the value that the C-funtion writes to the int pointers are not passed back to perl.

    Example in the generated C-file: 

    XS_EUPXS(XS_TellStick_tdSensor); /* prototype to pass -Wmissing-protot
+ypes */
XS_EUPXS(XS_TellStick_tdSensor)
{
  dVAR; dXSARGS;
  if (items != 6)
    croak_xs_usage(cv,  "protocol, protocolLen, model, modelLen, id, d
+ataTypes");
  {
    char *  protocol = (char *)SvPV_nolen(ST(0))
;
    int     protocolLen = (int)SvIV(ST(1))
;
    char *  model = (char *)SvPV_nolen(ST(2))
;
    int     modelLen = (int)SvIV(ST(3))
;
    int     id = (int)SvIV(ST(4))
;
    int     dataTypes = (int)SvIV(ST(5))
;
    int     RETVAL;
    dXSTARG;

    printf("id1: %d\n", id);
    RETVAL = tdSensor(protocol, protocolLen, model, modelLen, &id, &da
+taTypes);
    printf("id2: %d\n", id);
    XSprePUSH; PUSHi((IV)RETVAL);
  }
  XSRETURN(1);
}

    [download]
    I added two printf for debugging (best debug tool...) and when I run it with like this: 
    ...
my $sensorId = 1;
TellStick::tdSensor($protocol, 25, $model, 25, $sensorId, $dataTypes);
 print "protocol: $protocol, model: $model, sensorId: $sensorId, dataT
+ypes: $dataTypes\n";

I get the output

id1: 1
id2: 51
protocol: mandolyn, model: temperaturehumidity, sensorId: 1, dataTypes
+: 2

    [download]
    So it gets the correct value (51), but that is not passed back.

    It looks like the XS code only sees the id parameter as an input value (casting it right), but no output is generated (except the return value). The string arguments (protocol, model), seems to work in modifying what the value is pointing to.

    Should I somehow use "int *" in the XS anyway, but writing some special code for INPUT/OUTPUT in the typemap? Or is there some other way of returing the correct value?
[reply]
[d/l]
[select]

      So, you'll need to add just a bit more: 

      int
tdSensor(protocol, protocolLen, model, modelLen, id, dataTypes)
        char *  protocol
        int     protocolLen
        char *  model
        int     modelLen
        int &   id
        int &   dataTypes
CODE:
    RETVAL = tdSensor(protocol, protocolLen, model, modelLen, &id, &da
+taTypes);
OUTPUT:
    RETVAL
    id
    dataTypes

      [download]

      Sorry, it has been many years since I learned this stuff.

      - tye        

[reply]
[d/l]

        Problem: You can't write to protocol or model safely because one of the force functions wasn't used. You might end up changing variables and constants you didn't mean to change. (They might have fixed that in 5.24?) 

        use strict;
use warnings;

use feature qw( say );

use Inline C => <<'__EOS__';

   void testing(char* s) {
      s[0] = 'y';
   }

__EOS__

my $x = "x";
my $y = $x;
say "$x-$y";
testing($y);
say "$x-$y";

        [download]
        x-x
y-y   <-- Changed both $x and $y

        [download]
        or 
        for (1..2) {
   my $y = "x";
   say $y;
   testing($y);
}

        [download]
        x
y     <-- Changed the constant!

        [download]

        Note: The caller will need to do s/\0.*// for $protocol, $model;

[reply]
[d/l]
[select]
        Thank you!

        This also works! (I got another variant further down in the thread.)

        This was my first question to Perlmonks and I'm impressed by the speed and quality of the answers!

[reply]

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