Thread: Passing parameters to a C function

Passing parameters to a C function

From

"Rodrigo Sakai"

Date:

30 May 2007, 14:33:12

<div class="Section1"><p class="MsoNormal"><font face="Arial" size="2"><span style="font-size:10.0pt;
font-family:Arial">    Hello,</span></font><p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:10.0pt;
font-family:Arial"> </span></font><p class="MsoNormal"><font face="Arial" size="2"><span style="font-size:10.0pt;
font-family:Arial">   I have a question about passing parameters to a C function.  Imagine the example by
PostgreSQL:</span></font><pclass="MsoNormal"><font face="Arial" size="2"><span style="font-size:10.0pt; 
font-family:Arial"> </span></font><p class="MsoNormal" style="margin-left:.5in"><font face="Courier New" size="2"><span
style="font-size:10.0pt;font-family:"CourierNew"">PG_FUNCTION_INFO_V1(complex_add);</span></font><p class="MsoNormal"
style="margin-left:.5in"><fontface="Courier New" size="2"><span style="font-size:10.0pt;font-family:"Courier
New""> </span></font><pclass="MsoNormal" style="margin-left:.5in"><font face="Courier New" size="2"><span
style="font-size:10.0pt;font-family:"CourierNew"">Datum</span></font><p class="MsoNormal"
style="margin-left:.5in"><fontface="Courier New" size="2"><span style="font-size:10.0pt;font-family:"Courier
New"">complex_add(PG_FUNCTION_ARGS)</span></font><pclass="MsoNormal" style="margin-left:.5in"><font face="Courier New"
size="2"><spanstyle="font-size:10.0pt;font-family:"Courier New"">{</span></font><p class="MsoNormal"
style="margin-left:.5in"><fontface="Courier New" size="2"><span style="font-size:10.0pt;font-family:"Courier
New"">     Complex    *a = (Complex *) PG_GETARG_POINTER(0);</span></font><p class="MsoNormal"
style="margin-left:.5in"><fontface="Courier New" size="2"><span style="font-size:10.0pt;font-family:"Courier
New"">     Complex    *b = (Complex *) PG_GETARG_POINTER(1);</span></font><p class="MsoNormal"
style="margin-left:.5in"><fontface="Courier New" size="2"><span style="font-size:10.0pt;font-family:"Courier
New"">     Complex    *result;</span></font><p class="MsoNormal" style="margin-left:.5in"><font face="Courier New"
size="2"><spanstyle="font-size:10.0pt;font-family:"Courier New""> </span></font><p class="MsoNormal"
style="margin-left:.5in"><fontface="Courier New" size="2"><span style="font-size:10.0pt;font-family:"Courier
New"">     result = (Complex *) palloc(sizeof(Complex));</span></font><p class="MsoNormal"
style="margin-left:.5in"><fontface="Courier New" size="2"><span style="font-size:10.0pt;font-family:"Courier
New"">     result->x = a->x + b->x;</span></font><p class="MsoNormal" style="margin-left:.5in"><font
face="CourierNew" size="2"><span style="font-size:10.0pt;font-family:"Courier New"">      result->y = a->y +
b->y;</span></font><pclass="MsoNormal" style="margin-left:.5in"><font face="Courier New" size="2"><span
style="font-size:10.0pt;font-family:"CourierNew"">      PG_RETURN_POINTER(result);</span></font><p class="MsoNormal"
style="margin-left:.5in"><fontface="Courier New" size="2"><span style="font-size:10.0pt;font-family:"Courier
New"">}</span></font><pclass="MsoNormal"><font face="Arial" size="2"><span style="font-size:10.0pt; 
font-family:Arial"> </span></font><p class="MsoNormal"><font face="Arial" size="2"><span style="font-size:10.0pt;
font-family:Arial"> So, in the same .C file I have to write another function that calls this one, for example:
</span></font><pclass="MsoNormal"><font face="Arial" size="2"><span style="font-size:10.0pt; 
font-family:Arial"> </span></font><p class="MsoNormal" style="margin-left:.5in"><font face="Courier New" size="2"><span
style="font-size:10.0pt;font-family:"CourierNew"">void</span></font><p class="MsoNormal" style="margin-left:.5in"><font
face="CourierNew" size="2"><span style="font-size:10.0pt;font-family:"Courier New"">test_main()</span></font><p
class="MsoNormal"style="margin-left:.5in"><font face="Courier New" size="2"><span
style="font-size:10.0pt;font-family:"CourierNew"">{</span></font><p class="MsoNormal" style="margin-left:.5in"><font
face="CourierNew" size="2"><span style="font-size:10.0pt;font-family:"Courier New"">    Complex *a;</span></font><p
class="MsoNormal"style="margin-left:.5in"><font face="Courier New" size="2"><span
style="font-size:10.0pt;font-family:"CourierNew"">    Complex *b;</span></font><p class="MsoNormal"
style="margin-left:.5in"><fontface="Courier New" size="2"><span style="font-size:10.0pt;font-family:"Courier
New""> </span></font><pclass="MsoNormal" style="margin-left:.5in"><font face="Courier New" size="2"><span
style="font-size:10.0pt;font-family:"CourierNew"">    a = complex_in("(4.01, 3.77 )");</span></font><p
class="MsoNormal"style="margin-left:.5in"><font face="Courier New" size="2"><span
style="font-size:10.0pt;font-family:"CourierNew"">    printf("a = %s\n", complex_out(a));</span></font><p
class="MsoNormal"style="margin-left:.5in"><font face="Courier New" size="2"><span
style="font-size:10.0pt;font-family:"CourierNew""> </span></font><p class="MsoNormal" style="margin-left:.5in"><font
face="CourierNew" size="2"><span style="font-size:10.0pt;font-family:"Courier New"">    b =
complex_in("(1.0,2.0)");</span></font><pclass="MsoNormal" style="margin-left:.5in"><font face="Courier New"
size="2"><spanstyle="font-size:10.0pt;font-family:"Courier New"">    printf("b = %s\n",
complex_out(b));</span></font><pclass="MsoNormal" style="margin-left:.5in"><font face="Courier New" size="2"><span
style="font-size:10.0pt;font-family:"CourierNew""> </span></font><p class="MsoNormal" style="margin-left:.5in"><font
face="CourierNew" size="2"><span style="font-size:10.0pt;font-family:"Courier New"">    printf("a +  b = %s\n",
complex_out(complex_add(a,b)));</span></font><pclass="MsoNormal" style="margin-left:.5in"><font face="Courier New"
size="2"><spanstyle="font-size:10.0pt;font-family:"Courier New"">}</span></font><p class="MsoNormal"
style="margin-left:.5in"><fontface="Courier New" size="2"><span style="font-size:10.0pt;font-family:"Courier
New""> </span></font><pclass="MsoNormal"><font face="Arial" size="2"><span style="font-size:10.0pt; 
font-family:Arial">But using version 1 calling convention it won’t work! So, how can I pass the ‘a’ and ‘b’ variables
incomplex_add(?, ?)?</span></font><p class="MsoNormal"><font face="Arial" size="2"><span style="font-size:10.0pt; 
font-family:Arial"> </span></font><p class="MsoNormal"><font face="Arial" size="2"><span style="font-size:10.0pt;
font-family:Arial">Thanks in advance!</span></font><p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:10.0pt;
font-family:Arial"> </span></font><p class="MsoNormal"><font face="Arial" size="2"><span style="font-size:10.0pt;
font-family:Arial"> </span></font><p class="MsoNormal"><font face="Arial" size="2"><span style="font-size:10.0pt;
font-family:Arial"> </span></font><p class="MsoNormal"><font face="Arial" size="2"><span style="font-size:10.0pt;
font-family:Arial"> </span></font></div>

Re: Passing parameters to a C function

From

Martijn van Oosterhout

Date:

30 May 2007, 14:41:38

On Wed, May 30, 2007 at 11:26:01AM -0300, Rodrigo Sakai wrote:
>    I have a question about passing parameters to a C function.  Imagine the
> example by PostgreSQL:
>
> PG_FUNCTION_INFO_V1(complex_add);


> But using version 1 calling convention it won't work! So, how can I pass the
> 'a' and 'b' variables in complex_add(?, ?)?

Use the DirectFunctionCalln functions in fmgr.

Have a nice day,
--
Martijn van Oosterhout   <kleptog@svana.org>   http://svana.org/kleptog/
> From each according to his ability. To each according to his ability to litigate.

Re: Passing parameters to a C function

From

Tom Lane

Date:

30 May 2007, 15:01:36

Martijn van Oosterhout <kleptog@svana.org> writes:
> On Wed, May 30, 2007 at 11:26:01AM -0300, Rodrigo Sakai wrote:
>> But using version 1 calling convention it won't work! So, how can I pass the
>> 'a' and 'b' variables in complex_add(?, ?)?

> Use the DirectFunctionCalln functions in fmgr.

There are boatloads of examples in the existing datatype code, for
instance this function in geo_ops.c, which is just applying
close_sb() followed by dist_pb():

Datum
dist_sb(PG_FUNCTION_ARGS)
{   LSEG        *lseg = PG_GETARG_LSEG_P(0);   BOX         *box = PG_GETARG_BOX_P(1);   Point       *tmp;   Datum
result;
 
   tmp = DatumGetPointP(DirectFunctionCall2(close_sb,                                            LsegPGetDatum(lseg),
                                        BoxPGetDatum(box)));   result = DirectFunctionCall2(dist_pb,
           PointPGetDatum(tmp),                                BoxPGetDatum(box));
 
   PG_RETURN_DATUM(result);
}

All that casting to and from Datum is a bit of a pain, but it's worth it
to have a uniform, portable function API.

I strongly suggest adding some appropriate cast macros for your own
datatype, like the ones being used here, instead of dealing directly
with low-level operations like PG_GETARG_POINTER.
        regards, tom lane