SvSETMAGIC( ST(2) );    /* Is this correct? */
should be
SvSETMAGIC( bufsv );

The idea is to call tied($buf)->STORE() or similar if applicable.

Thanks. I had meant to replace ST(2) with bufsv (as I did other places). In the mean time I had read the docs on SvSETMAGIC() which indicated that usage was correct.

I'll update my original code block since you quote my original line above.

- tye        

Thanks ikegami. I am no XS or Perl internals expert so I appreciate the advice. I'll amend the code and come back here if that is all right.

Interesting comments tye. The code as I have it in DBD::ODBC does indeed pass is \$buf from Perl and works fine. The only bit from the DBD::Pg code I omitted was the sv_setpvn. Logically, I think the method I'm creating should pass in a reference to a scalar and that scalar should be grown to fit the returned lob data so I'll play with your suggested changes. Thanks. As it stands I have:

void odbc_lob_read(sth, colno, buf, length, attr = NULL)
     SV *sth
     int colno
     char *buf
     UV length
     SV *attr;
    PROTOTYPE: $$$$;$
    PREINIT:
     SV * const bufsv = SvROK(ST(2)) ? SvRV(ST(2)) : ST(2);
     UV ret_len;
     IV sql_type = 0;
    CODE:
     if (length == 0) {
         croak("Cannot retrieve 0 length lob");
     }
     if (attr) {
         SV **svp;
         DBD_ATTRIBS_CHECK("odbc_lob_read", sth, attr);
         DBD_ATTRIB_GET_IV(attr, "Type", 4, svp, sql_type);
     }
     sv_setpvn(bufsv, "", 0);                   /* ensure we can grow 
+ok */

     /* length is the length in chrs/bytes depending on the underlying
      * datatype. i.e., it is usually bytes but if we are built to sup
+port
      * unicode and the column is not a binary type, we will convert t
+o
      * UTF8 encoding so we need at least 6 * as many bytes. */
     buf = SvGROW(bufsv, length * + 1);
     ret_len = odbc_st_lob_read(sth, colno, bufsv, length, sql_type);
     if (ret_len > 0) {
         SvCUR_set(bufsv, ret_len);      /* set length in SV */
         *SvEND(bufsv) = '\0';           /* NUL terminate */
         /*sv_setpvn(ST(3), buf, 4);*/
         SvSETMAGIC(ST(2));
     }
     ST(0) = (ret_len >= 0) ? sv_2mortal(newSViv(ret_len)) : &PL_sv_un
+def;
[download]

and Perl code

my $s = $h->prepare(q{select 'frederick'});
$s->execute;
$s->bind_col(1, undef, {BindAsLOB=>1});
$s->fetch;
# SQL_SUCCESS = 0
# SQL_SUCCESS_WITH_INFO = 1
# SQL_NO_DATA = 100
while(my $len = $s->odbc_lob_read(1, \my $x, 8)) {
    print "len=$len, x=$x\n";
}
[download]

Heh, rereading the same basic code in your implementation made me realize that there is another minor bug. SvSETMAGIC() doesn't get called if the 'read' is successful but returns no data. That suggests the following simplification:

void odbc_lob_read(sth, colno, bufsv, length, attr = NULL)
     SV *sth
     int colno
     SV *bufsv
     UV  length
     SV *attr;
    PROTOTYPE: $$$$;$
    PREINIT:
     char *buf;
     UV ret_len;
     IV sql_type = 0;
    CODE:
     if (length == 0) {
         croak("Cannot retrieve 0 length lob");
     }
     if (SvROK(bufsv)) {
         bufsv = SvRV(bufsv);
     }
     if (attr) {
         SV **svp;
         DBD_ATTRIBS_CHECK("odbc_lob_read", sth, attr);
         DBD_ATTRIB_GET_IV(attr, "Type", 4, svp, sql_type);
     }
     sv_setpvn(bufsv, "", 0);                   /* ensure we can grow 
+ok */
     buf = SvGROW(bufsv, length + 1);
     ret_len = odbc_st_lob_read(sth, colno, bufsv, length, sql_type);
     if (ret_len < 0) {
         ST(0) = &PL_sv_undef;
     } else {
         SvCUR_set(bufsv, ret_len);      /* set length in SV */
         *SvEND(bufsv) = '\0';           /* NUL terminate */
         SvSETMAGIC(bufsv);
         ST(0) = sv_2mortal(newSViv(ret_len));
     }
     XSRETURN(1);
[download]

(Update: Added XSRETURN(1) after prompting in the chatterbox from ikegami.)

- tye        

Thanks again tye. I ended up with the following:

SV *
odbc_lob_read(sth, colno, bufsv, length, attr = NULL)
     SV  *sth
     int colno
     SV  *bufsv
     UV  length
     SV  *attr;
    PROTOTYPE: $$$$;$
    PREINIT:
     char *buf;
     UV   ret_len;
     IV   sql_type = 0;
    INIT:
     if (length == 0) {
         croak("Cannot retrieve 0 length lob");
     }
    CODE:
     if (attr) {
         SV **svp;
         DBD_ATTRIBS_CHECK("odbc_lob_read", sth, attr);
         DBD_ATTRIB_GET_IV(attr, "Type", 4, svp, sql_type);
     }
     if (SvROK(bufsv)) {
        bufsv = SvRV(bufsv);
     }
     sv_setpvn(bufsv, "", 0);                   /* ensure we can grow 
+ok */

     buf = SvGROW(bufsv, length + 1);
     ret_len = odbc_st_lob_read(sth, colno, bufsv, length, sql_type);
     if (ret_len > 0) {
         SvCUR_set(bufsv, ret_len);      /* set length in SV */
         *SvEND(bufsv) = '\0';           /* NUL terminate */
         SvSETMAGIC(bufsv);
         RETVAL = newSViv(ret_len);
     } else {
         XSRETURN_UNDEF;
     }
     OUTPUT:
        RETVAL
[download]

Thanks again for your suggestions.

Most grateful for you input tye++ - this is why I love perlmonks. I obviously need to work on this more before releasing it.