PostgreSQL and embedded Ruby

ruby-talk
1

Hello,

INTRO: I would like to build simple application in C to deal with
PostgreSQL, but part of work should be done by RUBY.

PROBLEM: I don’t want to make new connection with PGSQL in Ruby script
but attach existing one (‘conn’ variable as below) from C.

QUESTION: Is it possible to attach to Postgres connection from C in Ruby
script? And how to do it?

Szymon Drejewicz

···

#=======================================================================
#include <stdlib.h>
#include <libpq-fe.h>
#include <ruby.h>
#include “postgres.c”

int main(int argc, char** argv) {
PGresult *result;
PGconn *conn;
int nFields;
int i;
int j;
conn = PQconnectdb("");

if(PQstatus(conn) == CONNECTION_OK) {
printf(“connection made\n”);
result = PQexec(conn, “SELECT COUNT(*) FROM studenci”);

 printf("Wy¶wietlam wynik za pomoc± C:\n");
 /* first, print out the attribute names */
 nFields = PQnfields(result);
 for (i = 0; i < nFields; i++)
   printf("%-15s", PQfname(result, i));
 printf("\n\n");
 /* next, print out the rows */
 for (i = 0; i < PQntuples(result); i++) {
   for (j = 0; j < nFields; j++)
printf("%-15s", PQgetvalue(result, i, j));
   printf("\n");
 }

 printf("Teraz spróbujê przekazaæ wynik SELECTA do skryptu Ruby'ego,

który wy¶wietli to samo co powy¿ej:");
ruby_init();
ruby_options(argc,argv);
/* Udostêpniam Ruby’emu po³±czenie z PostgreSQL’em /
ruby_init_loadpath();
rb_require(“postgres”);
//-----------------PROBLEM BEGIN
VALUE DBconn = Data_Wrap_Struct(rb_cPGconn, 0, free_pgconn, conn);
// rb_obj_call_init(DBconn,0,0);
VALUE connection = conn;
rb_define_variable("$conn",&connection);
rb_eval_string(“puts ‘\nTEST\n’”);
rb_eval_string("a = $conn.query('select count() from
studenci’)[0][0] \n puts a");
//-----------------PROBLEM END
}
else
printf(“connection failed: %s\n”, PQerrorMessage(conn));

PQfinish(conn);
return EXIT_SUCCESS;
}

2

     VALUE DBconn = Data_Wrap_Struct(rb_cPGconn, 0, free_pgconn, conn);
     // rb_obj_call_init(DBconn,0,0);
     VALUE connection = conn;
     rb_define_variable("$conn",&connection);

                                   ^^^^^^^^^^^
                                   &DBconn

     rb_eval_string("puts '\nTEST\n'");
     rb_eval_string("a = $conn.query('select count(*) from
studenci')[0][0] \n puts a");

Guy Decoux

3

ts wrote:

“S” == Szymon Drejewicz drejewic@wsisiz.edu.pl writes:

 VALUE DBconn = Data_Wrap_Struct(rb_cPGconn, 0, free_pgconn, conn);
 //    rb_obj_call_init(DBconn,0,0);
 VALUE connection = conn;
 rb_define_variable("$conn",&connection);

                               ^^^^^^^^^^^
                               &DBconn

 rb_eval_string("puts '\nTEST\n'");
 rb_eval_string("a = $conn.query('select count(*) from

studenci’)[0][0] \n puts a");

Guy Decoux

Thanks for reply. I’ve changed this, but:

-e:1: method call on terminated object (NotImplementedError)
-e:1: [BUG] Segmentation fault

···

Szymon Drejewicz

4

-e:1: method call on terminated object (NotImplementedError)
-e:1: [BUG] Segmentation fault

pigeon% cat a.c
#include <stdlib.h>
#include <libpq-fe.h>
#include <ruby.h>

int main(int argc, char** argv) {
   PGresult *result;
   PGconn *conn;
   int nFields;
   int i;
   int j;
   VALUE rb_cPGconn;
   conn = PQconnectdb("");

   if(PQstatus(conn) == CONNECTION_OK) {
     printf("connection made\n");
     result = PQexec(conn, "SELECT COUNT(*) FROM studenci");

     nFields = PQnfields(result);
     for (i = 0; i < nFields; i++)
       printf("%-15s", PQfname(result, i));
     printf("\n\n");
     for (i = 0; i < PQntuples(result); i++) {
       for (j = 0; j < nFields; j++)
        printf("%-15s", PQgetvalue(result, i, j));
       printf("\n");
     }

     ruby_init();
     // ruby_options(argc,argv);
     ruby_init_loadpath();
     rb_require("postgres");
     rb_cPGconn = rb_const_get(rb_cObject, rb_intern("PGconn"));
     //-----------------PROBLEM BEGIN
     {
         VALUE DBconn = Data_Wrap_Struct(rb_cPGconn, 0, 0, conn);
     // rb_obj_call_init(DBconn,0,0);
         VALUE connection = conn;
         rb_define_variable("$conn",&DBconn);
         rb_eval_string("puts '\nTEST\n'");
         rb_eval_string("a = $conn.query('select count(*) from studenci')[0][0] \n puts a");
     }
     //-----------------PROBLEM END
   }
   else
     printf("connection failed: %s\n", PQerrorMessage(conn));

   PQfinish(conn);
   return EXIT_SUCCESS;
}

pigeon%

pigeon% ./a.out
connection made
count

1

TEST
1
pigeon%

Guy Decoux