[mercury-users] Strange problem

[Pieter Laeremans]

Hello,

I'm trying to create a module that enables me to store and retrieve
data in a postgres database.

So I created a module psql.m and used the foreign language interface
to include the C-library.

But when I compile and run the program using just a main predicate
that only contains an "io__nl" I get a segmentation fault. 

When I tested the program with an  io__write_string as the first
statement, there didn't appeared any output.  

Can you give me a clue about how I should debug this ?

thanks in advance,

Pieter

-----------[Mmakefile]-------------
MLLIBS=-lpq
MLFLAGS=--make-shared-lib
GRADE=hlc.gc

-----------[psql.m]-------------------

:- module psql.

:- interface.

:- import_module io.

:- type host == string.
:- type port == int.
:- type options == string.
:- type database == string.
:- type user == string.
:- type password == string.

:- type connection.
:- type connection_result  --->
     ok(connection);
     error.


:- pred create_connection(host::in,port::in, database::in, user::in, password::in, connection_result::uo, io__state::di, io__state::uo  ) is det.
:- pred create_connection(database::in, connection_result::out, io__state::di, io__state::uo) is det.

:- pred close_connection(connection::di, io__state::di, io__state::uo) is det.

:- pred main(io__state::di, io__state::uo) is det.

:- implementation.

:- import_module string.

main-->
     io__nl.
%     io__write_string("PSQL module testing...\n"),
     % create_connection("cudi", Result),
%      { Result = ok(Connection) ->
%        B = "Connectie gelukt!\n"
%      ;
%        B = "Connectie mislukt!\n"
%      },

%     io__write_string(B),


%%:- type connection == c_pointer.

:- pragma foreign_decl("C", "

		      #include <stdio.h>
		      #include <postgresql/libpq-fe.h>		      
		      
		      ").

% A connection is equal to this type:
:- pragma foreign_type("C", connection, "PGconn*").
     

:- pred pg_connectdb(string::in, connection::uo, int::out, io__state::di, io__state::uo) is det.
:- pragma foreign_proc("C", pg_connectdb(ConnectionString::in, Connection::uo, Errno::out, IO0::di, IO::uo), [promise_pure], "
       Connection = PQconnectdb(ConnectionString);
       if (PQstatus(Connection) == CONNECTION_BAD){
		      PQfinish(Connection);
		      Errno = 1;
       }else{
	Errno = 0;	      
       }
       IO = IO0;
				 
").

create_connection(Host, Port, Database, User, Password, ConnectionResult) -->
     {ConnectionString = "host='" ++ Host ++ "'port='" ++ string__int_to_string(Port)
     ++ "'dbname='" ++ Database ++ "'user='" ++ User ++ "'Password='" ++ Password ++ "'"},
     io__write_string("We gaan een connectie maken\n"),
     make_connection(ConnectionString, ConnectionResult).

create_connection(Database, ConnectionResult) -->
     {ConnectionString = "dbname='" ++ Database ++ "'"},
     make_connection(ConnectionString, ConnectionResult).

:- pred make_connection(string::in, connection_result::uo, io__state::di, io__state::uo) is det.
make_connection(ConnectionString, ConnectionResult) -->
     pg_connectdb(ConnectionString, Connection, Errno),
     { Errno = 0 ->
       ConnectionResult = ok(Connection)
     ;
       ConnectionResult = error
     }.


:- pragma foreign_proc("C", close_connection(Connection::di, IO0::di, IO::uo), [promise_pure],

		       "
		       PQfinish(Connection);
		       IO=IO0;

		       ").
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