Обсуждение: Extracting data from BYTEA column to binary file using libpq

Hello everybody out there using PostgreSQL,

What is the problem with the following C++ code for the extraction of
data from a BYTEA column to a binary file?

#include <stdlib.h>
#include <stdio.h>
#include <iostream>
#include <fstream>
#include "libpq-fe.h"
using namespace std;

main ()
{
   PGconn *conn;
   conn = PQconnectdb("hostaddr='databaseserver.com' port='5432'
dbname='test_db' user='test_user' password='secret'");
   int size;
   const char* contents;
   PGresult* res;
   res = PQexecParams(conn,
   "SELECT filecontent FROM pictures WHERE picture_id='3'",
   0, NULL,NULL,NULL,NULL,
   1);

   if (res && PQresultStatus(res)==PGRES_TUPLES_OK)
   {
     size = PQgetlength(res, 0, 0);
     contents = PQgetvalue(res, 0, 0);
   }
   ofstream myFile ("picture.jpg", ios::out | ios::binary);
   myFile.write (contents);
   myFile.close();
}

Thanks in advance,
Julia

    Julia Jacobson wrote:

>   ofstream myFile ("picture.jpg", ios::out | ios::binary);
>   myFile.write (contents);

You must specify the number of bytes to write.

Best regards,
--
Daniel
PostgreSQL-powered mail user agent and storage: http://www.manitou-mail.org

On Tue, Sep 14, 2010 at 6:01 PM, Julia Jacobson <julia.jacobson@arcor.de> wrote:
> Hello everybody out there using PostgreSQL,
>
> What is the problem with the following C++ code for the extraction of data
> from a BYTEA column to a binary file?
>
> #include <stdlib.h>
> #include <stdio.h>
> #include <iostream>
> #include <fstream>
> #include "libpq-fe.h"
> using namespace std;
>
> main ()
> {
>  PGconn *conn;
>  conn = PQconnectdb("hostaddr='databaseserver.com' port='5432'
> dbname='test_db' user='test_user' password='secret'");
>  int size;
>  const char* contents;
>  PGresult* res;
>  res = PQexecParams(conn,
>  "SELECT filecontent FROM pictures WHERE picture_id='3'",
>  0, NULL,NULL,NULL,NULL,
>  1);
>
>  if (res && PQresultStatus(res)==PGRES_TUPLES_OK)
>  {
>    size = PQgetlength(res, 0, 0);
>    contents = PQgetvalue(res, 0, 0);
>  }
>  ofstream myFile ("picture.jpg", ios::out | ios::binary);
>  myFile.write (contents);
>  myFile.close();
> }
>
> Thanks in advance,
> Julia
>
> --
> Sent via pgsql-general mailing list (pgsql-general@postgresql.org)
> To make changes to your subscription:
> http://www.postgresql.org/mailpref/pgsql-general
>

Hi,

In addition to what Daniel Verite said, I think you should use numeric
IP address instead of the host name, or consider replacing 'hostaddr'
with 'host' if you plan to use host names.
'hostaddr' is meant to be used when you want to avoid the name resolution step.

 conn = PQconnectdb("host='databaseserver.com' port='5432'
dbname='test_db' user='test_user' password='secret'");

regards,

diego