Обсуждение: code example for PQgetCopyData

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From: pgsql-general-owner@postgresql.org [mailto:pgsql-general-owner@postgresql.org] On Behalf Of Dave Huber
Sent: Thursday, December 03, 2009 9:18 AM
To: pgsql-general@postgresql.org
Subject: [GENERAL] code example for PQgetCopyData

Does anybody have a snippet where they use PQgetCopyData? I must be calling it wrong as it keep crashing my program. I’ve attached my code below. I am writing this for a Code Interface Node in LabVIEW.

 

Thanks,

Dave

 

MgErr CINRun(LStrHandle conninfo, LStrHandle copystr, TD1Hdl resultValues) {

      MgErr err = noErr;

      PGconn *pConn;

      PGresult* pResult = NULL;

      char* szCopyStr = NULL;

      char* errormsg = NULL;

      char** buffer = NULL;               // for retrieving the data

      int nLen;                                 // length of returned data

     

      // connect to the database

      char* szConnInfo = new char[LHStrLen(conninfo)+1];

      LToCStr((*(conninfo)), (CStr)szConnInfo);

      pConn = PQconnectdb(szConnInfo);

      delete [] szConnInfo;

 

      // check for errors connecting to database

      if (PQstatus(pConn) != CONNECTION_OK)

      {

            DbgPrintf("Connection to database failed: %s", PQerrorMessage(pConn));

      }

      else

      {

            // start the copy command

            szCopyStr = new char[LHStrLen(copystr)+1];

            LToCStr((*(copystr)), (CStr)szCopyStr);

 

            pResult = PQexec(pConn, szCopyStr);

            delete [] szCopyStr;

     

            // get the data

            int i = 0;

            while (nLen = PQgetCopyData(pConn, buffer, false) > 0)

            {

     

                  if (err = SetCINArraySize((UHandle)resultValues, 2, ++i))

                        goto out;

                  if (err = NumericArrayResize(uB, 1L, (UHandle*)(&(*resultValues)->elt[i-1]), nLen-1))

                        goto out;

                  LStrLen(*(*resultValues)->elt[i-1]) = nLen-1;                                 // set the Labview String size

                  MoveBlock(*buffer, LStrBuf(*(*resultValues)->elt[i-1]), nLen-1);  // copy the data to a new string

 

                  PQfreemem(*buffer);                                                                             // free the memory from getCopy

           

            }

            (*resultValues)->dimSize = i;

     

out:       

            PQclear(pResult);

 

            // see if there were errors

            if (nLen == -2)

            {

                  DbgPrintf("Copy Out failed: %s", PQerrorMessage(pConn));

            }

 

      }

      // close the connection

      PQfinish(pConn);

 

      return err;

}

 

Where is it blowing up?

 

 

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Dave Huber <DHuber@letourneautechnologies.com> writes:
> Does anybody have a snippet where they use PQgetCopyData? I must be calling it wrong as it keep crashing my program.
I'veattached my code below. I am writing this for a Code Interface Node in LabVIEW. 

One thing you're missing is that you should check that the result from
the PQexec actually shows successful entry into COPY_OUT state.
But I think the crash is because you're confused about the indirection
level.  buffer should be char *, not char **, and the argument ought to
be &buffer so that the function can assign to buffer.

            regards, tom lane

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From: Dave Huber [mailto:DHuber@letourneautechnologies.com]
Sent: Thursday, December 03, 2009 11:35 AM
To: 'bret_stern@machinemanagement.com'; pgsql-general@postgresql.org
Subject: RE: [GENERAL] code example for PQgetCopyData

 

> Where is it blowing up?

 

I’m sorry, I wasn’t clear. It bombs on the PQgetCopyData call. If I comment out the entire while loop, the program runs fine. If I simply comment out the contents of the while loop…kablooey!

 

Dave

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Tom,

Thanks for the help. Setting buffer to a char * fixed the crashing problem.

Now, I have a different issue. The result from PQgetCopyData is always 1 for every row of data returned. Does this not
workfor return data "WITH BINARY"? If I issue the same copy command to a file instead of STDOUT and examine the file
contents,there is most definitely data for each row. The command going to the PQexec function is: 

COPY (SELECT * FROM event_log_table) TO STDOUT WITH BINARY
Or
COPY (SELECT * FROM event_log_table) TO E'C:\\testfile' WITH BINARY

To reiterate, nLen in the following code always == 1:


                pResult = PQexec(pConn, szCopyStr);
                delete [] szCopyStr;

                // make sure we are in the copy out state before reading                        if (PGRES_COPY_OUT ==
PQresultStatus(pResult))
                {
                        // get the data
                        int i = 0;
                        while (nLen = PQgetCopyData(pConn, &buffer, false) > 0)
                        {
                ...


Thanks again.

Dave


Tom wrote:

One thing you're missing is that you should check that the result from
the PQexec actually shows successful entry into COPY_OUT state.
But I think the crash is because you're confused about the indirection
level.  buffer should be char *, not char **, and the argument ought to
be &buffer so that the function can assign to buffer.

                        regards, tom lane



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inerror, please immediately advise the sender by reply email message to the sender and delete all copies of this
message.
THIS E-MAIL IS NOT AN OFFER OR ACCEPTANCE: Notwithstanding the Uniform Electronic Transactions Act or any other law of
similarimport, absent an express statement to the contrary contained in this e-mail, neither this e-mail nor any
attachmentsare an offer or acceptance to enter into a contract, and are not intended to bind the sender, LeTourneau
Technologies,Inc., or any of its subsidiaries, affiliates, or any other person or entity. 
WARNING: Although the company has taken reasonable precautions to ensure no viruses are present in this email, the
companycannot accept responsibility for any loss or damage arising from the use of this email or attachments. 

Dave Huber <DHuber@letourneautechnologies.com> writes:
> Now, I have a different issue. The result from PQgetCopyData is always
> 1 for every row of data returned. Does this not work for return data
> "WITH BINARY"?

Weird, it should be the row length in bytes.  Are you maybe testing on
empty rows?

            regards, tom lane