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Re: PQescapeBytea & PQunescapeBytea

From: "Frost, Mr(dot) Michael (contractor)" <frost(at)nrlmry(dot)navy(dot)mil>
To: "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>, <pgsql-interfaces(at)postgresql(dot)org>
Subject: Re: PQescapeBytea & PQunescapeBytea
Date: 2005-05-04 17:09:15
Message-ID: 5188F15076BC7A4DBB705747602058C11AAB96@mailhost (view raw or flat)
Thread:
Lists: pgsql-interfaces
Thanks for the quick response Tom.

Here is some sample code that illustrates what I am trying to do.
Basically, I have an array of floating point values that I want to store
in a bytea field.  To do this, I cast the float * to unsigned char *,
run the unsigned char *'s through PQescapeBytea, and then insert the
record.  To extract it back out, I retrieve the record, run it through
PQunescapeBytea, and then cast it back to float.

The array of floats is just an example.  In practice, it would just be a
pointer some arbitrary data.

Thanks,
Michael Frost
Computer Sciences Corporation
Phone:  831-656-4723
Fax:  831-656-4769
Email:  frost(at)nrlmry(dot)navy(dot)mil

////////////////////////////////////////////////////////////////////////
//
#include "postgres.h"
#include <stdio.h>

exec sql include sqlca;

int main ( int argc, char **argv ) {
   void insertData ( );
   void getData ( );

   printf ( "Calling insertData\n" );
   insertData ( );

   printf ( "Calling getData\n" );
   getData ( );

}

void getData ( ) {
   int newSize = 0;
   float *prsData = 0;
exec sql begin declare section;
   char myData[2000];
   int nGridID = 0;
   int id = 1;
   char *pUnescapedData = 0;
exec sql end declare section;

   exec sql connect to mike;

   memset ( myData, 0, 2000 );
   exec sql select gridid, thedata into :nGridID, :myData from grids
where gridid = :id;

   pUnescapedData = PQunescapeBytea ( myData, &newSize );

   prsData = ( float * ) pUnescapedData;

   exec sql disconnect;

}

void insertData ( ) {
   float *pData = 0;
   int    i = 0;
   unsigned char *pTemp = 0;
   int    newSize = 0;
   int    size = 40;
exec sql begin declare section;
   unsigned char *pEncodeData = 0, *myData = 0;
   int            nGridID = 1;
exec sql end declare section;

   exec sql connect to mike;

   pData = ( float * ) calloc ( 1, 10 * sizeof ( float ) );
   for ( i = 0; i < 10; i++ )
      pData[i] = ( float ) i;

   pTemp = ( unsigned char * ) calloc ( 1, 10 * sizeof ( float )  + 4 );
   memcpy ( &pTemp[1], pData, 10 * sizeof ( float ) );
   memcpy ( pTemp, &size, 4 );

   pEncodeData = PQescapeBytea ( pTemp, 10 * sizeof ( float ) + 4,
&newSize );

   myData = ( unsigned char * ) calloc ( 1, newSize );
   memcpy ( myData, pEncodeData, newSize );

   exec sql delete from grids;
   exec sql insert into grids ( gridid, thedata ) values ( :nGridID,
:myData );

   exec sql disconnect;
}

/*
     Table "public.grids"
 Column  |  Type   | Modifiers
---------+---------+-----------
 gridid  | integer |
 thedata | bytea   |
*/
///////////////////////////////////////////////////////////////////////
-----Original Message-----
From: Tom Lane [mailto:tgl(at)sss(dot)pgh(dot)pa(dot)us]
Sent: Wednesday, May 04, 2005 7:30 AM
To: Frost, Mr. Michael (contractor)
Cc: pgsql-interfaces(at)postgresql(dot)org
Subject: Re: [INTERFACES] PQescapeBytea & PQunescapeBytea 


"Frost, Mr. Michael (contractor)" <frost(at)nrlmry(dot)navy(dot)mil> writes:
> [ PQescapeBytea & PQunescapeBytea are not inverses ]

They're not supposed to be.  PQescapeBytea creates something that can be
inserted into a SQL command as a string literal.  PQunescapeBytea
deconstructs something that has been returned as a SELECT result.
There are two different levels of backslashing involved.

This doesn't directly answer your original problem, but I think we'll
need to see more of your code to figure out what you were doing wrong.

			regards, tom lane

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Next:From: Tom LaneDate: 2005-05-04 18:19:49
Subject: Re: PQescapeBytea & PQunescapeBytea
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Subject: Re: [INTERFACES] calculated identity field in views, again...

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