Re: libpq: PQexecParams, binaryFormat and float

[Richard Frankland <rfrankla(at)yahoo(dot)com>]

Hi, This is my first reply/post to postgres forums. I hope I'm doing it OK.
If not, please help.

So, onto how I see the problem and a possible solution.
As Tom Lane pointed out. Using a "union" is another solution to this problem.

Here is a simple testing program and output:

#include <stdio.h>
/* for ntohl/htonl */
#include <netinet/in.h>

void show_hex (void * vptr) {
    unsigned char * xptr;
    int k;
    /* display each byte in heX */
    xptr = (unsigned char *)vptr;
    for (k=0; k<4; k++)
    {
        printf("%.2X  ", *(xptr+k));
    }
    printf("\n");
}

int
main(int argc, char **argv)
{
    float         starting_value = 1.23;
    float         loadavg;
    unsigned long reversed_float;

    /* prove we really are starting with 1.23 */
    printf( "%f \n", starting_value);
    show_hex(&starting_value);

    /* see man page for return type of htonl() */
    reversed_float = htonl(starting_value);
    show_hex(&reversed_float);
    
    /* implicit conversion shown explicitly (i.e. has no effect) */
    reversed_float = htonl((long)starting_value);
    show_hex(&reversed_float);
    
    /* and another implicit conversion */
    loadavg = reversed_float; //htonl(starting_value);
    show_hex(&loadavg);

    /* again, shown explicitly */ 
    loadavg = (float)(unsigned long)16777216; //htonl(starting_value);
    show_hex(&loadavg);
    loadavg = (float)(unsigned long)htonl((long)starting_value); //htonl(starting_value);
    show_hex(&loadavg);
    /* show what was stored into loadavg */
    printf( "%f \n", loadavg);

    /* show this is the same as the original problem */
    loadavg = htonl(1.23);
    printf( "%f \n", loadavg);

    /* A (not the only one) correct solution                   */
    /* Casting a pointer:                                      */
    /*                       address of source                 */
    /*                                       |                 */
    /* pointer to UNSIGNED type              |                 */
    /*                       |               |                 */
    /*   dereference pointer |               |                 */
    /*                     | |               |                 */
    /*                     | |               |                 */
    /*                     | |               |                 */
    reversed_float = htonl(*(unsigned long *)&starting_value);

    show_hex(&reversed_float);
    
    /*
     *   paramValues[0] = (unsigned char *) &reversed_float;
     */
}

------------------------------------------------------
$ ./tst
1.230000 
A4  70  9D  3F  
00  00  00  01  
00  00  00  01  
00  00  80  4B  
00  00  80  4B  
16777216.000000 
16777216.000000 
3F  9D  70  A4  
$




"Sun, Way" <Way(dot)Sun(at)sciatl(dot)com> wrote:     
 Hi,
 I'm having trouble inserting a float into pg using binary format. 
 Hope someone on the list can help me point out what I'm doing wrong....
     float    loadavg;
     loadavg = htonl(1.23);
     paramValues[0] = (char *) &loadavg
    paramLengths[0] = sizeof(loadavg);
    paramFormats[0] = 1;        
     res = PQexecParams(conn,
                       "INSERT INTO cpu (loadavg) VALUES ($1::real)",
                       1,       
                       NULL,    
                       paramValues,
                       paramLengths,
                       paramFormats,
                       1);      
 From psql, select returns the value as 1.20765e-38 instead of 1.23.....:(
 Thanks
 Way


 
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