libpq: PQexecParams, binaryFormat and float

It seems entirely unlikely that that will produce a byte-reversed float
value; what I think will happen is that the byte-reversed value of
integer 1 will be converted into a host-format float and stored into
loadavg.

AFAIK you can't really do this in C without a union. Take a look at
pq_sendfloat4 and pq_getmsgfloat4 in the backend for examples.

regards, tom lane

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" 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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