Обсуждение: libpq binary data

Hello,
I try to get the epoch value of a date via the 
PQexecParams(conn, "SELECT extract(epoch from date + time) as epoch, content 
FROM daten .....);
In that database, the timestamp ist stored in the two fields date and time.
I want to get this data in binary form. The 
PQfsize(res, 1); 
tells me, that the size of the returned data is 8 byte (in contrast to the 
standard size of epoch, which is meant to be 4 byte)
I don't manage to get the epoch valule (seconds since 1970) from that returned 
value. After ntohll (which I wrote as a wrapper around ntohl for long int, see 
below) it is a very huge value (4743709917079142400) but it should be 
1412179252 as I get it from the psql interface, when I type in the same 
command.
What am I missing or doing wrong?
Thanks for any help in advance

Thilo

unsigned long int ntohll(long int x)
{if (ntohl(1) == 1)    return x;else    return (long int) (ntohl((int)((x << 32) >> 32))) << 32 | (long 
int)ntohl(((int)(x >> 32)));
}

     thilo@riessner.de wrote:

> I try to get the epoch value of a date via the 
> PQexecParams(conn, "SELECT extract(epoch from date + time) as epoch, content
> FROM daten .....);
> In that database, the timestamp ist stored in the two fields date and time.
> I want to get this data in binary form. The 
> PQfsize(res, 1); 
> tells me, that the size of the returned data is 8 byte (in contrast to the 
> standard size of epoch, which is meant to be 4 byte)
> I don't manage to get the epoch valule (seconds since 1970) from that
> returned value.

The doc about EXTRACT(field FROM source)  says:
"The extract function returns values of type double precision"

In C, that would presumably map to the "double" 64 bits floating point type.

For code converting the binary representation to a host variable, you may get
bits from  postgres itself in backend/libpq/pqformat.c

Otherwise, a working example could look like this
(without guarantee that it's suitable for your platform):

#include <stdint.h>
union {   uint64_t i;   double fp;
} swap;
uint64_t ibe = *((uint64_t*)PQgetvalue(result, row, column);
swap.i = be64toh(ibe);

And your result would be in swap.fp

Or you if prefer getting an int from postgres, cast the result of extract()
to an integer in the SQL query itself.

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

Hello again,
after trying the PQftype function on the date row, I realized, that the type 
is 701 which is a double. Treating the pointer as a double solved my problem, 
now I get the right number of seconds. But it is still necessary to swap the 
byte order from network to host with the function "ntohll" as shown below.
Hope this helps someone stucking in a similar problem.

Thilo

> Hello,
> I try to get the epoch value of a date via the
> PQexecParams(conn, "SELECT extract(epoch from date + time) as epoch, content
> FROM daten .....);
> In that database, the timestamp ist stored in the two fields date and time.
> I want to get this data in binary form. The
> PQfsize(res, 1);
> tells me, that the size of the returned data is 8 byte (in contrast to the
> standard size of epoch, which is meant to be 4 byte)
> I don't manage to get the epoch valule (seconds since 1970) from that
> returned value. After ntohll (which I wrote as a wrapper around ntohl for
> long int, see below) it is a very huge value (4743709917079142400) but it
> should be 1412179252 as I get it from the psql interface, when I type in
> the same command.
> What am I missing or doing wrong?
> Thanks for any help in advance
> 
> Thilo
> 
> unsigned long int ntohll(long int x)
> {
>     if (ntohl(1) == 1)
>         return x;
>     else
>         return (long int) (ntohl((int)((x << 32) >> 32))) << 32 | (long
> int)ntohl(((int)(x >> 32)));
> }