Thread: More probs with BLOBs

We are getting a strange thing happening if the lo_export(attr, path)
destination is a fifo.

First, in the normal case, there is output to the file but also
feedback to stdout (or somewhere, appearing on the VT screen) of the
number 1.

-----------
         1
(1 row)

Now, if the destination file is a fifo and a psql -c command is given
from the shell to lo_export(attr, '/path/to/fifo') we don't get that
feedback, but then a cat of the fifo to a normal file (cat fifo >
file) writes the BLOB content to the destination file but also sends

-----------
         1
(1 row)

(or just "1" with psql -t) to the console, apparently (not stderr).

So it seems to us that the lo_export output can't be piped (via a
named pipe) to another process that reads from the pipe and expects
the valid BLOB contents.

Beyond my understanding.  What is happening here, and can the echoing
to the fifo be avoided?  Ideas, anyone?

(PostgreSQL 9.1.2, Linux 2.6.29.)

Elle

On Wed, 15 Aug 2012, Neanderthelle Jones wrote:

> So it seems to us that the lo_export output can't be piped (via a
> named pipe) to another process that reads from the pipe and expects
> the valid BLOB contents.

I should have said that the BLOB is a PDF and the file that results
from cat fifo >my.pdf is a valid PDF file (containing only an image
object) that xpdf displays and prints.

But that is after the adornment has gone to the console.

Elle

Neanderthelle Jones wrote:
> We are getting a strange thing happening if the lo_export(attr, path)
> destination is a fifo.
>
> First, in the normal case, there is output to the file but also
> feedback to stdout (or somewhere, appearing on the VT screen) of the
> number 1.
>
> -----------
>          1
> (1 row)
>
> Now, if the destination file is a fifo and a psql -c command is given
> from the shell to lo_export(attr, '/path/to/fifo') we don't get that
> feedback, but then a cat of the fifo to a normal file (cat fifo >
> file) writes the BLOB content to the destination file but also sends
>
> -----------
>          1
> (1 row)
>
> (or just "1" with psql -t) to the console, apparently (not stderr).
>
> So it seems to us that the lo_export output can't be piped (via a
> named pipe) to another process that reads from the pipe and expects
> the valid BLOB contents.
>
> Beyond my understanding.  What is happening here, and can the echoing
> to the fifo be avoided?  Ideas, anyone?
>
> (PostgreSQL 9.1.2, Linux 2.6.29.)

It works fine here (9.1.3 on 2.4.21):

$ mkfifo fifo && chmod 0777 fifo
$ psql -U postgres
test#> \lo_import '/path/to/some/bmp'
lo_import 47248

test#> \q
$ psql -U postgres -c "SELECT lo_export(47248, '/path/to/fifo')" >out
2>err &
$ cat fifo >f
$ file f
f: PC bitmap data, Windows 3.x format, 173 x 115 x 8
$ cat out
lo_export
-----------
         1
(1 row)

The file "err" is empty.

Yours,
Laurenz Albe