Re: issue with reading hostname

On 4/22/24 13:13, Atul Kumar wrote:
> Hi,
>
> I have postgresql  version 15 running on centos7.
>
> I have below query that reads hostname from /tmp directory:
>
> psql postgres -A -t -p 5432 -h /tmp/ -c 'SELECT pg_is_in_recovery();'
>
>
> so below are my questions:
>
> 1. Is the psql client reading the socket file that resides in the /tmp
> directory to fetch the hostname ?
>
> 2. I saw the socket file in /tmp and it is empty. Then how is the psql
> client still reading the socket file successfully for hostname ?
>
>
> this is my socket looks ( the size is 0 as the file is empty):
>
> srwxrwxrwx. 1 postgres postgres      0 Apr 22 12:47 .s.PGSQL.5432
>
>
> Please help me clarify these doubts.

https://www.postgresql.org/docs/current/libpq-connect.html#LIBPQ-PARAMKEYWORDS

"host

Name of host to connect to. If a host name looks like an absolute
path name, it specifies Unix-domain communication rather than TCP/IP
communication; the value is the name of the directory in which the
socket file is stored. (On Unix, an absolute path name begins with a
slash. On Windows, paths starting with drive letters are also
recognized.) If the host name starts with @, it is taken as a
Unix-domain socket in the abstract namespace (currently supported on
Linux and Windows). The default behavior when host is not specified, or
is empty, is to connect to a Unix-domain socket in /tmp (or whatever
socket directory was specified when PostgreSQL was built). On Windows,
the default is to connect to localhost.

A comma-separated list of host names is also accepted, in which
case each host name in the list is tried in order; an empty item in the
list selects the default behavior as explained above. See Section
34.1.1.3 for details.
"

The simplistic explanation is that the socket is the "host".

>
>
>
> Regards.
>
>
>
>
>
>

--
Adrian Klaver
adrian(dot)klaver(at)aklaver(dot)com