Thread: Example of a pg_gethostname() function. Feedback?

I had a need recently to get the server hostname of some databases
that we were unit-testing and couldn't find a built-in function for
it. One of my coworkers put together a C function that seems to work
well.
Does anyone have any suggestions or improvements to the code below?

Some testing was done with both 9.0 and and 9.1, linux, x86_64

To use:

select pg_gethostname();


/*
        A PostgreSQL function for getting the hostname.

        File: `pg_config --libdir`/pg_gethostname.c

        To compile:
                //gcc -fpic -c pg_gethostname.c
                //gcc -shared -o pg_gethostname.so pg_gethostname.o

        Note: the compile options above did work, amended steps:
                // make sure pg_config is in your path
                gcc  -I`pg_config --includedir-server` -fpic -c
pg_gethostname.c -L`pg_config --libdir`
                gcc -shared -o pg_gethostname.so pg_gethostname.o

        To create the function in PostgreSQL.

        //DROP FUNCTION IF EXISTS pg_gethostname();
        CREATE OR REPLACE FUNCTION pg_gethostname() RETURNS text
                AS 'pg_gethostname'
                LANGUAGE C IMMUTABLE STRICT;

*/

#include "postgres.h"
#include <unistd.h>
#include <string.h>
#include "fmgr.h"

#include "utils/palloc.h"
#include "utils/elog.h"
#include "storage/bufpage.h"

#ifdef PG_MODULE_MAGIC
PG_MODULE_MAGIC;
#endif

#define MAX_HOSTNAME_LENGTH 255


PG_FUNCTION_INFO_V1( pg_gethostname );

Datum pg_gethostname( PG_FUNCTION_ARGS );

Datum pg_gethostname( PG_FUNCTION_ARGS )
{
        text *t;
        char server_hostname[MAX_HOSTNAME_LENGTH];
        size_t length;

        if ( gethostname( server_hostname, MAX_HOSTNAME_LENGTH ) != 0 )
        {
                // things are not okay
                strncpy( server_hostname, "UNKNOWN", MAX_HOSTNAME_LENGTH );
        }

        length = strnlen( server_hostname, MAX_HOSTNAME_LENGTH );
        t = (text *) palloc(VARHDRSZ + length );
        SET_VARSIZE( t, VARHDRSZ + length );
        memcpy( VARDATA(t), server_hostname, length );

        PG_RETURN_TEXT_P( t );
}

On 24/12/2011 2:47 AM, bricklen wrote:
> I had a need recently to get the server hostname of some databases
> that we were unit-testing and couldn't find a built-in function for
> it. One of my coworkers put together a C function that seems to work
> well.
> Does anyone have any suggestions or improvements to the code below?
Only that it might be less hassle to wrap gethostname from pl/perl or
pl/python rather than adding a new C function, particularly if this is
only for DB testing and is not performance critical.

--
Craig Ringer

On Fri, Dec 23, 2011 at 8:22 PM, Craig Ringer <ringerc@ringerc.id.au> wrote:
> Only that it might be less hassle to wrap gethostname from pl/perl or
> pl/python rather than adding a new C function, particularly if this is only
> for DB testing and is not performance critical.
>
> --
> Craig Ringer

Hi Craig,

That was my first thought. However, in two of our servers, Perl is
mis-configured in some way that it causes the db to abort and crash
when a simple plperlu function is run. I reported a test case a few
months ago and that was Tom's conclusion and I didn't have the time or
inclination to pursue it. We rarely use any pl language other than
plpgsql, so I deemed it safer in this instance to avoid using plperlu
altogether.


Cheers,

Bricklen


Note, my coworker revised the C function slightly:
- Used HOST_NAME_MAX from limits.h, which is portable.
- Return an empty string on failure instead of "UNKNOWN" because that
makes more sense and allows for the host name "UNKNOWN".
- Fixed the comments.


/*
    A PostgreSQL function for getting the hostname.

    To compile...  (make sure pg_config is in your PATH)
    gcc  -I$(pg_config --includedir-server) -fpic -c pg_gethostname.c
-L$(pg_config --libdir)
    gcc --shared -o pg_gethostname.so pg_gethostname.o

    To create the funciton in PostgreSQL...
    CREATE OR REPLACE FUNCTION pg_gethostname() RETURNS text
        AS 'pg_gethostname'
        LANGUAGE C IMMUTABLE STRICT;

*/
#include "postgres.h"
#include <limits.h>
#include <unistd.h>
#include <string.h>
#include "fmgr.h"

#include "utils/palloc.h"
#include "utils/elog.h"
#include "storage/bufpage.h"

#ifdef PG_MODULE_MAGIC
PG_MODULE_MAGIC;
#endif




PG_FUNCTION_INFO_V1( pg_gethostname );

Datum pg_gethostname( PG_FUNCTION_ARGS );

Datum pg_gethostname( PG_FUNCTION_ARGS )
{
    text *t;
    char server_hostname[HOST_NAME_MAX];
    size_t length;

    if ( gethostname( server_hostname, HOST_NAME_MAX ) != 0 )
    {
        // returning an empty string for the hostname if it fails makes
        // sense because if it succeeded, we would have a name
        server_hostname[0] = '\0';
    }

    length = strnlen( server_hostname, HOST_NAME_MAX );
    t = (text *) palloc(VARHDRSZ + length );
    SET_VARSIZE( t, VARHDRSZ + length );
    memcpy( VARDATA(t), server_hostname, length );

    PG_RETURN_TEXT_P( t );
}