Обсуждение: sin() in PostgreSQL?

Is there a sin() function in PostgreSQL? I need to create a view in
PostgreSQL that uses sin(x), and the only real mathematical functions I
see supported are dexp() and dpow(). I tried using dexp(I) to get the
exponential of an imaginary number (which would make writing sin() easy),
but this didn't work, since dexp() expects a floating point argument. Is
there an easy way to get sin()?

I tried the following, one at a time, to create sin() for PostgreSQL:

CREATE FUNCTION sin(float8) RETURNS float8 AS '/usr/lib/libm.so' LANGUAGE 'c';
CREATE FUNCTION sin(float4) RETURNS float4 AS '/usr/lib/libm.so' LANGUAGE 'c';
CREATE FUNCTION sin(float4) RETURNS float8 AS '/usr/lib/libm.so' LANGUAGE 'c';
CREATE FUNCTION sin(float8) RETURNS float4 AS '/usr/lib/libm.so' LANGUAGE 'c';

Each of these gave different and odd results (and a 'segmentation fault'
at one point), but none of them gave the right answer. What am I doing
wrong?

On Mon, 31 Jan 2000 mathprof@bigfoot.com wrote:

> I tried the following, one at a time, to create sin() for PostgreSQL:
>
> CREATE FUNCTION sin(float8) RETURNS float8 AS '/usr/lib/libm.so' LANGUAGE 'c';
> CREATE FUNCTION sin(float4) RETURNS float4 AS '/usr/lib/libm.so' LANGUAGE 'c';
> CREATE FUNCTION sin(float4) RETURNS float8 AS '/usr/lib/libm.so' LANGUAGE 'c';
> CREATE FUNCTION sin(float8) RETURNS float4 AS '/usr/lib/libm.so' LANGUAGE 'c';
>
> Each of these gave different and odd results (and a 'segmentation fault'
> at one point), but none of them gave the right answer. What am I doing
> wrong?

 Yes, it is probably wrong. Very offen PG's buildin functions allocate memory
for result and IMHO your trial wrong mixing pointers. See a backend/utils/atd
in PG source as example.

                        Karel

Karel Zak - Zakkr wrote:
>
> On Mon, 31 Jan 2000 mathprof@bigfoot.com wrote:
>
> > I tried the following, one at a time, to create sin() for PostgreSQL:
> >
> > CREATE FUNCTION sin(float8) RETURNS float8 AS '/usr/lib/libm.so' LANGUAGE 'c';
> > CREATE FUNCTION sin(float4) RETURNS float4 AS '/usr/lib/libm.so' LANGUAGE 'c';
> > CREATE FUNCTION sin(float4) RETURNS float8 AS '/usr/lib/libm.so' LANGUAGE 'c';
> > CREATE FUNCTION sin(float8) RETURNS float4 AS '/usr/lib/libm.so' LANGUAGE 'c';
> >
> > Each of these gave different and odd results (and a 'segmentation fault'
> > at one point), but none of them gave the right answer. What am I doing
> > wrong?
>
>  Yes, it is probably wrong. Very offen PG's buildin functions allocate memory
> for result and IMHO your trial wrong mixing pointers. See a backend/utils/atd
> in PG source as example.

More specifically, only 1,2 and 4 byte integer values may be passed by
call-by-value to userdefined C functions in PostgreSQL. That is, floats
are passed by reference (pointers) while the math lib passes them by
value - you have to write your own wrappers around the mathlib sin
function.

Sevo

Thanks! Based on this, I created a file called "test.c" as follows:

#include <math.h>
float pg_sin(float *x) {return(sin(*x));}

and compiled it using: "gcc test.c -lm -shared" to create a shared object
file. I then tried the same steps as below (substituting
"/usr/lib/libm.so" with the path to my "a.out" file created by gcc), but
this still didn't work? I'm getting different wrong values, but they're
still wrong nonetheless?

On Tue, 1 Feb 2000, Sevo Stille wrote:

> Date: Tue, 01 Feb 2000 13:06:37 +0100
> From: Sevo Stille <sevo@ip23.net>
> To: Karel Zak - Zakkr <zakkr@zf.jcu.cz>
> Cc: mathprof@bigfoot.com, pgsql-general@postgreSQL.org
> Subject: Re: [GENERAL] Linking in sin() as a C function
>
> Karel Zak - Zakkr wrote:
> >
> > On Mon, 31 Jan 2000 mathprof@bigfoot.com wrote:
> >
> > > I tried the following, one at a time, to create sin() for PostgreSQL:
> > >
> > > CREATE FUNCTION sin(float8) RETURNS float8 AS '/usr/lib/libm.so' LANGUAGE 'c';
> > > CREATE FUNCTION sin(float4) RETURNS float4 AS '/usr/lib/libm.so' LANGUAGE 'c';
> > > CREATE FUNCTION sin(float4) RETURNS float8 AS '/usr/lib/libm.so' LANGUAGE 'c';
> > > CREATE FUNCTION sin(float8) RETURNS float4 AS '/usr/lib/libm.so' LANGUAGE 'c';
> > >
> > > Each of these gave different and odd results (and a 'segmentation fault'
> > > at one point), but none of them gave the right answer. What am I doing
> > > wrong?
> >
> >  Yes, it is probably wrong. Very offen PG's buildin functions allocate memory
> > for result and IMHO your trial wrong mixing pointers. See a backend/utils/atd
> > in PG source as example.
>
> More specifically, only 1,2 and 4 byte integer values may be passed by
> call-by-value to userdefined C functions in PostgreSQL. That is, floats
> are passed by reference (pointers) while the math lib passes them by
> value - you have to write your own wrappers around the mathlib sin
> function.
>
> Sevo
>
> ************
>

mathprof@bigfoot.com wrote:
>
> Thanks! Based on this, I created a file called "test.c" as follows:
>
> #include <math.h>
> float pg_sin(float *x) {return(sin(*x));}
>
> and compiled it using: "gcc test.c -lm -shared" to create a shared object
> file. I then tried the same steps as below (substituting
> "/usr/lib/libm.so" with the path to my "a.out" file created by gcc), but
> this still didn't work? I'm getting different wrong values, but they're
> still wrong nonetheless?


The return has to be a pointer too. Try:

#include <math.h>
#include "postgres.h"
#include "utils/palloc.h"

float *pg_sin(float *x){
    float *f = (float *)palloc(sizeof(float));
     *f = sin(*x);
    return f;
}


Sevo

--
Sevo Stille
sevo@ip23.net