How to send a NULL argument through a DirectFunctionCall?

Question

I'm modifying the function Datum create_vlabel(PG_FUNCTION_ARGS), within Apache AGE.

But sometimes it's needed that one of the arguments to this function be NULL, and there's a check to that, like this:

if (!PG_ARGISNULL(2)) { ... }

The thing is, other functions need to call create_vlabel with the DirectFunctionCall command, but I can't figure out how to pass a NULL value through this command.

Everything I tried either broke the system or was not recognized as a proper NULL by the if clause mentioned above.

Here's everything I tried that did not work:

DirectFunctionCall1(create_vlabel, NULL) 
DirectFunctionCall1(create_vlabel, CStringGetDatum("NULL"))
DirectFunctionCall1(create_vlabel, CStringGetDatum(""))
DirectFunctionCall1(create_vlabel, CStringGetDatum(NULL))
DirectFunctionCall1(create_vlabel, PointerGetDatum(NULL))

So is there a proper way of sending a NULL argument through a DirectFunctionCall?

Laurenz Albe · Accepted Answer · 2023-02-28 11:56:52Z

2

The comments in src/include/fmgr.h are quite clear on this topic:

/* These are for invocation of a specifically named function with a
 * directly-computed parameter list.  Note that neither arguments nor result
 * are allowed to be NULL.  Also, the function cannot be one that needs to
 * look at FmgrInfo, since there won't be any.
 */
extern Datum DirectFunctionCall1Coll(PGFunction func, Oid collation,
                                     Datum arg1);

[...]

#define DirectFunctionCall1(func, arg1) \
    DirectFunctionCall1Coll(func, InvalidOid, arg1)

If you need to call an SQL function will NULL values, you have to use FunctionCallInvoke(). Look at src/backend/tcop/fastpath.c for inspiration:

define fcinfo with
```
LOCAL_FCINFO(fcinfo, FUNC_MAX_ARGS);
```
initialize it with InitFunctionCallInfoData()
set fcinfo->nargs and fcinfo->args[i]->isnull and fcinfo->args[i]->value for each argument
call FunctionCallInvoke(fcinfo)

answered Feb 28, 2023 at 11:56

Laurenz Albe

257k22 gold badges312 silver badges388 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

jian Over a year ago

Hi, there. Sorry. I failed, can you give a full minimum example just like cstring_to_text with cstring as null.

Laurenz Albe Over a year ago

@jian Too lazy, sorry.

Collectives™ on Stack Overflow

How to send a NULL argument through a DirectFunctionCall?

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related