PostgreSQL module - How to map an array of parameters into the function?

Question

Am trying to convert a little bit of C test code into a PostgreSQL v1 module

Code originally designed as a simple command line, which takes a variable number - an array - of text arguments; from 3 to 7

the original code's declarations are commented; I'm now trying to convert that into a PG shared object function. All of the arguments to the function will be text strings (base command, options, etc.)

how can I declare/pass the array into the function?

    PG_FUNCTION_INFO_V1(embed_0);

Datum
embed_0(PG_FUNCTION_ARGS)

// THIS was the declaration when it was a C executable:
// int main(int argc, char *argv[])
{
// don't think mapping argc to a PG type is needed here, right?
// (argc is not a parameter?)
int     *argc; // = PG_GETARG_INT32(0);
char    argv[] = PG_GETARG_TEXT_P(0);

int i;
Object *pName, *pCall, *pPart1, *pPart2, *pArgs, *pValue;

Craig Ringer · Accepted Answer · 2012-10-02 01:09:59Z

If you have a variable number of arguments you need to either:

Declare it VARIADIC;
Create n different signatures for the function with different numbers of arguments; or
In C create only the most general form, the 7-argument variety, and then create SQL function wrappers for the fewer-arguments cases that call the most general form.

If you really only need 3,4,5,6, and 7-argument versions I'd do something like:

CREATE FUNCTION embed0(text,text,text,text,text,text,text) RETURNS text
LANGUAGE 'C' AS 'embed0','embed0';

CREATE OR REPLACE FUNCTION embed0(text,text,text) RETURNS text AS $$
SELECT embed0($1,$2,$2,NULL,NULL,NULL,NULL);
$$ LANGUAGE 'SQL';

// ... etc

If 7 args was just an arbitrary upper bound and you can actually take any number of arguments you should instead just write:

CREATE FUNCTION embed0(text,text,text,VARIADIC text) RETURNS text
LANGUAGE 'C' AS 'embed0','embed0';

and handle the variable arguments in your C function. See the PostgreSQL source code for the concat function for how. Its implementation is text_concat in src/backend/utils/adt/varlena.c on line 3842 in current git head; your line number will differ. Most of the work is done in concat_internal.

Another example is the format function, with C implementation text_format (via lookup in pg_proc.h), located in varlena.c (according to git grep '^text_format'; Pg coding style rules specify that the function name must begin on the hard left of the line), line 3953 in current git. While a more complicated function it might be better as an example for you because it does all the work in one place rather than splitting out for a helper function call. It's declared in pg_proc.h but if it were declared in SQL it'd look something like:

CREATE FUNCTION format(text, VARIADIC text) RETURNS text AS 'internal','text_format';

There you'll see that VARIADIC arguments are accessed like any other from C, using the PG_GETARG_...(argno) macros. The PG_NARGS() macro reports the number of arguments passed. VARIADIC arguments may be null so you have to use PG_ARGISNULL(argno) and handle the case of a null argument.

So: I'd write it as a VARIADIC function using PG_NARGS, PG_GETARG_TEXT_P, PG_ARGISNULL. Because Pg's VARIADIC functions cannot be called implicitly with zero variadic arguments, I'd write a wrapper for the 3-argument case that does:

CREATE OR REPLACE FUNCTION embed_0(text,text,text) RETURNS text AS $$
SELECT embed_0($1,$2,$2, VARIADIC ARRAY[]::text[]);
$$ LANGUAGE 'SQL';

, passing an empty array as the variadic parameter. That way you can call it with 3 args too.

BTW, when coding be aware that the string in a Pg text is not null terminated, unlike those passed to main(). You must use the lengths PostgreSQL provides.. See src/include/fmgr.h, the tutorial, and the text handling in the functions in the source code. Don't use strlen, strcat, strcpy, etc because they expect null-terminated strings.

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