String manipulation by C language function of PostgreSQL

Question

I'd like to create a postgresql c-Language function whose arguments and return values are varchar and which uses Char [] during processing.

But, it does not work as expected.

I made a program of c like this.

#include <postgres.h>
#include <port.h>
#include <fmgr.h>
#include <stdlib.h>

#ifdef PG_MODULE_MAGIC
PG_MODULE_MAGIC;
#endif

// Required for windows.
extern PGDLLEXPORT Datum VARCHAR_CHAR_ARRAY_VARCHAR(PG_FUNCTION_ARGS);

PG_FUNCTION_INFO_V1(VARCHAR_CHAR_ARRAY_VARCHAR);

Datum VARCHAR_CHAR_ARRAY_VARCHAR(PG_FUNCTION_ARGS) {
    // Get arg.
    VarChar  *arg1 = (VarChar *)PG_GETARG_VARCHAR_P(0);

    // VarChar to Char[].
    char *c = (char *)VARDATA(arg1);

    elog(NOTICE, "VarChar to Char[]."); 
    elog(NOTICE, c);//Log1

    // Do something.(e.g. replace)
    // Since it uses another system, it must be Char [].

    //Char[] to VarChar.
    VarChar *rtn = (VarChar *)VARDATA(c);
    elog(NOTICE, "Char[] to VarChar.");
    elog(NOTICE, rtn);//Log2

    // Return VarChar.
    PG_RETURN_VARCHAR_P(rtn);
}

And, created such a script.

CREATE OR REPLACE FUNCTION public.VARCHAR_CHAR_ARRAY_VARCHAR(character varying)
  RETURNS character varying AS
'$libdir/test/VARCHAR_CHAR_ARRAY_VARCHAR.dll', 'VARCHAR_CHAR_ARRAY_VARCHAR'
  LANGUAGE c VOLATILE STRICT;

The execution result is like this.

SELECT VARCHAR_CHAR_ARRAY_VARCHAR('a');
-- Expected value: 'a'
-- Postgresql terminated abnormally... :(

SELECT VARCHAR_CHAR_ARRAY_VARCHAR('Action');
-- Expected return value: 'Action'
-- Actual return value  :'n'
-- Log1  :'NOTICE:  n'
-- Log2  :''

SELECT VARCHAR_CHAR_ARRAY_VARCHAR('ActionAction');
-- Expected return value: 'ActionAction'
-- Actual return value  : ''  <-Why?
-- Log1  :''
-- Log2  :''

SELECT VARCHAR_CHAR_ARRAY_VARCHAR('Action Action');
-- Expected return value: 'Action Action'
-- Actual return value  : 'n Action'
-- Log1  :'NOTICE:  Action Action'
-- Log2  :'NOTICE:  ction'

SELECT VARCHAR_CHAR_ARRAY_VARCHAR('1234567890');
-- Expected return value: '1234567890'
-- Actual return value  : '6789'
-- Log1  :'NOTICE:  1234567890'
-- Log2  :'NOTICE:  90'

Execution environment:

Windows Server 2012 R2 x64
PostgreSQL 9.5.9, compiled by Visual C++ build 1800, 64-bit

I'm using C language for the first time, so I'm glad to present sample code.

I'm sorry, my English is not good.

How does it "not work as expected"? You show us the actual results but what are the expected results? Please (re-)read about how to ask good questions. — Some programmer dude
– Some programmer dude, Commented Oct 27, 2017 at 4:16
Sorry, I added an expected value. Anyway, if the input value is return as it is OK. (It is mandatory that it is internally converted to char [].) — Yuwaz
– Yuwaz, Commented Oct 27, 2017 at 4:22

Craig Ringer · Accepted Answer · 2017-10-27 06:35:01Z

5

Manually constructing a text or VarChar like that won't work. You can't just

VarChar *rtn = (VarChar *)VARDATA(c);

because what you're doing there is trying to interpret the char* value c as if it were a VarChar*, reading the first few bytes as a VARLENA header, and then getting some substring within it as the value. Not going to work.

It's much easier to use cstring_to_text and text_to_cstring to convert. text* is exactly compatible with VarChar*. (They're in utils/builtins.h).

e.g.

 PG_RETURN_VARCHAR_P(cstring_to_text(c));

To do it manually, have to palloc a string with strlen bytes + VARHDRSIZE, SET_VARSIZE, then strcpy the data to the VARDATA offset. See src/include/utils/varlena.h and src/backend/utils/adt/varlena.c. It's fiddly and not necessary when PostgreSQL has helper functions for the purpose.

Personally I recommend ignoring varchar completely for C-level programming. Define your functions as accepting and returning text.

edited Oct 27, 2017 at 6:35

answered Oct 27, 2017 at 4:47

Craig Ringer

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1 Comment

Yuwaz Over a year ago

Thank you for answering. However, I could not find cstring_from_text. Perhaps, is text_to_cstring?

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