[HACKERS] C Extension woes

[Tim Hawes]

Hello all,

I am trying to write an extension in C that returns a simple environment 
variable. The code compiles without any complaint or warning, and it 
loads fine into the database, however, when I run the function, I get 
disconnected from the server.


Here is my C code:

#include 
#include 
PG_MODULE_MAGIC;

#include 
#include 

PG_FUNCTION_INFO_V1(pl_masterkey);

text * pl_masterkey(PG_FUNCTION_ARGS)
{
 char *e_var = getenv("PGMASTERKEY");
 size_t length = VARSIZE(e_var) - VARHDRSZ;

 text * mkey = (text *) palloc(length);
 VARATT_SIZEP(mkey) = length;
 memcpy(VARDATA(mkey), e_var, length);

 return mkey;
}

And here is the SQL I use to create the function in PostgreSQL:

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

And the results:

select pl_masterkey();
server closed the connection unexpectedly
   This probably means the server terminated abnormally
   before or while processing the request.
The connection to the server was lost. Attempting reset: Failed.
!>

Thanks ahead of time for any and all help.


--
Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-hackers

Re: [HACKERS] C Extension woes

[Tim Hawes]

Thank you for your replies, however, it still is not working, see below...

Andrew Chernow wrote:

Tim Hawes wrote:


text * pl_masterkey(PG_FUNCTION_ARGS)
{
 char *e_var = getenv("PGMASTERKEY");
 size_t length = VARSIZE(e_var) - VARHDRSZ;




The VARSIZE macro is for variable length structures, like a text or 
bytea which contains a length and data member.  You are using this 
macro on a regular C string "e_var".  Try this instead:


size_t length = e_var != NULL ? strlen(e_var) : 0;


@Jan:
 It appears the cstring_to_text function is unique to the latest 
PostgreSQL code. I do not have a def for that for PostgreSQL 8.2, and 
currently I am stuck working with that version. I changed the return 
value to Datum, and I had previously only copied from the examples at: 
http://www.postgresql.org/docs/8.2/interactive/xfunc-c.html


@Andrew:
 here is my new code:
Datum pl_masterkey(PG_FUNCTION_ARGS)
{
 char *e_var = getenv("PGMASTERKEY");
 size_t length = e_var != NULL ? strlen(e_var) : 0;

 text * mkey = (text *) palloc(length);
 VARATT_SIZEP(mkey) = length;
 memcpy(VARDATA(mkey), e_var, length);

 PG_RETURN_TEXT_P(mkey);
}
now gets:
select pl_masterkey();
ERROR:  invalid memory alloc request size 4294967293


--
Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-hackers

Re: [HACKERS] C Extension woes

[Tim Hawes]

Ok, that worked!
Thank you very much, Jan and others who gave their input.
I did see Tom's input for the VARHDRSZ and tried that, but forgot to add 
that again

when I called VARATT_SIZEP

Jan Urbański wrote:

Tim Hawes wrote:


@Jan:
 It appears the cstring_to_text function is unique to the latest 
PostgreSQL code. I do not have a def for that for PostgreSQL 8.2, and 


Oh, I'm sorry, I forgot about that. cstring_to_text has been added 
only recently (it's not even it 8.3, silly me).



Datum pl_masterkey(PG_FUNCTION_ARGS)
{
 char *e_var = getenv("PGMASTERKEY");
 size_t length = e_var != NULL ? strlen(e_var) : 0;

 text * mkey = (text *) palloc(length);
 VARATT_SIZEP(mkey) = length;
 memcpy(VARDATA(mkey), e_var, length);

 PG_RETURN_TEXT_P(mkey);
}


You forgot to palloc space for the varlena header. You need an extra 
VARHDRSZ bytes for the header, in addition to what is needed for your 
data.

Try something like this:

Datum
pl_masterkey(PG_FUNCTION_ARGS)
{
  char *e_var;
  size_t length;
  text *mkey;

  e_var = getenv("PGMASTERKEY");
  length = e_var ? strlen(e_var) : 0;
  mkey = (text *) palloc(VARHDRSZ + length);

  VARATT_SIZEP(mkey) = VARHDRSZ + length;
  memcpy(VARDATA(mkey), e_var, length);

  PG_RETURN_TEXT_P(mkey);
}




--
Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-hackers