Postgres Pro
Downloads
Home   >   mailing lists

Thread: C Extension woes

C Extension woes

From
Tim Hawes
Date:
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 <postgres.h>
#include <fmgr.h>
PG_MODULE_MAGIC;

#include <stdio.h>
#include <stdlib.h>

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
whileprocessing the request.
 
The connection to the server was lost. Attempting reset: Failed.
!>

Thanks ahead of time for any and all help.

Re: C Extension woes

From
Jan Urbański
Date:
Tim Hawes wrote:
> 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 <postgres.h>
> #include <fmgr.h>
> PG_MODULE_MAGIC;
> 
> #include <stdio.h>
> #include <stdlib.h>
> 
> 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;
> }

Oh, you confused a lot of things.
You need something like

Datum pl_masterkey(PG_FUNCTION_ARGS) {  char *e_var = getenv("PGMASTERKEY");
PG_RETURN_TEXT_P(cstring_to_text(e_var));
}

You don't need to mess with anything varlena-related (liek VARSIZE), 
it's all handled for you.
Also, read up on how to declare user-defined C functions in Postgres 
(they always need to return Datum).

Cheers,
Jan

-- 
Jan Urbanski
GPG key ID: E583D7D2

ouden estin

Re: C Extension woes

From
Andrew Chernow
Date:
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;

-- 
Andrew Chernow
eSilo, LLC
every bit counts
http://www.esilo.com/

Re: C Extension woes

From
Tom Lane
Date:
Andrew Chernow <ac@esilo.com> writes:
> Tim Hawes wrote:
>> 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".

The code on the output side is wrong too: if the useful data length is
"length", then the palloc has to ask for length + VARHDRSZ, and that's
also what's got to be put into the length word.
        regards, tom lane

Re: C Extension woes

From
Tim Hawes
Date:
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

Re: C Extension woes

From
Jan Urbański
Date:
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);
}

-- 
Jan Urbanski
GPG key ID: E583D7D2

ouden estin

Re: C Extension woes

From
Tim Hawes
Date:
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);
> }
>