Thread: Help with adding C-Language Functions
Hi,
I am trying to add a C-Language Function. I have created a small example
in file BlackboardFunctions.c.
#include <postgres.h>
#include <fmgr.h>
PG_FUNCTION_INFO_V1(add_one);
Datum
add_one(PG_FUNCTION_ARGS)
{ int32 arg = PG_GETARG_INT32(0);
PG_RETURN_INT32(arg + 1);
}
The make file:
make: cc -fpic -c BlackboardFunctions.c -I /home/grads/carrolls/source/postgresql-7.4.5/src/include/ cc
-shared-o BlackboardFunctions.so BlackboardFunctions.o
Everything compiles nicely and when i try to add the function using:
Blackboard=# CREATE FUNCTION add_one(integer) RETURNS integer
Blackboard-# AS '/home/grads/carrolls/gradstudies/implementation/BlackboardFunctions/BlackboardFunctions',
'add_one'
Blackboard-# LANGUAGE C STRICT;
I get the following error:
ERROR: could not find function "add_one" in file
"/home/grads/carrolls/gradstudies/implementation/BlackboardFunctions/BlackboardFunctions.so"
A grep of BlackboardFunction.so finds add_one.
I figure i must be doing something obviously wrong but for the life of me
i cant figure out what that might bt. Can anyone recommend a course
of action?
Thank you,
Seamus
I changed my file from BlackboardFunctions.c to all lowercase,
blackboardfunctions.c, and the database now finds the add_one function.
Crazy. Can someone explain the reason why this happened? Is this a
linker issue or something in postgresql.
Seamus
On Wed, 9 Feb 2005, Seamus Thomas Carroll wrote:
> Hi,
>
> I am trying to add a C-Language Function. I have created a small example in
> file BlackboardFunctions.c.
>
> #include <postgres.h>
> #include <fmgr.h>
>
> PG_FUNCTION_INFO_V1(add_one);
>
> Datum
> add_one(PG_FUNCTION_ARGS)
> {
> int32 arg = PG_GETARG_INT32(0);
>
> PG_RETURN_INT32(arg + 1);
> }
>
> The make file:
> make:
> cc -fpic -c BlackboardFunctions.c -I
> /home/grads/carrolls/source/postgresql-7.4.5/src/include/
> cc -shared -o BlackboardFunctions.so BlackboardFunctions.o
>
> Everything compiles nicely and when i try to add the function using:
> Blackboard=# CREATE FUNCTION add_one(integer) RETURNS integer
> Blackboard-# AS
> '/home/grads/carrolls/gradstudies/implementation/BlackboardFunctions/BlackboardFunctions',
> 'add_one'
> Blackboard-# LANGUAGE C STRICT;
>
> I get the following error:
> ERROR: could not find function "add_one" in file
> "/home/grads/carrolls/gradstudies/implementation/BlackboardFunctions/BlackboardFunctions.so"
>
> A grep of BlackboardFunction.so finds add_one.
> I figure i must be doing something obviously wrong but for the life of me i
> cant figure out what that might bt. Can anyone recommend a course of action?
>
> Thank you,
>
> Seamus
>
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On Thu, Feb 10, 2005 at 01:17:14AM -0700, Seamus Thomas Carroll wrote: > I changed my file from BlackboardFunctions.c to all lowercase, > blackboardfunctions.c, and the database now finds the add_one function. > > Crazy. Can someone explain the reason why this happened? Is this a > linker issue or something in postgresql. What platform are you using? I built and loaded your original example with mixed-case file names on FreeBSD and Solaris and had no problems. -- Michael Fuhr http://www.fuhr.org/~mfuhr/
Seamus Thomas Carroll <carrolls@cpsc.ucalgary.ca> writes:
> I changed my file from BlackboardFunctions.c to all lowercase,
> blackboardfunctions.c, and the database now finds the add_one function.
I'm betting that the actual sequence of events was more like this:
CREATE FUNCTION f1(...) as '/home/.../BlackboardFunctions' ...;[ test f1, it works, cool ][ add add_one to C source
file,recompile ]CREATE FUNCTION add_one(...) as '/home/.../BlackboardFunctions' ...;[ fails ][ rename file ]CREATE
FUNCTIONadd_one(...) as '/home/.../blackboardfunctions' ...;[ works ]
The reason the second try didn't work is that an existing backend will
not re-load an already loaded .so file, unless you force it to with the
LOAD command. It doesn't notice that you've modified the file. Had you
started a fresh session, things would have worked, too.
regards, tom lane
Thinking back I think i tried g++ first which caused the error to occur. When i then tried cc the .so was already loaded. I have c++ classes i want to use but when i complile using g++ i get the error (cant find function x in file.so). Does anyone have experience compiling c++ and loading the function? Seamus On Thu, 10 Feb 2005, Tom Lane wrote: > Seamus Thomas Carroll <carrolls@cpsc.ucalgary.ca> writes: >> I changed my file from BlackboardFunctions.c to all lowercase, >> blackboardfunctions.c, and the database now finds the add_one function. > > I'm betting that the actual sequence of events was more like this: > > CREATE FUNCTION f1(...) as '/home/.../BlackboardFunctions' ...; > [ test f1, it works, cool ] > [ add add_one to C source file, recompile ] > CREATE FUNCTION add_one(...) as '/home/.../BlackboardFunctions' ...; > [ fails ] > [ rename file ] > CREATE FUNCTION add_one(...) as '/home/.../blackboardfunctions' ...; > [ works ] > > The reason the second try didn't work is that an existing backend will > not re-load an already loaded .so file, unless you force it to with the > LOAD command. It doesn't notice that you've modified the file. Had you > started a fresh session, things would have worked, too. > > regards, tom lane >
Seamus Thomas Carroll <carrolls@cpsc.ucalgary.ca> writes:
> I have c++ classes i want to use but when i complile using g++ i get the
> error (cant find function x in file.so). Does anyone have experience
> compiling c++ and loading the function?
The immediate problem is that you didn't declare the function as extern
"C" so it doesn't follow C naming conventions. In general, though, it's
fairly difficult to use any interesting C++ capabilities in a Postgres
backend module, because the main backend does not contain the C++
library. So, for instance, I'd not expect C++ exceptions or RTTI to
work at all well.
regards, tom lane
Hi Tom,
I am trying to LOAD the .so file and i have replaced the add_one function(
this one compiled and tested after adding the extern "C"{}) with the
concat_text function.
It compiles but when i try the LOAD command i get the following error:
ERROR: could not load library "/home/grads/.../BlackboardFunctions.so":
/home/grads/.../BlackboardFunctions.so: undefined symbol: _Z16pg_detoast_datumP7varlena
I am guessing this has something to do with the varlena in c.h.
Seamus
On Thu, 10 Feb 2005, Tom Lane wrote:
> Seamus Thomas Carroll <carrolls@cpsc.ucalgary.ca> writes:
>> I have c++ classes i want to use but when i complile using g++ i get the
>> error (cant find function x in file.so). Does anyone have experience
>> compiling c++ and loading the function?
>
> The immediate problem is that you didn't declare the function as extern
> "C" so it doesn't follow C naming conventions. In general, though, it's
> fairly difficult to use any interesting C++ capabilities in a Postgres
> backend module, because the main backend does not contain the C++
> library. So, for instance, I'd not expect C++ exceptions or RTTI to
> work at all well.
>
> regards, tom lane
>
Seamus Thomas Carroll <carrolls@cpsc.ucalgary.ca> writes:
> It compiles but when i try the LOAD command i get the following error:
> ERROR: could not load library "/home/grads/.../BlackboardFunctions.so":
> /home/grads/.../BlackboardFunctions.so: undefined symbol: _Z16pg_detoast_datumP7varlena
extern "C" { ... } around all the Postgres include files, too, no doubt.
You might wish to go and read some information about connecting C and C++,
since you've obviously never done it before ;-)
regards, tom lane