Re: [MonetDB-users] Difficulty with External C Function

30 Nov 2007

      En hoe snel it vanaf huis?
Niels Nes wrote:
...
On Fri, Nov 30, 2007 at 10:22:14AM -0700, Thomas J. Schulte wrote:
...
I'm having difficulty linking a custom external C function into
   MonetDB without crashing the database connection.  If anyone knows
   of a documented end-to-end example of adding external functions
   compiled with GCC, please let me know.  I've documented a minimal
   example below that crashes the connection for me each time.
Software:
Red Hat Linux knock-off CentOS, i386.
Most recent MonetDB download from Sourceforge (server 5.2.2, client
   1.20.2, common 1.20.0).
GCC 4.1.1

Create a simple C file named te.c.

int
TEtest( )
{
return 4;
}
mal c-functions return a str (exception or MAT_SUCCEED)
your example has an mal level output variable of type int
so your function signature should be
str TEtest(int *res) 
{
   *res = 4;
   return MAL_SUCCEED;
}
Niels
...

Compile file, move it to the proper directory, and create

symlinks.
$  gcc -c -fPIC te.c -o te.o
$  gcc -shared -Wl,-soname,lib_te.so.0 -o lib_te.so.0.0.0 te.o
   (also   tried   with   soname  =  libte.so.0,  leaving  output
   as lib_te.so.0.0.0)
$  cp lib_te.so.0.0.0 $mhome/lib/MonetDB5/lib
$  cd $mhome/lib/MonetDB5/lib
$  ln -s lib_te.so.0.0.0 lib_te.so.0
$  ln -s lib_te.so.0.0.0 lib_te.so

Create a MAL file, saved as $mhome/lib/MonetDB5/te.mal

module te;
command test( ):int
   address TEtest();
command doesnotexist( ):int
   address TEdoesnotexist;

Update mal_init.mal by adding the following 2 lines:

library te;
   include te;

Start  a  random  DB and try to invoke the test function.

Merovingian log show database dino killed by signal 11 afterwards.
$  cd $mhome/bin
$ ./merovingian &
   [1] xxxx
$  ./mclient -lmal --database=dino -h COS1
   mal> te.test( );
   MAPI = [1]monetdb@COS1:50001
   ACTION= read_line
   QUERY = te.test();
   ERROR = Connection terminated
If I instead invoke te.doesnotexist(), I get a different error
   (below.)  This leads me to think that MonetDB is findng and loading
   my compiled .so file, but it wasn't compiled correctly or otherwise
   isn't providing what MonetDB expects.
mal> te.doesnotexist();
   MAPI = [2]monetdb@COS1:50001
   QUERY = te.doesnotexist();
   ERROR = !TypeException:user.main[1]:'te.doesnotexist' undefined in:
   _1:int := te.doesnotexist()
Any pointers are appreciated.
Thomas Schulte
References

mailto:monetdb@COS1:50001
mailto:monetdb@COS1:50001

...

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Re: [MonetDB-users] Difficulty with External C Function

Martin Kersten