Use of inv_getsize in functions

Hi,

I am programming a new set of functions for manipulation blobs, one
of the functions is that I want to get the size of the blob.

The function is coded in C and goes like this:

Datum
blob_size(PG_FUNCTION_ARGS)
{
Oid blob_oid = PG_GETARG_OID(0);
uint32 blobsize = 0;
blobsize = inv_getsize( blob_oid );
PG_RETURN_UINT32( blobsize );
}

I have used other function calls like
inv_open with no problem, but when I load this modules I get:

undefined symbol: inv_getsize

No errors when compiling.

I use:
postgresql 7.4.7 on debian/woody i386

Best regards,

Søren