[SOLVED] postgresql c-extension loads another external library

postgresql c-extension loads another external library

I need to compare pHashes (phash.org) with a hamming distance function.

I tried the one from pg_similarity, but it doesn't seem to work right. (identical pHashes don't have a hamming distance of 0).

So I figured I'd just use a c-extension to use the ph_hamming_distance function that's part of the pHash library.

What I've got: phash.c

#include <postgres.h>
#include <pHash.h>
#include <fmgr.h>
#include <utils/bytea.h>
#include <utils/datum.h>

#ifdef PG_MODULE_MAGIC
PG_MODULE_MAGIC;
#endif

PG_FUNCTION_INFO_V1(phash_hamming);
Datum phash_hamming(PG_FUNCTION_ARGS) {

    bytea *bytea1 = PG_GETARG_BYTEA_P(0);
    bytea *bytea2 = PG_GETARG_BYTEA_P(1);
    //FIXME - length of bytea1 & bytea2 must be 4 bytes (64bits)

    ulong64 long1 = *((ulong64*) bytea1);
    ulong64 long2 = *((ulong64*) bytea2);

    int32 ret = ph_hamming_distance(long1, long2);

    PG_RETURN_INT32(ret);
}

Makefile

CXXFLAGS=-I/usr/include/postgresql/server
LDFLAGS=-Bstatic -lpHash
all: phash.o

phash.o:
    $(CXX) $(CXXFLAGS) -fpic -c phash.c
    $(CXX) $(LDFLAGS) -shared -o phash.so phash.o

install:
    cp phash.so `pg_config --pkglibdir`

clean:
    rm -f phash.o phash.so

SQL

 CREATE FUNCTION phash_hamming (bytea1 bytea, bytea2 bytea) RETURNS int AS '$libdir/phash' LANGUAGE C;

Error that I'm getting:

ERROR:  could not load library "/usr/lib/postgresql/phash.so": /usr/lib/postgresql/phash.so: undefined symbol: _Z16pg_detoast_datumP7varlena

I must not be linking right to postgresql somehow?

Solution

It's an old question, but...

There is no need to add extra wrapper file and compile it using gcc.

You need extern "C" both PostgreSQL headers and PostgreSQL macros.

extern "C" {
  #include <postgres.h>
  #include <fmgr.h>
  #ifdef PG_MODULE_MAGIC
  PG_MODULE_MAGIC
  #endif
}