PostgreSQL external C function link failed on Mac OSX - c++

Question

I'm trying to build an external PostgreSQL function on OSX 10.11 with both clang and gcc, but link failed with the following errors:
c++ -I/usr/local/Cellar/postgresql/9.5.3/include/server -fpic -c ./main.c
c++ -shared -o ttt.dylib main.o
Undefined symbols for architecture x86_64:
"_deconstruct_array", referenced from:
_psql_nearest in main.o
"_elog_finish", referenced from:
_psql_nearest in main.o
"_elog_start", referenced from:
_psql_nearest in main.o
"_get_typlenbyvalalign", referenced from:
_psql_nearest in main.o
"_pfree", referenced from:
_psql_nearest in main.o
"_pg_detoast_datum", referenced from:
_psql_nearest in main.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
It looks like I need to link my library with some of PostgreSQL libraries. What are these libraries?
main.cpp:
extern "C" {
#include <postgres.h>
#include <fmgr.h>
#include <utils/array.h>
#include <utils/lsyscache.h>
#include <catalog/pg_type.h>
#ifdef PG_MODULE_MAGIC
PG_MODULE_MAGIC;
#endif
PG_FUNCTION_INFO_V1(psql_nearest);
Datum psql_nearest(PG_FUNCTION_ARGS) {
if(PG_ARGISNULL(0) || PG_ARGISNULL(1)) {
elog(ERROR, "DOC2VEC: NULL INPUT DATA");
PG_RETURN_NULL();
}
ArrayType *_docVector = PG_GETARG_ARRAYTYPE_P(0);
Oid elTypeVals = ARR_ELEMTYPE(_docVector);
if (elTypeVals != FLOAT4OID) {
elog(ERROR, "DOC2VEC: INVALID INPUT DATA TYPE");
PG_RETURN_NULL();
}
int16 typeLenVals = 0;
bool typeByValVals = false;
char typeAlignVals = char(0);
get_typlenbyvalalign(elTypeVals, &typeLenVals, &typeByValVals, &typeAlignVals);
Datum *inputVals;
bool *nullVals;
int nVals;
deconstruct_array(_docVector, elTypeVals, typeLenVals, typeByValVals, typeAlignVals, &inputVals, &nullVals, &nVals);
pfree(inputVals);
pfree(nullVals);
PG_RETURN_NULL();
}
}

Answer 1

Thanks to PostgreSQL developers, they explained me the difference in Linux and OSX linking of external functions.
Instead of -shared you need -bundle -bundle_loader /path/to/postgres,
and there are some other linker flags that are advisable too.
Also, PostgreSQL expects the file extension for loadable modules to be .so even on OSX.
It's usually better to use PGXS to build extensions, instead of
learning such details for yourself.
Or you can crib from one of the extensions in the contrib/
source tree.

Answer 2

If you need to link, you will need the -L flag to point the linker to the path where the postgres libraries are located (the linker equivalent of the -I compiler flag). and the -l flag to actually link the libraries (one for each library); the library name without the lib prefix and without the extension.
In your case, something along the lines of -L/usr/local/Cellar/postgresql/9.5.3/lib -lpostgres
(There's a variety of library files in that directory; try -lpg to start with.
The reference to _pfree in your error message also suggest to link pgcommon, which contains the implementation of pgree (at least when using nm libpgcommon.a).
)
You may want to read up a bit more on compiling and linking in general; you do the right thing for compiling with the -I flag, but oddly then miss out on the linking step. And learning about make and Makefiles will come in handy.
I also don't understand the extern "C" { part for a .c file, which is clearly a C-only file. extern "C" is usually used in C++ files for compatibility with C.