Linker errors while creating a PostgreSQL C extension function.

Hello,

*I have this code for my C extension function.
*

xtern "C" { // C Headers must be inside exter "C" { } block.
#include <postgres.h>
#include <fmgr.h>
#include <utils/builtins.h>
#include <catalog/pg_type.h>
#include <utils/rel.h>
#include <utils/array.h>
#include <stdlib.h>
#include <stdint.h>

PG_MODULE_MAGIC;
}

// CPP Header must be outside extern "C" { } block.
#include <string>
#include <vector>
#include <iostream>
#include <fstream>
#include <sstream>
#include <iterator> // For the ostream_iterator

// External projects c++ libraries compiled and linked on running 'make'.
#include <seal/seal.h>
#include <thread>
#include <cppcodec/base64_rfc4648.hpp>

std::stringstream dec(std::string st){

// Decode the base64 string into a stringstream
auto decodeBase64 = cppcodec::base64_rfc4648::decode(st);
std::stringstream decodeBase64SS;
std::move(decodeBase64.begin(), decodeBase64.end(),
std::ostream_iterator<unsigned char>(decodeBase64SS));

return decodeBase64SS;
}

std::string enc(std::string st){

// Create a vector to hold the raw data
std::vector<uint8_t> encodeStream;

// Push all the characters from the raw data string into the vector
for (auto &ch : st){
encodeStream.push_back((unsigned char&&)(ch));
}

// Encode the vector as base64 string
std::string encodeBase64 =
cppcodec::base64_rfc4648::encode(encodeStream);
encodeStream.clear();
return encodeBase64;

}

std::string seal_diff_operation(std::string decodedLocalEncParamTmp,
std::string decodedLocalTmp1, std::string decodedLocalTmp2){

std::stringstream decodedLocalEncParam;
decodedLocalEncParam.str(decodedLocalEncParamTmp);
std::stringstream decodedLocalT1;
decodedLocalT1.str(decodedLocalTmp1);
std::stringstream decodedLocalT2;
decodedLocalT2.str(decodedLocalTmp2);

// Execute seal library operations
// Load the ecryption parameters
seal::EncryptionParameters IntegerEncryptorParms;
IntegerEncryptorParms.load(decodedLocalEncParam);
// Set Context and evaluator objects
seal::SEALContext context(IntegerEncryptorParms);
seal::Evaluator evaluator(context);
// Set the Encoder parameters
seal::IntegerEncoder encoder(context.plain_modulus());

// Create Ciphertexts and load Chipertext information into them
seal::Ciphertext number1Encoded;
seal::Ciphertext number2Encoded;
seal::Ciphertext diffEncodedResult;
number1Encoded.load(decodedLocalT1);
number2Encoded.load(decodedLocalT2);

// Do the diff operation on the Ciphertexts and prepare the result
for output
evaluator.sub(number1Encoded, number2Encoded, diffEncodedResult);
std::stringstream encResult;
diffEncodedResult.save(encResult);

std::string output = enc(encResult.str());
return output;

}

extern "C" { // Usage of CPP functions in the module must be inside extern
"C" { } block.
Datum seal_diff_cpp(PG_FUNCTION_ARGS){

// Get the inputs
text *t1 = PG_GETARG_TEXT_PP(0);
text *t2 = PG_GETARG_TEXT_PP(1);
text *encParam = PG_GETARG_TEXT_PP(2);
std::string localT1;
std::string localT2;
std::string localEncParam;
localT1 = text_to_cstring(t1);
localT2 = text_to_cstring(t2);
localEncParam = text_to_cstring(encParam);

// Decode the parameters
std::stringstream decodedLocalT1 = dec(localT1);
std::stringstream decodedLocalT2 = dec(localT2);
std::stringstream decodedLocalEncParam = dec(localEncParam);

// Encode the parameters
std::string encodedLocalT1 = enc(decodedLocalT1.str());
std::string encodedLocalT2 = enc(decodedLocalT2.str());
std::string outputParam =
seal_diff_operation(decodedLocalEncParam.str(), decodedLocalT1.str(),
decodedLocalT2.str());

// Return the result
PG_RETURN_TEXT_P(cstring_to_text_with_len(localT1.c_str(),
localT1.size()));
};

PG_FUNCTION_INFO_V1(seal_diff_cpp);
}

*And I use this Makefile to create the seal_diff_cpp.so file:
*

MODULES = seal_diff_cpp

PG_CONFIG = /usr/pgsql-10/bin/pg_config
PGXS = $(shell $(PG_CONFIG) --pgxs)
INCLUDEDIR = $(shell $(PG_CONFIG) --includedir-server)
INCLUDE_SEAL = /usr/local/include/seal
INCLUDE_SEAL_LIB = /usr/local/lib
INCLUDE_CPPCODEC = /usr/local/include/cppcodec
CXX = g++
CXXFLAGS = -std=c++17 -fPIC -Wall -Iinclude -Werror -g -O0 -pthread \
-I$(INCLUDEDIR) -I$(INCLUDE_SEAL) -I$(INCLUDE_CPPCODEC)
LDFLAGS = -L$(INCLUDE_SEAL_LIB) -llibseal.a -lpthread
include $(PGXS)
seal_diff_cpp.so: seal_diff_cpp.o
$(CXX) -Wl,--no-undefined -shared -o seal_diff_cpp.so
seal_diff_cpp.o $(LDFLAGS)

seal_diff_cpp.o: seal_diff_cpp.cpp
$(CXX) $(CXXFLAGS) -o seal_diff_cpp.o -c seal_diff_cpp.cpp

*But when I run the make command I get those linker errors
*

g++ -std=c++17 -fPIC -Wall -Iinclude -Werror -g -O0 -pthread
-I/usr/pgsql-10/include/server -I"/usr/local/include" -o seal_diff_cpp.o -c
seal_diff_cpp.cpp
g++ -Wl,--no-undefined -shared -o seal_diff_cpp.so seal_diff_cpp.o
-L/usr/pgsql-10/lib -L/usr/lib64 -Wl,--as-needed
-Wl,-rpath,'/usr/pgsql-10/lib',--enable-new-dtags
seal_diff_cpp.o: In function
`seal_diff_operation(std::__cxx11::basic_string<char,
std::char_traits&lt;char>, std::allocator<char> >,
std::__cxx11::basic_string<char, std::char_traits&lt;char>,
std::allocator<char> >, std::__cxx11::basic_string<char,
std::char_traits&lt;char>, std::allocator<char> >)':
/etc/opt/pgsql_c_functions/CPP/seal_extension/seal_diff_cpp.cpp:72:
undefined reference to `seal::EncryptionParameters::EncryptionParameters()'
/etc/opt/pgsql_c_functions/CPP/seal_extension/seal_diff_cpp.cpp:73:
undefined reference to `seal::EncryptionParameters::load(std::istream&)'
/etc/opt/pgsql_c_functions/CPP/seal_extension/seal_diff_cpp.cpp:75:
undefined reference to
`seal::SEALContext::SEALContext(seal::EncryptionParameters const&)'
/etc/opt/pgsql_c_functions/CPP/seal_extension/seal_diff_cpp.cpp:76:
undefined reference to `seal::Evaluator::Evaluator(seal::SEALContext
const&)'
/etc/opt/pgsql_c_functions/CPP/seal_extension/seal_diff_cpp.cpp:78:
undefined reference to
`seal::IntegerEncoder::IntegerEncoder(seal::SmallModulus const&, unsigned
long)'
/etc/opt/pgsql_c_functions/CPP/seal_extension/seal_diff_cpp.cpp:84:
undefined reference to `seal::Ciphertext::load(std::istream&)'
/etc/opt/pgsql_c_functions/CPP/seal_extension/seal_diff_cpp.cpp:85:
undefined reference to `seal::Ciphertext::load(std::istream&)'
/etc/opt/pgsql_c_functions/CPP/seal_extension/seal_diff_cpp.cpp:90:
undefined reference to `seal::Ciphertext::save(std::ostream&) const'
/etc/opt/pgsql_c_functions/CPP/seal_extension/seal_diff_cpp.cpp:78:
undefined reference to `seal::IntegerEncoder::~IntegerEncoder()'
/etc/opt/pgsql_c_functions/CPP/seal_extension/seal_diff_cpp.cpp:78:
undefined reference to `seal::IntegerEncoder::~IntegerEncoder()'
seal_diff_cpp.o: In function `seal_diff_cpp':
/etc/opt/pgsql_c_functions/CPP/seal_extension/seal_diff_cpp.cpp:106:
undefined reference to `pg_detoast_datum_packed'
/etc/opt/pgsql_c_functions/CPP/seal_extension/seal_diff_cpp.cpp:107:
undefined reference to `pg_detoast_datum_packed'
/etc/opt/pgsql_c_functions/CPP/seal_extension/seal_diff_cpp.cpp:108:
undefined reference to `pg_detoast_datum_packed'
/etc/opt/pgsql_c_functions/CPP/seal_extension/seal_diff_cpp.cpp:112:
undefined reference to `text_to_cstring'
/etc/opt/pgsql_c_functions/CPP/seal_extension/seal_diff_cpp.cpp:113:
undefined reference to `text_to_cstring'
/etc/opt/pgsql_c_functions/CPP/seal_extension/seal_diff_cpp.cpp:114:
undefined reference to `text_to_cstring'
/etc/opt/pgsql_c_functions/CPP/seal_extension/seal_diff_cpp.cpp:130:
undefined reference to `cstring_to_text_with_len'
seal_diff_cpp.o: In function
`seal::EncryptionParameters::~EncryptionParameters()':
/usr/local/include/seal/encryptionparams.h:48: undefined reference to
`seal::BigPoly::~BigPoly()'
seal_diff_cpp.o: In function `seal::Evaluator::sub(seal::Ciphertext const&,
seal::Ciphertext const&, seal::Ciphertext&)':
/usr/local/include/seal/evaluator.h:205: undefined reference to
`seal::Ciphertext::operator=(seal::Ciphertext const&)'
/usr/local/include/seal/evaluator.h:206: undefined reference to
`seal::Evaluator::sub(seal::Ciphertext&, seal::Ciphertext const&)'
seal_diff_cpp.o: In function `seal::SEALContext::~SEALContext()':
/usr/local/include/seal/context.h:116: undefined reference to
`seal::BigUInt::~BigUInt()'
collect2: error: ld returned 1 exit status
make: *** [Makefile:16: seal_diff_cpp.so] Error 1

I can't figure what am I doing wrong? The SEAL library is an external
library installed to /usr/local/include/seal and its libseal.a into
/usr/local/lib/. I was told by the SEAL library creators (Cryptography
Research Group at Microsoft), that all I need is the installd header files
in /usr/local/include/seal and the libseal.a in /usr/local/lib/ so I don't
understand why the linker can't link those functions.

Thanks a lot,
Tal

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