Can't find libpq library for C++ program using libpqxx

Question

I'm not a C++ developer and I don't know very much about compiling projects or makefiles, but I'm trying to compile and run someone else's C++ program. I'm using Ubuntu. Here's the makefile I have so far, based on this:

TARGET_EXEC ?= a.out

BUILD_DIR ?= ./build
SRC_DIRS ?= ./src

SRCS := $(shell find $(SRC_DIRS) -name *.cpp -or -name *.c -or -name *.s)
OBJS := $(SRCS:%=$(BUILD_DIR)/%.o)
DEPS := $(OBJS:.o=.d)

INC_DIRS := $(shell find $(SRC_DIRS) -type d)
INC_FLAGS := $(addprefix -I,$(INC_DIRS))

CPPFLAGS ?= $(INC_FLAGS) -MMD -MP
CC = gcc

$(BUILD_DIR)/$(TARGET_EXEC): $(OBJS)
    $(CC) $(OBJS) -o $@ $(LDFLAGS) /usr/lib/x86_64-linux-gnu/libstdc++.so.6 -I/usr/local/include/boost -I/usr/local/include/pqxx -I/usr/lib/postgresql/13/lib -I/usr/include/postgresql -I/usr/include/postgresql/libpq -I/usr/include/postgresql/internal -I/usr/include/postgresql/internal/libpq -L/usr/local/lib -lboost_filesystem -lpqxx

# assembly
$(BUILD_DIR)/%.s.o: %.s
    $(MKDIR_P) $(dir $@)
    $(AS) $(ASFLAGS) -c $< -o $@

# c source
# $(BUILD_DIR)/%.c.o: %.c
#   $(MKDIR_P) $(dir $@)
#   $(CC) $(CPPFLAGS) $(CFLAGS) -c $< -o $@ -lstdc++

# c++ source
$(BUILD_DIR)/%.cpp.o: %.cpp
    $(MKDIR_P) $(dir $@)
    $(CXX) $(CPPFLAGS) $(CXXFLAGS) -c $< -o $@


.PHONY: clean

clean:
    $(RM) -r $(BUILD_DIR)

-include $(DEPS)

MKDIR_P ?= mkdir -p

The part commented # c++ source seems to complete without issue, but then I run into problems with the long line above $(CC) $(OBJS) -o $@ $(LDFLAGS) ...:

/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::backendpid() const':
/usr/local/include/pqxx/src/connection_base.cxx:121: undefined reference to `PQbackendPID'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::sock() const':
/usr/local/include/pqxx/src/connection_base.cxx:129: undefined reference to `PQsocket'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::protocol_version() const':
/usr/local/include/pqxx/src/connection_base.cxx:214: undefined reference to `PQprotocolVersion'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::status() const':
/usr/local/include/pqxx/src/connection_base.cxx:965: undefined reference to `PQstatus'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::consume_input()':
/usr/local/include/pqxx/src/connection_base.cxx:501: undefined reference to `PQconsumeInput'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::is_busy() const':
/usr/local/include/pqxx/src/connection_base.cxx:507: undefined reference to `PQisBusy'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::set_verbosity(pqxx::connection_base::error_verbosity)':
/usr/local/include/pqxx/src/connection_base.cxx:551: undefined reference to `PQsetErrorVerbosity'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::err_msg() const':
/usr/local/include/pqxx/src/connection_base.cxx:660: undefined reference to `PQerrorMessage'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::set_notice_processor()':
/usr/local/include/pqxx/src/connection_base.cxx:672: undefined reference to `PQsetNoticeProcessor'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::clear_notice_processor()':
/usr/local/include/pqxx/src/connection_base.cxx:666: undefined reference to `PQsetNoticeProcessor'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::internal_set_trace()':
/usr/local/include/pqxx/src/connection_base.cxx:957: undefined reference to `PQtrace'
/usr/bin/ld: /usr/local/include/pqxx/src/connection_base.cxx:958: undefined reference to `PQuntrace'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::status() const':
/usr/local/include/pqxx/src/connection_base.cxx:965: undefined reference to `PQstatus'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::unesc_raw[abi:cxx11](char const*)':
/usr/local/include/pqxx/src/connection_base.cxx:1152: undefined reference to `PQunescapeBytea'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::status() const':
/usr/local/include/pqxx/src/connection_base.cxx:965: undefined reference to `PQstatus'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::internal::wait_read(pg_conn const*)':
/usr/local/include/pqxx/src/connection_base.cxx:129: undefined reference to `PQsocket'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::internal::wait_read(pg_conn const*, long, long)':
/usr/local/include/pqxx/src/connection_base.cxx:129: undefined reference to `PQsocket'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::internal::wait_write(pg_conn const*)':
/usr/local/include/pqxx/src/connection_base.cxx:129: undefined reference to `PQsocket'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::status() const':
/usr/local/include/pqxx/src/connection_base.cxx:965: undefined reference to `PQstatus'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::cancel_query()':
/usr/local/include/pqxx/src/connection_base.cxx:526: undefined reference to `PQgetCancel'
/usr/bin/ld: /usr/local/include/pqxx/src/connection_base.cxx:535: undefined reference to `PQcancel'
/usr/bin/ld: /usr/local/include/pqxx/src/connection_base.cxx:530: undefined reference to `PQfreeCancel'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::read_capabilities()':
/usr/local/include/pqxx/src/connection_base.cxx:1360: undefined reference to `PQserverVersion'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::protocol_version() const':
/usr/local/include/pqxx/src/connection_base.cxx:214: undefined reference to `PQprotocolVersion'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::set_client_encoding(char const*)':
/usr/local/include/pqxx/src/connection_base.cxx:1395: undefined reference to `PQsetClientEncoding'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::encoding_id() const':
/usr/local/include/pqxx/src/connection_base.cxx:1419: undefined reference to `PQclientEncoding'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::status() const':
/usr/local/include/pqxx/src/connection_base.cxx:965: undefined reference to `PQstatus'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::encrypt_password(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)':
/usr/local/include/pqxx/src/connection_base.cxx:94: undefined reference to `PQencryptPassword'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::quote_name(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)':
/usr/local/include/pqxx/src/connection_base.cxx:1180: undefined reference to `PQescapeIdentifier'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::status() const':
/usr/local/include/pqxx/src/connection_base.cxx:965: undefined reference to `PQstatus'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::clear_notice_processor()':
/usr/local/include/pqxx/src/connection_base.cxx:666: undefined reference to `PQsetNoticeProcessor'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::set_up_state()':
/usr/local/include/pqxx/src/connection_base.cxx:321: undefined reference to `PQsendQuery'
/usr/bin/ld: /usr/local/include/pqxx/src/connection_base.cxx:324: undefined reference to `PQgetResult'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::status() const':
/usr/local/include/pqxx/src/connection_base.cxx:965: undefined reference to `PQstatus'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::err_msg() const':
/usr/local/include/pqxx/src/connection_base.cxx:660: undefined reference to `PQerrorMessage'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::status() const':
/usr/local/include/pqxx/src/connection_base.cxx:965: undefined reference to `PQstatus'
/usr/bin/ld: /usr/local/include/pqxx/src/connection_base.cxx:965: undefined reference to `PQstatus'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::register_prepared(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)':
/usr/local/include/pqxx/src/connection_base.cxx:800: undefined reference to `PQprepare'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::start_exec(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)':
/usr/local/include/pqxx/src/connection_base.cxx:1079: undefined reference to `PQsendQuery'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::esc_raw[abi:cxx11](unsigned char const*, unsigned long)':
/usr/local/include/pqxx/src/connection_base.cxx:1139: undefined reference to `PQescapeByteaConn'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::reset()':
/usr/local/include/pqxx/src/connection_base.cxx:889: undefined reference to `PQreset'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::esc[abi:cxx11](char const*, unsigned long)':
/usr/local/include/pqxx/src/connection_base.cxx:1108: undefined reference to `PQescapeStringConn'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::end_copy_write()':
/usr/local/include/pqxx/src/connection_base.cxx:1054: undefined reference to `PQputCopyEnd'
/usr/bin/ld: /usr/local/include/pqxx/src/connection_base.cxx:1070: undefined reference to `PQgetResult'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::status() const':
/usr/local/include/pqxx/src/connection_base.cxx:965: undefined reference to `PQstatus'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::read_copy_line(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&)':
/usr/local/include/pqxx/src/connection_base.cxx:999: undefined reference to `PQgetCopyData'
/usr/bin/ld: /usr/local/include/pqxx/src/connection_base.cxx:1007: undefined reference to `PQgetResult'
/usr/bin/ld: /usr/local/include/pqxx/src/connection_base.cxx:1009: undefined reference to `PQgetResult'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::status() const':
/usr/local/include/pqxx/src/connection_base.cxx:965: undefined reference to `PQstatus'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::consume_input()':
/usr/local/include/pqxx/src/connection_base.cxx:501: undefined reference to `PQconsumeInput'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::get_notifs()':
/usr/local/include/pqxx/src/connection_base.cxx:565: undefined reference to `PQnotifies'
/usr/bin/ld: /usr/local/include/pqxx/src/connection_base.cxx:565: undefined reference to `PQnotifies'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::parameterized_exec(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, char const* const*, int const*, int const*, int)':
/usr/local/include/pqxx/src/connection_base.cxx:1448: undefined reference to `PQexecParams'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::exec_params(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, pqxx::internal::params const&)':
/usr/local/include/pqxx/src/connection_base.cxx:1460: undefined reference to `PQexecParams'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::prepared_exec(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, char const* const*, int const*, int const*, int)':
/usr/local/include/pqxx/src/connection_base.cxx:836: undefined reference to `PQexecPrepared'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::exec_prepared(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, pqxx::internal::params const&)':
/usr/local/include/pqxx/src/connection_base.cxx:852: undefined reference to `PQexecPrepared'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::exec(char const*, int)':
/usr/local/include/pqxx/src/connection_base.cxx:715: undefined reference to `PQexec'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::status() const':
/usr/local/include/pqxx/src/connection_base.cxx:965: undefined reference to `PQstatus'
/usr/bin/ld: /usr/local/include/pqxx/src/connection_base.cxx:965: undefined reference to `PQstatus'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::exec(char const*, int)':
/usr/local/include/pqxx/src/connection_base.cxx:721: undefined reference to `PQexec'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::status() const':
/usr/local/include/pqxx/src/connection_base.cxx:965: undefined reference to `PQstatus'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::write_copy_line(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)':
/usr/local/include/pqxx/src/connection_base.cxx:1041: undefined reference to `PQputCopyData'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::err_msg() const':
/usr/local/include/pqxx/src/connection_base.cxx:660: undefined reference to `PQerrorMessage'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::write_copy_line(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)':
/usr/local/include/pqxx/src/connection_base.cxx:1046: undefined reference to `PQendcopy'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::status() const':
/usr/local/include/pqxx/src/connection_base.cxx:965: undefined reference to `PQstatus'
/usr/bin/ld: /usr/local/include/pqxx/src/connection_base.cxx:965: undefined reference to `PQstatus'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::add_receiver(pqxx::notification_receiver*)':
/usr/local/include/pqxx/src/connection_base.cxx:452: undefined reference to `PQexec'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::clear_notice_processor()':
/usr/local/include/pqxx/src/connection_base.cxx:666: undefined reference to `PQsetNoticeProcessor'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::set_notice_processor()':
/usr/local/include/pqxx/src/connection_base.cxx:672: undefined reference to `PQsetNoticeProcessor'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::get_result()':
/usr/local/include/pqxx/src/connection_base.cxx:1086: undefined reference to `PQgetResult'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::dbname()':
/usr/local/include/pqxx/src/connection_base.cxx:627: undefined reference to `PQdb'
/usr/bin/ld: /usr/local/include/pqxx/src/connection_base.cxx:627: undefined reference to `PQdb'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::username()':
/usr/local/include/pqxx/src/connection_base.cxx:636: undefined reference to `PQuser'
/usr/bin/ld: /usr/local/include/pqxx/src/connection_base.cxx:636: undefined reference to `PQuser'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::hostname()':
/usr/local/include/pqxx/src/connection_base.cxx:645: undefined reference to `PQhost'
/usr/bin/ld: /usr/local/include/pqxx/src/connection_base.cxx:645: undefined reference to `PQhost'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::port()':
/usr/local/include/pqxx/src/connection_base.cxx:654: undefined reference to `PQport'
/usr/bin/ld: /usr/local/include/pqxx/src/connection_base.cxx:654: undefined reference to `PQport'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::err_msg() const':
/usr/local/include/pqxx/src/connection_base.cxx:660: undefined reference to `PQerrorMessage'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::cancel_query() [clone .cold]':
/usr/local/include/pqxx/src/connection_base.cxx:530: undefined reference to `PQfreeCancel'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection_base.o): in function `pqxx::connection_base::err_msg() const':
/usr/local/include/pqxx/src/connection_base.cxx:660: undefined reference to `PQerrorMessage'
/usr/bin/ld: /usr/local/include/pqxx/src/connection_base.cxx:660: undefined reference to `PQerrorMessage'
/usr/bin/ld: /usr/local/include/pqxx/src/connection_base.cxx:660: undefined reference to `PQerrorMessage'
/usr/bin/ld: /usr/local/include/pqxx/src/connection_base.cxx:660: undefined reference to `PQerrorMessage'
/usr/bin/ld: /usr/local/include/pqxx/src/connection_base.cxx:660: undefined reference to `PQerrorMessage'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection.o): in function `pqxx::connectionpolicy::do_disconnect(pg_conn*)':
/usr/local/include/pqxx/src/connection.cxx:72: undefined reference to `PQfinish'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection.o): in function `pqxx::connect_async::do_startconnect(pg_conn*)':
/usr/local/include/pqxx/src/connection.cxx:116: undefined reference to `PQconnectStart'
/usr/bin/ld: /usr/local/include/pqxx/src/connection.cxx:118: undefined reference to `PQstatus'
/usr/bin/ld: /usr/local/include/pqxx/src/connection.cxx:121: undefined reference to `PQerrorMessage'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection.o): in function `pqxx::connect_async::do_completeconnect(pg_conn*)':
/usr/local/include/pqxx/src/connection.cxx:164: undefined reference to `PQconnectPoll'
/usr/bin/ld: /usr/local/include/pqxx/src/connection.cxx:164: undefined reference to `PQconnectPoll'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection.o): in function `pqxx::connectionpolicy::normalconnect(pg_conn*)':
/usr/local/include/pqxx/src/connection.cxx:38: undefined reference to `PQconnectdb'
/usr/bin/ld: /usr/local/include/pqxx/src/connection.cxx:40: undefined reference to `PQstatus'
/usr/bin/ld: /usr/local/include/pqxx/src/connection.cxx:42: undefined reference to `PQerrorMessage'
/usr/bin/ld: /usr/local/include/pqxx/src/connection.cxx:43: undefined reference to `PQfinish'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection.o): in function `pqxx::connect_direct::do_startconnect(pg_conn*)':
/usr/local/include/pqxx/src/connection.cxx:88: undefined reference to `PQstatus'
/usr/bin/ld: /usr/local/include/pqxx/src/connection.cxx:90: undefined reference to `PQerrorMessage'
/usr/bin/ld: /usr/local/lib/libpqxx.a(connection.o): in function `pqxx::connect_async::do_completeconnect(pg_conn*)':
/usr/local/include/pqxx/src/connection.cxx:146: undefined reference to `PQerrorMessage'
/usr/bin/ld: /usr/local/lib/libpqxx.a(encodings.o): in function `pqxx::internal::name_encoding(int)':
/usr/local/include/pqxx/src/encodings.cxx:633: undefined reference to `pg_encoding_to_char'
/usr/bin/ld: /usr/local/include/pqxx/src/encodings.cxx:633: undefined reference to `pg_encoding_to_char'
/usr/bin/ld: /usr/local/lib/libpqxx.a(result.o): in function `pqxx::result::size() const':
/usr/local/include/pqxx/src/result.cxx:96: undefined reference to `PQntuples'
/usr/bin/ld: /usr/local/include/pqxx/src/result.cxx:96: undefined reference to `PQntuples'
/usr/bin/ld: /usr/local/lib/libpqxx.a(result.o): in function `pqxx::result::empty() const':
/usr/local/include/pqxx/src/result.cxx:102: undefined reference to `PQntuples'
/usr/bin/ld: /usr/local/lib/libpqxx.a(result.o): in function `pqxx::result::size() const':
/usr/local/include/pqxx/src/result.cxx:96: undefined reference to `PQntuples'
/usr/bin/ld: /usr/local/include/pqxx/src/result.cxx:96: undefined reference to `PQntuples'
/usr/bin/ld: /usr/local/lib/libpqxx.a(result.o):/usr/local/include/pqxx/src/result.cxx:96: more undefined references to `PQntuples' follow
/usr/bin/ld: /usr/local/lib/libpqxx.a(result.o): in function `pqxx::result::cmd_status() const':
/usr/local/include/pqxx/src/result.cxx:273: undefined reference to `PQcmdStatus'
/usr/bin/ld: /usr/local/lib/libpqxx.a(result.o): in function `pqxx::result::affected_rows() const':
/usr/local/include/pqxx/src/result.cxx:294: undefined reference to `PQcmdTuples'
/usr/bin/ld: /usr/local/lib/libpqxx.a(result.o): in function `pqxx::result::get_is_null(unsigned long, unsigned int) const':
/usr/local/include/pqxx/src/result.cxx:312: undefined reference to `PQgetisnull'
/usr/bin/ld: /usr/local/lib/libpqxx.a(result.o): in function `pqxx::result::get_length(unsigned long, unsigned int) const':
/usr/local/include/pqxx/src/result.cxx:319: undefined reference to `PQgetlength'
/usr/bin/ld: /usr/local/lib/libpqxx.a(result.o): in function `pqxx::result::errorposition() const':
/usr/local/include/pqxx/src/result.cxx:375: undefined reference to `PQresultErrorField'
/usr/bin/ld: /usr/local/lib/libpqxx.a(result.o): in function `pqxx::result::ThrowSQLError(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) const':
/usr/local/include/pqxx/src/result.cxx:153: undefined reference to `PQresultErrorField'
/usr/bin/ld: /usr/local/lib/libpqxx.a(result.o): in function `pqxx::result::columns() const':
/usr/local/include/pqxx/src/result.cxx:402: undefined reference to `PQnfields'
/usr/bin/ld: /usr/local/lib/libpqxx.a(result.o): in function `pqxx::result::column_type(unsigned int) const':
/usr/local/include/pqxx/src/result.cxx:325: undefined reference to `PQftype'
/usr/bin/ld: /usr/local/lib/libpqxx.a(result.o): in function `pqxx::result::column_table(unsigned int) const':
/usr/local/include/pqxx/src/result.cxx:336: undefined reference to `PQftable'
/usr/bin/ld: /usr/local/lib/libpqxx.a(result.o): in function `pqxx::result::columns() const':
/usr/local/include/pqxx/src/result.cxx:402: undefined reference to `PQnfields'
/usr/bin/ld: /usr/local/lib/libpqxx.a(result.o): in function `pqxx::result::table_column(unsigned int) const':
/usr/local/include/pqxx/src/result.cxx:352: undefined reference to `PQftablecol'
/usr/bin/ld: /usr/local/lib/libpqxx.a(result.o): in function `pqxx::result::columns() const':
/usr/local/include/pqxx/src/result.cxx:402: undefined reference to `PQnfields'
/usr/bin/ld: /usr/local/lib/libpqxx.a(result.o): in function `pqxx::result::size() const':
/usr/local/include/pqxx/src/result.cxx:96: undefined reference to `PQntuples'
/usr/bin/ld: /usr/local/lib/libpqxx.a(result.o): in function `pqxx::result::StatusError[abi:cxx11]() const':
/usr/local/include/pqxx/src/result.cxx:245: undefined reference to `PQresultStatus'
/usr/bin/ld: /usr/local/include/pqxx/src/result.cxx:259: undefined reference to `PQresultErrorMessage'
/usr/bin/ld: /usr/local/lib/libpqxx.a(result.o): in function `pqxx::result::inserted_oid() const':
/usr/local/include/pqxx/src/result.cxx:288: undefined reference to `PQoidValue'
/usr/bin/ld: /usr/local/lib/libpqxx.a(result.o): in function `pqxx::result::column_name(unsigned int) const':
/usr/local/include/pqxx/src/result.cxx:386: undefined reference to `PQfname'
/usr/bin/ld: /usr/local/lib/libpqxx.a(result.o): in function `pqxx::internal::clear_result(pg_result const*)':
/usr/local/include/pqxx/src/result.cxx:32: undefined reference to `PQclear'
/usr/bin/ld: /usr/local/lib/libpqxx.a(result.o): in function `pqxx::result::GetValue(unsigned long, unsigned int) const':
/usr/local/include/pqxx/src/result.cxx:304: undefined reference to `PQgetvalue'
/usr/bin/ld: /usr/local/lib/libpqxx.a(result.o): in function `pqxx::result::columns() const':
/usr/local/include/pqxx/src/result.cxx:402: undefined reference to `PQnfields'
/usr/bin/ld: /usr/local/lib/libpqxx.a(result.o): in function `pqxx::internal::clear_result(pg_result const*)':
/usr/local/include/pqxx/src/result.cxx:32: undefined reference to `PQclear'
/usr/bin/ld: /usr/local/lib/libpqxx.a(result.o): in function `pqxx::result::StatusError[abi:cxx11]() const':
/usr/local/include/pqxx/src/result.cxx:265: undefined reference to `PQresultStatus'
/usr/bin/ld: /usr/local/lib/libpqxx.a(result.o): in function `pqxx::result::columns() const':
/usr/local/include/pqxx/src/result.cxx:402: undefined reference to `PQnfields'
/usr/bin/ld: /usr/local/lib/libpqxx.a(row.o): in function `pqxx::result::column_number(char const*) const':
/usr/local/include/pqxx/src/row.cxx:209: undefined reference to `PQfnumber'
/usr/bin/ld: /usr/local/lib/libpqxx.a(util.o): in function `pqxx::describe_thread_safety()':
/usr/local/include/pqxx/src/util.cxx:35: undefined reference to `PQisthreadsafe'
/usr/bin/ld: /usr/local/lib/libpqxx.a(util.o): in function `pqxx::internal::freepqmem(void const*)':
/usr/local/include/pqxx/src/util.cxx:108: undefined reference to `PQfreemem'
collect2: error: ld returned 1 exit status
make: *** [Makefile:17: build/a.out] Error 1

From what I can tell, some necessary parts of the libpq library can't be found. I don't know if I'm supposed to be using -I or -L, /usr/lib/postgresql/13/ or /usr/include/postgresql or something else, so I've just been trying different combinations, but to no avail. In addition to installing PostgreSQL, I installed libpq-dev with apt-get, but I don't know what that actually did.

"/usr/local/lib" indicates that all the Postgres stuff was manually installed, and for some unclear reason it is not the standard, fully-configured installable package that's included in every Linux distribution. Writing and correctly linking C++ code with advanced database libraries is hard enough, but having to work with manually-installed, custom library version, with unknown installation issues and a likely conflict with the system-installed Postgres library, will likely make it a very difficult task even for experienced C++ developers. — Sam Varshavchik, Oct 22 '20 at 12:46
@SamVarshavchik I followed these instructions for installing PostgreSQL: https://www.postgresql.org/download/linux/ubuntu/. I'm not sure if it was installed already. What do you suggest I do about it? — Zyxl, Oct 22 '20 at 12:55
Related: [https://www.digitalocean.com/community/tutorials/how-to-install-and-use-postgresql-on-ubuntu-20-04](https://www.digitalocean.com/community/tutorials/how-to-install-and-use-postgresql-on-ubuntu-20-04) — drescherjm, Oct 22 '20 at 13:06
@SamVarshavchik I just checked and I don't think I actually have any Postgre stuff in usr/local/lib, just boost and pqxx. — Zyxl, Oct 22 '20 at 13:36
What I suggest is that you investigate how to install the pqxx package from your Linux distribution's package repository. There's never any need to download popular Linux software and manually build it, since it's a near certainty that it's already included in every mainstream Linux distribution. Those installation instructions are really for experienced Linux distribution developers who are preparing ***the package itself*** for the Linux distribution itself. All you have to do is install the pre-built, pre-configured package itself. — Sam Varshavchik, Oct 22 '20 at 13:43
Same goes for Boost. Boost is already included in every Linux distribution that I know off. Manually installing Boost yourself, in most cases, only creates instability due to the likely conflict with the already-installed Boost components, which are almost certainly a different version that was installed manually. All this does is create more confusion and unneeded work, for yourself. Every popular Linux package, like Boost and Postgres, is already included in every Linux distribution. Just needs to be installed from the distribution's package repository. — Sam Varshavchik, Oct 22 '20 at 13:45
Additionally, most, if not all, Linux distributions rely on packaging formats that encode information about dependencies among packages, so that at worst, you cannot unknowingly install a package without also installing all its dependencies. But most systems take that a step further by taking care of installing missing packages dependencies automatically. It is difficult to overstate how much easier that makes things, even for people who know exactly what they're doing. You don't get any of that when you install directly from source. — John Bollinger, Oct 22 '20 at 17:15

Can't find libpq library for C++ program using libpqxx

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