Cannot connect to PostgreSQL using C++ - c++

Question

I am trying to run PostgreSQL on my mac. PostgreQL itself works fine and I can create database and table and stuff but when I try to connect to PostgreSQL using C++ with something like:
#include <stdio.h>
#include </Library/PostgreSQL/8.4/include/libpq-fe.h>
#include <string>
int main() {
PGconn *conn;
PGresult *res;
int rec_count;
conn = PQconnectdb("dbname=ljdata host=localhost user=dataman);
if (PQstatus(conn) == CONNECTION_BAD) {
puts("We were unable to connect to the database");
exit(0);
}
res = PQexec(conn, "update people set phonenumber=\'5055559999\' where id=3");
and compile with something like:
g++ -lpq db.cpp -o db
I get the error
ld: library not found for -lpq
and if I compile without lpq, I get
Undefined symbols:
"_PQclear", referenced from:
_main in ccpjNCAU.o
_main in ccpjNCAU.o"
I have already included the libpq-fe.h, shouldn't it work? Does anybody know what went wrong?

Answer 1

g++ can't find the pq library. You have to specify where to look for it, with a capital -L:
g++ -L/path/to/pq/lib -lpq db.cpp -o db
where pq is /path/to/pq/lib/libpq.a (or whatever the extension is)
Here's what you probably want to do:
change the include line to not have the path.
#include "libpq-fe.h"
Add the include path to the commandline
g++ -I/Library/PostgreSQL/8.4/include db.cpp
Build intermediary object files
g++ -I/Library/PostgreSQL/8.4/include db.cpp -c -o db.o
Link it together as a separate step
g++ -L/Library/PostgreSQL/8.4/lib db.o -lpq
Build with debug info using -g
Put it all together, for two separate build steps: compile and link:
g++ -I/Library/PostgreSQL/8.4/include db.cpp -c -g -o db.o
g++ -L/Library/PostgreSQL/8.4/lib db.o -lpq -o db

Answer 2

libpq-fe.h is a user library, not a system library, and therefore you should use "..." instead of <...>, like this:
#include "/Library/PostgreSQL/8.4/include/libpq-fe.h"
Take a look at this link. And make sure libpq-fe.h can actually be found by your compiler.

Answer 3

Had the same problem, You need to add the path of the library to /etc/ld.so.conf, do it and you'll see.
Good luck