Cannot connect to PostgreSQL using 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?

Tim · Accepted Answer · 2011-03-14 22:57:58Z

4

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

edited Mar 14, 2011 at 22:57

answered Mar 14, 2011 at 22:17

Tim

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14 Comments

Confused Over a year ago

So if my file is in home, I should just do g++ -L/Library/PostgreSQL/8.4/include/libpq-fe.h? That doesn't work either.

Tim Over a year ago

-L/path/to/lib/dir not /path/to/header/file. Where is libpq located?

Confused Over a year ago

libpq-fe.h and libpq-events.h are in Library/PostgreSQL/8.4/include/ but there is one more, "libpq-fs.h" in Library/PostgreSQL/8.4/include/libpq

Tim Over a year ago

those are header files - they end in .h. Where is the actual library file?

Tim Over a year ago

try looking in Library/PostgreSQL/8.4/lib is it there? If so, try compiling with -L~/Library/PostgreSQL/8.4/lib

|

darioo · Accepted Answer · 2011-03-14 22:19:30Z

0

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.

answered Mar 14, 2011 at 22:19

darioo

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1 Comment

Confused Over a year ago

I think my compiler can find libpq-fe.h. Once I change #include "/Library/PostgreSQL/8.4/include/libpq-fe.h" to #include "Library/PostgreSQL/8.4/include/libpq-fe.h", I get error: ‘PGconn’ was not declared in this scope error: ‘conn’ was not declared in this scope. As PGconn has been defined, doesn't that mean the libpq-fe.h can be found by the compiler?

Coding Mash · Accepted Answer · 2012-10-26 11:36:20Z

0

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

edited Oct 26, 2012 at 11:36

Coding Mash

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answered Apr 16, 2012 at 22:25

OmarJarid

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