Question

I am using libpq-fe to connect to the sql server as explained here.
I made a small file to check login using this api.

Here is the file psql.cpp:

#include <iostream>
#include <libpq-fe.h>
#include <string>
using namespace std;

void CloseConn(PGconn *conn)
{
  PQfinish(conn);
  getchar();
}

PGconn *ConnectDB(string user="postgres",string password="123321",string dbname="bridge",string hostaddr="127.0.0.1",string port="5432")
{
  PGconn *conn = NULL;
  string s = "user=" + user + " password=" + password + " dbname=" +dbname + " hostaddr=" + hostaddr + " port=" + port;

  // Make a connection to the database
  conn = PQconnectdb(s.c_str());

  // Check to see that the backend connection was successfully made
  if (PQstatus(conn) != CONNECTION_OK)
  {
    cout << "Connection to database failed.\n";
    CloseConn(conn);
  }

  cout << "Connection to database - OK\n";

  return conn;
}


void login_check(PGconn *conn, string username, string password)
{
  string query = "SELECT * FROM login where plid='" + username + "'";

  PGresult *res = PQexec(conn,query.c_str());

  if(PQresultStatus(res) == PGRES_TUPLES_OK)//successful completion of a command returning data
  {
    cout << "query executed successfully\n";
    int row = PQntuples(res); // number of rows in the output of the query
    cout<<row<<endl;
    if (row != 1)
    {
      //wrong username
    }
    else
    {      
      cout<<PQgetvalue(res,0,1)<<endl;
      if( !(string(PQgetvalue(res,0,1)).compare(password)) )//return 0 on equality
      {
        cout<<"valid user";
      }
    }
  }

  // Clear result
  PQclear(res);
}

I put hits main function in this file and everything works fine:

int main()
{
  PGconn *conn = NULL;
  //conn = ConnectDB("postgres","123321","bridge","127.0.0.1","5432");
  conn = ConnectDB("postgres","123321","bridge","127.0.0.1","5432");

  if (conn != NULL) {
    login(conn, "11111000", "abcd");
    CloseConn(conn);
  }

  return 0;

}

Now I want to include this file in another .cpp file so I made a psql.h file:

#include <libpq-fe.h>
#include <string>
using namespace std;

PGconn *ConnectDB(string ,string, string, string, string);
void login_check(PGconn, string, string);
void CloseConn(PGconn);

For using the psql.h file the changes I have done in psql.cpp header are:

#include "psql.h"
#include <iostream>

void CloseConn(PGconn *conn)
{
...
...

and I removed the main function from this.

Now in my new file – dispatcher.cpp I want to include this file, So change in its header:

#include "psql.h"

And I put the contents of the main function that I have mentioned above in the main function of this file. When I compile this file I got the error:

dispatcher.cpp: In function ‘void login(int)’:
dispatcher.cpp:154:45: error: parameter 1 of ‘void login_check(PGconn, std::string, std::string)’ has incomplete type ‘PGconn {aka pg_conn}’
dispatcher.cpp:155:23: error: parameter 1 of ‘void CloseConn(PGconn)’ has incomplete type ‘PGconn {aka pg_conn}’

My makefile:

dispatcher:dispatcher.o access.o psql.o
    g++ dispatcher.o access.o psql.o -pthread -I /usr/include/postgresql -lpq -o ./bin/dispatcher

dispatcher.o:dispatcher.cpp
    g++ -I /usr/include/postgresql -lpq -c dispatcher.cpp

access.o:access.cpp access.h
    g++ -c access.cpp

psql.o:psql.cpp psql.h
    g++ -c psql.cpp -I /usr/include/postgresql -lpq

You can ignore access.o it only contain accessories. I know I am c programming using c++. Why I am getting this error. Is it anything specific to CPP. The login() is a simple function that I am calling from the dispatcher's main and its defination is same as of the main I have mentioned above.

Answer 1

void login_check(PGconn, string, string);
void CloseConn(PGconn);

This doesn’t match your functions. Use:

void login_check(PGconn*, string, string);
void CloseConn(PGconn*);

You can have pointers to incomplete types, but not incomplete types, in function arguments. The compiler needs to know the size of the objects to set up the call/stack properly. It can’t do that with incomplete types, but it can with pointers to such.