undefined reference to 'XXX' after using #define _GNU

I used search.h for my c program, where I need to put #define _GNU_SOURCE to the first line in order to introduce multiple hash tables. But after that, errors like undefined reference to 'log10' and undefined reference to 'PQntuples' popped up. I certainly need all the packages there, how should I now compile the program? Any help would be deeply appreciated! Thanks.

The headers:

#define _GNU_SOURCE

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <math.h>
#include <string.h>
// library for psql
#include <libpq-fe.h>
#include <unistd.h>
#include <time.h>
#include <search.h>

int main(void){
    char host[] = "localhost";
    char port[] = "5432";
    char db_name[] = "db_name";
    char user[] = "test_usr";
    char password[] = "123456";
    sprintf(db_str, "host=%s port=%s dbname=%s user=%s password=%s",
             host, port, db_name, user, password);
    PGconn *db_connection = DBConnect(db_str);

    struct hsearch_data htab;
    hcreate_r(10, &htb);
    ENTRY e, *ep;
    e.key = "test";
    e.data = (void *) 1;
    hsearch_r(e, ENTER, &ep, &htab);
}

And this was how I compile the file:

gcc -Wall -Wextra -I/home/userX/postgresql/include -L/home/userX/postgresql/lib -lm -lpq -g my_program.c

CodePudding user response：

Specifiy the libraries at the end of the command line

gcc -Wall -Wextra -I/home/userX/postgresql/include \
      -L/home/userX/postgresql/lib -g my_program.c -lpq -lm
//                                                 ^^^^^^^^

gcc looks at required symbols left to right on the command line.
With the libraries before the source files, when gcc processes the -llib argument it does not have any requirements and therefore does not "extract" any function from the library.