C++ multiple definition error when compiling in G++ - c++

Question

I'm running into some issues compiling and I'm not sure why. I get a multiple definition error for a few things, however, as far as I'm aware, I only define them once. I'm confused as to why this is happening and where these definitions are happening and why its not working. I'm not that familiar with C++ so I'm not sure what I'm doing wrong here and would appreciate some assistance.
make
/usr/bin/g++ -c -o object/author.o src/author.cpp -g -I./include
/usr/bin/g++ -o ---- object/connection_manager.o object/author.o. object/control_header_lib.o object/network_util.o object/main.o object/control_handler.o -g -I./include
object/main.o:(.bss+0x4): multiple definition of `control_socket'
object/connection_manager.o:(.bss+0x104): first defined here
object/main.o: In function `main':
/home/Documents/---/src/main.cpp:9: multiple definition of `router_socket'
object/connection_manager.o:/home/Documents/---/src. /connection_manager.cpp:35: first defined here
object/main.o: In function `main':
/home/Documents/---/src/main.cpp:9: multiple definition of `data_socket'
object/connection_manager.o:/home/Documents/---/src. /connection_manager.cpp:35: first defined here
object/control_handler.o:(.bss+0x0): multiple definition of `CONTROL_PORT'
object/main.o:/home/Documents/---/src/main.cpp:9: first defined here
object/connection_manager.o: In function `main_loop()':
/home/Documents/---/src/connection_manager.cpp:33: undefined reference to `new_control_conn(int)'
/home/Documents/---/src/connection_manager.cpp:54: undefined reference to `isControl(int)'
/home/Documents/---/src/connection_manager.cpp:56: undefined reference to `control_recv_hook(int)'
collect2: error: ld returned 1 exit status
Makefile:19: recipe for target '----' failed
make: *** [----] Error 1
main.cpp
#include "../include/global.h"
#include "../include/connection_manager.h"
using namespace std;
int sockfd;
int main(int argc, char **argv)
{
if(argc != 2){
ERROR("Incorrect number of args...\n");
}
printf("control port: %d\n", atoi(argv[1]));
sscanf(argv[1], "%" SCNu16, &CONTROL_PORT);
init();
return 0;
}
global.h
#ifndef GLOBAL_H_
#define GLOBAL_H_
#include <stdio.h>
#include <stdlib.h>
#include <inttypes.h>
#define ERROR(err_msg) {perror(err_msg); exit(EXIT_FAILURE);}
#define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)]))
#endif
connection_manager.cpp
#include <sys/select.h>
#include "../include/connection_manager.h"
#include "../include/global.h"
#include "../include/control_handler.h"
int control_socket, router_socket, data_socket;
void init()
{
control_socket = create_control_sock();
}
connection_manager.h
#ifndef CONNECTION_MANAGER_H_
#define CONNECTION_MANAGER_H_
void init();
#endif
control_handler.cpp
#include <sys/socket.h>
#include <netinet/in.h>
#include <strings.h>
#include <sys/queue.h>
#include <unistd.h>
#include <string.h>
#include <list>
#include "../include/global.h"
#include "../include/network_util.h"
#include "../include/control_header_lib.h"
#include "../include/author.h"
#ifndef PACKET_USING_STRUCT
#define CNTRL_CONTROL_CODE_OFFSET 0x04
#define CNTRL_PAYLOAD_LEN_OFFSET 0x06
#endif
struct ControlConn // linked list for active control connections
{
int sockfd;
// ..
}*connection, *conn_temp;
std::list<ControlConn> control_conn_list;
int create_control_sock()
{
int sock;
struct sockaddr_in control_addr;
socklen_t addrlen = sizeof(control_addr);
sock = socket(AF_INET, SOCK_STREAM, 0);
if (sock < 0){
ERROR("Failed to create socket...\n");
}
bzero(&control_addr, addrlen);
control_addr.sin_family = AF_INET;
control_addr.sin_addr.s_addr = htonl(INADDR_ANY);
control_addr.sin_port = htons(CONTROL_PORT);
if (bind(sock, (struct sockaddr *)&control_addr, addrlen) < 0){
ERROR("Bind failed...");
}
if (listen(sock, 5) < 0){
ERROR("Listen failed...");
}
return sock;
}
control_handler.h
#ifndef CONTROL_HANDLER_H_
#define CONTROL_HANDLER_H_
int create_control_sock();
int new_control_conn(int sock_index);
bool isControl(int sock_index);
bool control_recv_hook(int sock_index);
#endif

Answer 1

In header file you need to just declare variable:
extern uint16_t CONTROL_PORT;
Then in corresponding cpp (global.cpp) file you should define it:
uint16_t CONTROL_PORT;