An "external function" is one that isn't in the current file of your source code. When you use external functions (as you are doing), you need to tell the compiler which library contains the functions you are calling.

Nevertheless, that is what you appear to be doing.
Code:
steve@steve01:~/Projects/ClientApp/src$ g++ -o ../bin/client ClientApp.cpp -I ../include \
-L ../lib -lSocketFunctions
The only thing I can think of is that your library is compiled as C with gcc, while your ClientApp.cpp is compiled as C++ with g++. If this is the case, either compile both as C++ with g++ or try something like this:
Code:
#include <iostream>
extern "C" {
#include "SocketApp.h"
}

using namespace std;

int main(int argc, char** argv)
{
	cout << "******* This is the client application	********" << endl;
	cout << endl;

	int sockfd = 0;

	//Create a new socket
	sockfd = CreateSocket();
	
	cout << "sockfd: " << sockfd << endl;

	return 0;
}