使用标题编译C程序(新手)

gcd.h

#ifndef GCD_H
#define GCD_H

long gcd(long a,long b);

#endif

gcd.cc

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

using namespace std;

long gcd(long a,long b) {
    if( a == 0 && b == 0 ) {
        cerr << "WARNING: gcd(0,0) not defined" << endl;
        return 0;
    }

    if( a < 0 ) a = -a;
    if( b < 0 ) b = -b;

    if( b == 0 ) retrun a;
    if( a == 0 ) return b;

    long c = a % b;

    return gcd(b,c);
}

和gcdtest.cc

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

using namespace std;

int main() {
    long a,b;

    cout << "Enter the first number > ";
    cin >> a;
    cout << "Enter the second number > ";
    cin >> b;

    long d = gcd(a,b);

    cout << "The gcd of " << a << " and " << b << " is " << d << endl;

    return 0;
}

所有这些文件都驻留在同一目录中.当我尝试编译gcdtest.cc时

$g++ -Wall gcdtest.cc -o gcdtest.exe

我收到以下错误：

$g++ -Wall gcdtest.cc -o gcdtest.exe
Undefined symbols for architecture x86_64:
      "gcd(long,long)",referenced from:
          _main in ccgbCyQO.o
ld: symbol(s) not found for architecture x86_64
collect2: ld returned 1 exit status

我是C的新手,并没有完全grokked编译和链接.我究竟做错了什么？