Ambiguous reference for type pthread_mutex_t

I announced

pthread_mutex_t mutex=PTHREAD_MUTEX_INITIALIZER;

      

        
        
        
      

    

in the global space of my main program main.cpp.

However, when I try to refer to it in a function definition, for example

void foo(void)
{
    pthread_mutex_lock(&mutex);
    ...
}

      

        
        
        
      

    

the following error message appears:

reference to "mutex" is ambiguous

Any idea what's going on? If it helps, it gets recorded in xcode and I have included it in the program header.