Undefined reference problem

[David Holmes - Sun Microsystems]

Newbie question I guess but it's got me perplexed :)

In os::Linux I'm adding:

static pthread_condattr_t _cond_attr[1];
public:
static pthread_condattr_t cond_attr() { return _cond_attr; }

it compiles fine, but at link time I get an error:

Undefined reference to: os::Linux::_cond_attr

??? What did I miss?

Thanks,
David