Pthreads function return values

Most pthreads functions return 0 on success, and an error number of failure.
Note that the pthreads functions do not set errno. For each of the pthreads
functions that can return an error, POSIX.1-2001 specifies that the function
can never fail with the error EINTR.

Auf der anderen Seite finde ich in der mutex-Klasse von boost.thread:

void lock()
{
    int res;
    do
    {
        res = pthread_mutex_lock(&m);
    } while (res == EINTR);
...

Wie passt das zusammen?