fork + wait gets EINTR on OSX

I fail at fork 101. I expect this to fork a child process, and output both child and parent printfs:

pid_t fpid;

if ((fpid = fork()) < 0)
{
    printf("fork: %s\n", strerror(errno));
    exit(-1);
}

if (0 == fpid) // child
{
    printf("\nI am the child\n");
}
else
{
    printf("\nI am the parent\n");
    pid_t wpid;
    while ((wpid = waitpid(WAIT_ANY, NULL, 0)))
    {
        if (errno == ECHILD)
            break;
        else if (wpid < 0)
            printf("wait: %s\n", strerror(errno));
    }
}

Instead, I get this output:

I am the parent

wait: Interrupted system call

So my question is: why doesn't the child get a chance to live and run? Won't someone please think of the children! Also, where does the EINTR come from? Obviously, this is somehow related to my first question.

Furthermore, when I run that code in a standalone program, it works correctly, but not when inside a larger program of mine; what could the larger program do to upset waitpid?

FWIW this is on OSX 10.9.