I forgot about that part of libdispatch. You still can't do synchronous IO but you can use libdispatch semaphores, wait on a queue, etc. and that should not block the thread, right?
#define DISPATCH_SOURCE_TYPE_DATA_ADD
#define DISPATCH_SOURCE_TYPE_DATA_OR
#define DISPATCH_SOURCE_TYPE_MACH_RECV
#define DISPATCH_SOURCE_TYPE_MACH_SEND
#define DISPATCH_SOURCE_TYPE_PROC
#define DISPATCH_SOURCE_TYPE_READ
#define DISPATCH_SOURCE_TYPE_SIGNAL
#define DISPATCH_SOURCE_TYPE_TIMER
#define DISPATCH_SOURCE_TYPE_VNODE
#define DISPATCH_SOURCE_TYPE_WRITE
#define DISPATCH_SOURCE_TYPE_MEMORYPRESSURE