UMTX_SHM_CREAT
fd = _umtx_op(NULL, UMTX_OP_SHM, doalloc ? UMTX_SHM_CREAT :
if (__bitcount(flags & (UMTX_SHM_CREAT | UMTX_SHM_LOOKUP |
error = (flags & UMTX_SHM_CREAT) != 0 ?