in pathconf, -1, not 0, means unsupported.. syncio presumably works, too.

src/conf/fpathconf.c

@@ -14,9 +14,9 @@ long fpathconf(int fd, int name)
 		[_PC_CHOWN_RESTRICTED] = 1,
 		[_PC_NO_TRUNC] = 1,
 		[_PC_VDISABLE] = 0,
-		[_PC_SYNC_IO] = 0,
-		[_PC_ASYNC_IO] = 0,
-		[_PC_PRIO_IO] = 0,
+		[_PC_SYNC_IO] = 1,
+		[_PC_ASYNC_IO] = -1,
+		[_PC_PRIO_IO] = -1,
 		[_PC_SOCK_MAXBUF] = -1,
 		[_PC_FILESIZEBITS] = FILESIZEBITS,
 		[_PC_REC_INCR_XFER_SIZE] = PAGE_SIZE,