+#ifndef NFDBITS
+#define NFDBITS (sizeof(fd_mask)*8)
+#endif
+
+/* Divide positive x by y, rounding up. */
+#define DIV_ROUNDUP(x, y) (((x)+((y)-1))/(y))
+
+/* How many bytes to allocate for N fds? */
+#define SELECT_ALLOC_SIZE(n) \
+ (DIV_ROUNDUP(n, NFDBITS) * sizeof(fd_mask))
+