futex.2: Note that at least one bit must be set in mask for BITSET operations

At least one bit must be set in the 'val3' mask supplied for the
FUTEX_WAIT_BITSET and FUTEX_WAKE_BITSET operations.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

diff --git a/man2/futex.2 b/man2/futex.2
index 7e065e7ab69ce02ab33b2537be50f72fa3f9737d..5146a1146cfadd8b9664c5b6e7875f31d400f734 100644 (file)
--- a/man2/futex.2
+++ b/man2/futex.2
@@ -694,7 +694,8 @@ This operation is like
 except that
 .I val3
 is used to provide a 32-bit bit mask to the kernel.
-This bit mask is stored in the kernel-internal state of the waiter.
+This bit mask, in which at least one bit must be set, 
+is stored in the kernel-internal state of the waiter.
 See the description of
 .BR FUTEX_WAKE_BITSET
 for further details.
@@ -722,7 +723,8 @@ This operation is the same as
 except that the
 .I val3
 argument is used to provide a 32-bit bit mask to the kernel.
-This bit mask is used to select which waiters should be woken up.
+This bit mask, in which at least one bit must be set,
+is used to select which waiters should be woken up.
 The selection is done by a bit-wise AND of the "wake" bit mask
 (i.e., the value in
 .IR val3 )