--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,12 @@
+2012-09-13  Paolo Bonzini  <bonzini@gnu.org>
+
+	poll/select: document portability problems not fixed by Gnulib.
+	* doc/posix-functions/poll.texi: poll does not work well on
+	pipes under Windows.  It has the same limitations as select on
+       BeOS.
+	* doc/posix-functions/select.texi: select does not work well
+       on pipes under Windows.
+
 2012-09-10  Paul Eggert  <eggert@cs.ucla.edu>
 
 	fcntl-h: check for AIX 7.1 bug with O_NOFOLLOW and O_CREAT

--- a/doc/posix-functions/poll.texi
+++ b/doc/posix-functions/poll.texi
@@ -19,4 +19,12 @@
 
 Portability problems not fixed by Gnulib:
 @itemize
+@item
+Under BeOS, Gnulib's @code{poll} replacement can only be called on descriptors
+created by the @code{socket} function, not on regular file descriptors.
+
+@item
+Under Windows, when passing a pipe, Gnulib's @code{poll} replacement might
+return 0 even before the timeout has passed.  Programs using it with pipes can
+thus busy wait.
 @end itemize

--- a/doc/posix-functions/select.texi
+++ b/doc/posix-functions/select.texi
@@ -31,6 +31,10 @@
 On BeOS, @code{select} can only be called on descriptors created by the
 @code{socket} function, not on regular file descriptors.
 @item
+Under Windows, when passing a pipe, Gnulib's @code{select} replacement might
+return 0 even before the timeout has passed.  Programs using it with pipes can
+thus busy wait.
+@item
 On Solaris 2.6 and older, @code{select} applied to a file descriptor opened
 for reading and associated with @code{/dev/null} hangs, waiting for input,
 when instead it should return immediately.