@itemize
@item
@samp{O_CLOEXEC}, @samp{O_DIRECTORY}, @samp{O_DSYNC}, @samp{O_NOCTTY},
-@samp{O_NOFOLLOW}, @samp{O_NONBLOCK}, @samp{O_RSYNC}, @samp{O_SYNC},
+@samp{O_NOFOLLOW}, @samp{O_RSYNC}, @samp{O_SYNC},
and @samp{O_TTY_INIT} are not defined on some platforms. Gnulib defines
-these macros to 0.
+these macros to 0, which is generally safe.
+
+@item
+@samp{O_NONBLOCK} is not defined on some platforms. If the
+@samp{nonblocking} module is in use, gnulib guarantees a working
+non-zero value; otherwise, the gnulib replacement is 0.
@item
@samp{O_EXEC} and @samp{O_SEARCH} are not defined on some platforms.
Gnulib defines these macros to @samp{O_RDONLY}, which is typically 0.
+@item
+The @samp{O_ACCMODE} mask mistakenly omits @samp{O_SEARCH} and
+@samp{O_EXEC} on some platforms:
+Cygwin.
+
@item
@samp{O_BINARY}, @samp{O_TEXT} (not specified by POSIX, but essential for
portability to Woe32 platforms) are defined on some platforms but not on