nonblocking: provide O_NONBLOCK for mingw Mingw is the only known platform that lacks O_NONBLOCK (possibly via the alternate spelling O_NDELAY). But mingw also lacks any files where open() needs to enforce non-blocking behavior, and lacks openat(), so it is relatively simple to provide a non-zero flag. A future patches will make use of O_NONBLOCK for pipe2. * modules/nonblocking (Depends-on): Add open. (configure.ac): Set new witness macro. * m4/fcntl_h.m4 (gl_FCNTL_H_DEFAULTS): Provide default for it. * modules/fcntl-h (Makefile.am): Substitute it. * lib/fcntl.in.h (O_NONBLOCK): Guarantee non-zero definition when nonblocking module is in use. * lib/nonblocking.c: Adjust portability test. * lib/open.c (open): Don't let native open see gnulib flag. * tests/test-fcntl-h.c (main): Enhance test. * tests/test-open.h (test_open): Likewise. * doc/posix-headers/fcntl.texi (fcntl.h): Document the replacement. Signed-off-by: Eric Blake <eblake@redhat.com>

--- a/doc/posix-headers/fcntl.texi
+++ b/doc/posix-headers/fcntl.texi
@@ -9,9 +9,14 @@
 @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.