--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+2007-04-13  Bruno Haible  <bruno@clisp.org>
+
+	* modules/fwritable: New file.
+	* lib/fwritable.h: New file.
+	* lib/fwritable.c: New file.
+	* m4/fwritable.m4: New file.
+
 2007-04-13  Bruno Haible  <bruno@clisp.org>
 
 	* modules/freadable-tests: New file.

new file mode 100644
--- /dev/null
+++ b/lib/fwritable.c
@@ -0,0 +1,38 @@
+/* Retrieve information about a FILE stream.
+   Copyright (C) 2007 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 2, or (at your option)
+   any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License along
+   with this program; if not, write to the Free Software Foundation,
+   Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.  */
+
+#include <config.h>
+
+/* Specification.  */
+#include "fwritable.h"
+
+bool
+fwritable (FILE *fp)
+{
+  /* Most systems provide FILE as a struct and the necessary bitmask in
+     <stdio.h>, because they need it for implementing getc() and putc() as
+     fast macros.  */
+#if defined _IO_ferror_unlocked     /* GNU libc, BeOS */
+  return (fp->_flags & _IO_NO_WRITES) == 0;
+#elif defined __sferror             /* FreeBSD, NetBSD, OpenBSD, MacOS X, Cygwin */
+  return (fp->_flags & (__SRW | __SWR)) != 0;
+#elif defined _IOERR                /* AIX, HP-UX, IRIX, OSF/1, Solaris, mingw */
+  return (fp->_flag & (_IORW | _IOWRT)) != 0;
+#else
+ #error "Please port gnulib fwritable.c to your platform!"
+#endif
+}

new file mode 100644
--- /dev/null
+++ b/lib/fwritable.h
@@ -0,0 +1,43 @@
+/* Retrieve information about a FILE stream.
+   Copyright (C) 2007 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 2, or (at your option)
+   any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License along
+   with this program; if not, write to the Free Software Foundation,
+   Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.  */
+
+#include <stdbool.h>
+#include <stdio.h>
+
+/* Return true if the stream STREAM supports writing, false if it supports
+   only reading.
+   STREAM must not be wide-character oriented.
+   The result doesn't change until the stream is closed or re-opened.  */
+
+#if HAVE___FWRITABLE /* glibc >= 2.2, Solaris >= 7 */
+
+# include <stdio_ext.h>
+# define fwritable(stream) (__fwritable (stream) != 0)
+
+#else
+
+# ifdef __cplusplus
+extern "C" {
+# endif
+
+extern bool fwritable (FILE *stream);
+
+# ifdef __cplusplus
+}
+# endif
+
+#endif

new file mode 100644
--- /dev/null
+++ b/m4/fwritable.m4
@@ -0,0 +1,13 @@
+# fwritable.m4 serial 1
+dnl Copyright (C) 2007 Free Software Foundation, Inc.
+dnl This file is free software; the Free Software Foundation
+dnl gives unlimited permission to copy and/or distribute it,
+dnl with or without modifications, as long as this notice is preserved.
+
+AC_DEFUN([gl_FUNC_FWRITABLE],
+[
+  AC_CHECK_FUNCS_ONCE([__fwritable])
+  if test $ac_cv_func___fwritable = no; then
+    AC_LIBOBJ([fwritable])
+  fi
+])

new file mode 100644
--- /dev/null
+++ b/modules/fwritable
@@ -0,0 +1,25 @@
+Description:
+fwritable() function: Determine whether a FILE stream supports writing.
+
+Files:
+lib/fwritable.h
+lib/fwritable.c
+m4/fwritable.m4
+
+Depends-on:
+stdbool
+
+configure.ac:
+gl_FUNC_FWRITABLE
+
+Makefile.am:
+
+Include:
+"fwritable.h"
+
+License:
+LGPL
+
+Maintainer:
+Bruno Haible, Eric Blake
+