--- a/lib/fflush.c
+++ b/lib/fflush.c
@@ -29,6 +29,55 @@
 
 #undef fflush
 
+static inline void
+clear_ungetc_buffer (FILE *fp)
+{
+#if defined __sferror               /* FreeBSD, NetBSD, OpenBSD, MacOS X, Cygwin */
+# if defined __NetBSD__ || defined __OpenBSD__
+  struct __sfileext
+    {
+      struct  __sbuf _ub; /* ungetc buffer */
+      /* More fields, not relevant here.  */
+    };
+#  define HASUB(fp) (((struct __sfileext *) (fp)->_ext._base)->_ub._base != NULL)
+# else
+#  define HASUB(fp) ((fp)->_ub._base != NULL)
+# endif
+  if (HASUB (fp))
+    {
+      fp->_p += stream->_r;
+      fp->_r = 0;
+    }
+#endif
+}
+
+#if defined __sferror && defined __SNPT /* FreeBSD, NetBSD, OpenBSD, MacOS X, Cygwin */
+
+static inline int
+disable_seek_optimization (FILE *fp)
+{
+  int saved_flags = fp->_flags & (__SOPT | __SNPT);
+  fp->_flags = (fp->_flags & ~__SOPT) | __SNPT;
+  return saved_flags;
+}
+
+static inline void
+restore_seek_optimization (FILE *fp, int saved_flags)
+{
+  fp->_flags = (fp->_flags & ~(__SOPT | __SNPT)) | saved_flags;
+}
+
+#endif
+
+static inline void
+update_fpos_cache (FILE *fp)
+{
+#if defined __sferror               /* FreeBSD, NetBSD, OpenBSD, MacOS X, Cygwin */
+  fp->_offset = pos;
+  fp->_flags |= __SOFF;
+#endif
+}
+
 /* Flush all pending data on STREAM according to POSIX rules.  Both
    output and seekable input streams are supported.  */
 int
@@ -80,24 +129,7 @@
      _IOERR, because an ungetc() on this platform prepends the pushed-back
      bytes to the buffer without an indication of the limit between the
      pushed-back bytes and the read-ahead bytes.  */
-#if defined __sferror               /* FreeBSD, NetBSD, OpenBSD, MacOS X, Cygwin */
-  {
-# if defined __NetBSD__ || defined __OpenBSD__
-    struct __sfileext
-      {
-	struct  __sbuf _ub; /* ungetc buffer */
-	/* More fields, not relevant here.  */
-      };
-    if (((struct __sfileext *) stream->_ext._base)->_ub._base != NULL)
-# else
-    if (stream->_ub._base != NULL)
-# endif
-      {
-	stream->_p += stream->_r;
-	stream->_r = 0;
-      }
-  }
-#endif
+  clear_ungetc_buffer (stream);
 
   /* POSIX does not specify fflush behavior for non-seekable input
      streams.  Some implementations purge unread data, some return
@@ -122,12 +154,11 @@
     /* Disable seek optimization for the next fseeko call.  This tells the
        following fseeko call to seek to the desired position directly, rather
        than to seek to a block-aligned boundary.  */
-    int saved_flags = stream->_flags & (__SOPT | __SNPT);
-    stream->_flags = (stream->_flags & ~__SOPT) | __SNPT;
+    int saved_flags = disable_seek_optimization (stream);
 
     result = fseeko (stream, pos, SEEK_SET);
 
-    stream->_flags = (stream->_flags & ~(__SOPT | __SNPT)) | saved_flags;
+    restore_seek_optimization (stream, saved_flags);
   }
   return result;
 
@@ -138,10 +169,7 @@
     return EOF;
   /* After a successful lseek, update the file descriptor's position cache
      in the stream.  */
-# if defined __sferror           /* FreeBSD, NetBSD, OpenBSD, MacOS X, Cygwin */
-  stream->_offset = pos;
-  stream->_flags |= __SOFF;
-# endif
+  update_fpos_cache (stream);
 
   return 0;
 

--- a/m4/fflush.m4
+++ b/m4/fflush.m4
@@ -1,4 +1,4 @@
-# fflush.m4 serial 5
+# fflush.m4 serial 6
 
 # Copyright (C) 2007-2008 Free Software Foundation, Inc.
 # This file is free software; the Free Software Foundation
@@ -56,4 +56,12 @@
   AC_REQUIRE([gl_STDIO_H_DEFAULTS])
   REPLACE_FFLUSH=1
   REPLACE_FSEEKO=1
+  gl_PREREQ_FFLUSH
 ])
+
+# Prerequisites of lib/fflush.c.
+AC_DEFUN([gl_PREREQ_FFLUSH],
+[
+  AC_REQUIRE([AC_C_INLINE])
+  :
+])