Changes to m4/modules/m4.c,v

[Eric Blake]

CVSROOT:        /sources/m4
Module name:    m4
Changes by:     Eric Blake <ericb>      06/12/15 04:23:48

Index: modules/m4.c
===================================================================
RCS file: /sources/m4/m4/modules/m4.c,v
retrieving revision 1.93
retrieving revision 1.94
diff -u -b -r1.93 -r1.94
--- modules/m4.c        11 Nov 2006 16:21:25 -0000      1.93
+++ modules/m4.c        15 Dec 2006 04:23:48 -0000      1.94
@@ -486,9 +486,33 @@
 {
   FILE *debug_file = m4_get_debug_file (context);
 
-  if (debug_file != stdout) fflush (stdout);
-  if (debug_file != stderr) fflush (stderr);
-  if (debug_file != NULL)   fflush (debug_file);
+  if (debug_file != stdout)
+    fflush (stdout);
+  if (debug_file != stderr)
+    fflush (stderr);
+  if (debug_file != NULL)
+    fflush (debug_file);
+  /* POSIX requires that if m4 doesn't consume all input, but stdin is
+     opened on a seekable file, that the file pointer be left at the
+     next character on exit (but places no restrictions on the file
+     pointer location on a non-seekable file).  It also requires that
+     fflush() followed by fseek() on an input file set the underlying
+     file pointer.  However, fflush() on a non-seekable file can lose
+     buffered data, which we might otherwise want to process after
+     syscmd.  Hence, we must check whether stdin is seekable.  We must
+     also be tolerant of operating with stdin closed, so we don't
+     report any failures in this attempt.  The stdio-safer module and
+     friends are essential, so that if stdin was closed, this lseek is
+     not on some other file that we have since opened.  Mingw has bugs
+     when using fseek on text files, so we only strive for POSIX
+     behavior when we detect a UNIX environment.  */
+#if UNIX
+  if (lseek (STDIN_FILENO, 0, SEEK_CUR) >= 0
+      && fflush (stdin) == 0)
+    {
+      fseek (stdin, 0, SEEK_CUR);
+    }
+#endif /* UNIX */
 }
 
 M4BUILTIN_HANDLER (syscmd)