Re: Austin group ruling on ungetc vs. fflush

[Bruno Haible]

Hi Eric,

> A while ago, we wrote some unit tests that failed on a number of systems
> with different behaviors of fflush after ungetc (particularly if you used
> ungetc to push back a different byte than what was originally read).  At
> the time, Bruno ended up commenting the tests out until we had an official
> ruling from the POSIX folks on what should happen.
> 
> Well, the topic finally came up in yesterday's meeting:
> 
> https://www.opengroup.org/sophocles/show_mail.tpl?CALLER=index.tpl&source=L&listname=austin-group-l&id=11808
> 
> For more details, browse to
> http://www.opengroup.org/austin/aardvark/latest/xshbug3.txt
> and search for Enhancement Request Number 17
> 
> in particular, the new wording states that after fflush, "the file offset
> of the underlying open file description shall be set to the file position
> of the stream, and any characters pushed back onto the stream by ungetc()
> or ungetwc() that have not subsequently been read from the stream shall be
> discarded."

Thanks a lot for pursuing this issue until it came to a resolution!!

> We ought to go ahead and reinstate the proper unit tests for these
> behaviors, as well as improve the fflush and other modules (and perhaps
> add an ungetc module) to make this behavior consistent across platforms.

I agree. The appended patch enables two fflush-after-ungetc tests. It
passes on AIX, HP-UX, IRIX, OSF/1, Solaris, but fails on glibc, *BSD,
mingw platforms. More work to be done for these platforms...


2009-01-15  Bruno Haible  <address@hidden>

        * tests/test-fflush2.sh: Invoke test-fflush2 twice.
        * tests/test-fflush2.c (ASSERT): Always fail.
        (main): Add two tests for fflush() after ungetc(), taking into account
        the Austin Group's clarification.
        Suggested by Eric Blake.

*** tests/test-fflush2.c.orig   2009-01-15 12:16:35.000000000 +0100
--- tests/test-fflush2.c        2009-01-15 12:00:16.000000000 +0100
***************
*** 1,5 ****
  /* Test of POSIX compatible fflush() function.
!    Copyright (C) 2008 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
--- 1,5 ----
  /* Test of POSIX compatible fflush() function.
!    Copyright (C) 2008-2009 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
***************
*** 20,28 ****
  
  #include <stdlib.h>
  
! /* This test can only be made to work on specific platforms.  */
! #if defined _IO_ferror_unlocked || defined __sferror /* GNU libc, BeOS; 
FreeBSD, NetBSD, OpenBSD, MacOS X, Cygwin */
! # define ASSERT(expr) \
    do                                                                       \
      {                                                                      \
        if (!(expr))                                                         \
--- 20,26 ----
  
  #include <stdlib.h>
  
! #define ASSERT(expr) \
    do                                                                       \
      {                                                                      \
        if (!(expr))                                                         \
***************
*** 33,89 ****
          }                                                                  \
      }                                                                      \
    while (0)
- #else
- # define ASSERT(expr) \
-   do                                                                       \
-     {                                                                      \
-       if (!(expr))                                                         \
-         {                                                                  \
-           printf ("Skipping test: expected failure on this platform\n");     \
-           exit (77);                                                       \
-         }                                                                  \
-     }                                                                      \
-   while (0)
- #endif
  
  int
  main (int argc, char **argv)
  {
- #if 0
-   /* Check fflush after a backup ungetc() call.  This is case 1 in terms of
-      <http://lists.gnu.org/archive/html/bug-gnulib/2008-03/msg00131.html>.
-      The Austin Group has not yet decided how this should behave.  */
- #endif
- #if 0
-   /* Check fflush after a non-backup ungetc() call.  This is case 2 in terms 
of
-      <http://lists.gnu.org/archive/html/bug-gnulib/2008-03/msg00131.html>.
-      The Austin Group has not yet decided how this should behave.  */
-   /* Check that fflush after a non-backup ungetc() call discards the ungetc
-      buffer.  This is mandated by POSIX
-      <http://www.opengroup.org/susv3/functions/ungetc.html>:
-        "The value of the file-position indicator for the stream after
-         reading or discarding all pushed-back bytes shall be the same
-         as it was before the bytes were pushed back."  */
    int c;
  
!   c = fgetc (stdin);
!   ASSERT (c == '#');
  
!   c = fgetc (stdin);
!   ASSERT (c == '!');
  
!   /* Here the file-position indicator must be 2.  */
  
!   c = ungetc ('@', stdin);
!   ASSERT (c == '@');
  
!   fflush (stdin);
  
!   /* Here the file-position indicator must be 2 again.  */
  
!   c = fgetc (stdin);
!   ASSERT (c == '/');
! #endif
  
!   return 0;
  }
--- 31,115 ----
          }                                                                  \
      }                                                                      \
    while (0)
  
  int
  main (int argc, char **argv)
  {
    int c;
  
!   if (argc > 1)
!     switch (argv[1][0])
!       {
!       case '1':
!       /* Check fflush after a backup ungetc() call.  This is case 1a in
!          terms of
!          <http://lists.gnu.org/archive/html/bug-gnulib/2008-03/msg00131.html>,
!          according to the Austin Group's resolution on 2009-01-08.  */
! 
!       c = fgetc (stdin);
!       ASSERT (c == '#');
! 
!       c = fgetc (stdin);
!       ASSERT (c == '!');
! 
!       /* Here the file-position indicator must be 2.  */
! 
!       c = ungetc ('!', stdin);
!       ASSERT (c == '!');
! 
!       fflush (stdin);
! 
!       /* Here the file-position indicator must be 1.  */
! 
!       c = fgetc (stdin);
!       ASSERT (c == '!');
! 
!       c = fgetc (stdin);
!       ASSERT (c == '/');
! 
!       return 0;
! 
!       case '2':
!       /* Check fflush after a non-backup ungetc() call.  This is case 2a in
!          terms of
!          <http://lists.gnu.org/archive/html/bug-gnulib/2008-03/msg00131.html>,
!          according to the Austin Group's resolution on 2009-01-08.  */
!       /* Check that fflush after a non-backup ungetc() call discards the
!          ungetc buffer.  This is mandated by POSIX
!          <http://www.opengroup.org/susv3/functions/ungetc.html>:
!            "The value of the file-position indicator for the stream after
!             reading or discarding all pushed-back bytes shall be the same
!             as it was before the bytes were pushed back."
!          <http://www.opengroup.org/austin/aardvark/latest/xshbug3.txt>
!            "[After fflush(),] the file offset of the underlying open file
!             description shall be set to the file position of the stream, and
!             any characters pushed back onto the stream by ungetc() or
!             ungetwc() that have not subsequently been read from the stream
!             shall be discarded."  */
! 
!       c = fgetc (stdin);
!       ASSERT (c == '#');
! 
!       c = fgetc (stdin);
!       ASSERT (c == '!');
! 
!       /* Here the file-position indicator must be 2.  */
  
!       c = ungetc ('@', stdin);
!       ASSERT (c == '@');
  
!       fflush (stdin);
  
!       /* Here the file-position indicator must be 1.  */
  
!       c = fgetc (stdin);
!       ASSERT (c == '!');
  
!       c = fgetc (stdin);
!       ASSERT (c == '/');
  
!       return 0;
!       }
  
!   return 1;
  }
*** tests/test-fflush2.sh.orig  2009-01-15 12:16:35.000000000 +0100
--- tests/test-fflush2.sh       2009-01-15 11:24:32.000000000 +0100
***************
*** 2,8 ****
  
  # Execute the test only with seekable input stream.
  # The behaviour of fflush() on a non-seekable input stream is undefined.
! ./test-fflush2${EXEEXT} < "$srcdir/test-fflush2.sh" || exit $?
  #cat "$srcdir/test-fflush2.sh" | ./test-fflush2${EXEEXT} || exit $?
  
  exit 0
--- 2,9 ----
  
  # Execute the test only with seekable input stream.
  # The behaviour of fflush() on a non-seekable input stream is undefined.
! ./test-fflush2${EXEEXT} 1 < "$srcdir/test-fflush2.sh" || exit $?
! ./test-fflush2${EXEEXT} 2 < "$srcdir/test-fflush2.sh" || exit $?
  #cat "$srcdir/test-fflush2.sh" | ./test-fflush2${EXEEXT} || exit $?
  
  exit 0