Re: seekable stdin test failure on OS X

[Gary V. Vaughan]

Hi Eric,

Here is the relevant code from modules/m4.c in CVS HEAD:


   521       behavior when we detect a UNIX environment.  */
   522  #if UNIX
   523    if (lseek (STDIN_FILENO, 0, SEEK_CUR) >= 0
   524        && fflush (stdin) == 0)
   525      {
   526        fseek (stdin, 0, SEEK_CUR);
   527      }
   528  #endif /* UNIX */
   529  }

On 2 Apr 2007, at 14:24, Eric Blake wrote:

508 /* POSIX requires that if m4 doesn't consume all input, but stdin is 509 opened on a seekable file, that the file pointer be left at the 510 next character on exit (but places no restrictions on the file 511 pointer location on a non-seekable file). It also requires that 512 fflush() followed by fseek() on an input file set the underlying 513 file pointer. However, fflush() on a non-seekable file can lose 514 buffered data, which we might otherwise want to process after 515 syscmd. Hence, we must check whether stdin is seekable. We must 516 also be tolerant of operating with stdin closed, so we don't 517 report any failures in this attempt. The stdio-safer module and 518 friends are essential, so that if stdin was closed, this lseek is 519 not on some other file that we have since opened. Mingw has bugs 520 when using fseek on text files, so we only strive for POSIX

m4 already called fseek, but currently
it just discarded the results; can you step through m4_sysval_flush to

of the input rather than at the end?

determine if the fseek was accurately returning the offset in the middle

Okay, at line 524 fflush returns -1 and sets errno to EBADF.

Looking at the man page for Mac OS fflush, it is only supposed to work
for output streams where it flushes the write buffer.  The equivalent
function for input streams is fpurge.  And indeed, if I change 524 to
call "fpurge (stdin)", the test passes.

If it does work, how many other
gnulib-using programs would benefit from a fix?

At worst we need an autoconf test to see whether fflush works on stdin,

to fflush otherwise?

Cheers,
        Gary
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but maybe all that is needed is to call fpurge when present, and fallback

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