lib/fwriteerror.c

/* Detect write error on a stream.
   Copyright (C) 2003-2006, 2008-2026 Free Software Foundation, Inc.
   Written by Bruno Haible <bruno@clisp.org>, 2003.

   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 3 of the License, 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, see <https://www.gnu.org/licenses/>.  */

#include <config.h>

/* Specification.  */
#include "fwriteerror.h"

#include <errno.h>

static int
do_fwriteerror (FILE *fp, bool ignore_ebadf)
{
  /* State to allow multiple calls to fwriteerror (stdout).  */
  static bool stdout_closed = false;

  if (fp == stdout)
    {
      if (stdout_closed)
        return 0;

      /* If we are closing stdout, don't attempt to do it later again.  */
      stdout_closed = true;
    }

  /* This function returns an error indication if there was a previous failure
     or if fclose failed, with two exceptions:
       - Ignore an fclose failure if there was no previous error, no data
         remains to be flushed, and fclose failed with EBADF.  That can
         happen when a program like cp is invoked like this 'cp a b >&-'
         (i.e., with standard output closed) and doesn't generate any
         output (hence no previous error and nothing to be flushed).
       - Ignore an fclose failure due to EPIPE.  That can happen when a
         program blocks or ignores SIGPIPE, and the output pipe or socket
         has no readers now.  The EPIPE tells us that we should stop writing
         to this output.  That's what we are doing anyway here.

     Need to
     1. test the error indicator of the stream,
     2. flush the buffers both in userland and in the kernel, through fclose,
        testing for error again.  */

  /* Clear errno, so that on non-POSIX systems the caller doesn't see a
     wrong value of errno when we return -1.  */
  errno = 0;

  if (ferror (fp))
    {
      if (fflush (fp))
        goto close_preserving_errno; /* errno is set here */
      /* The stream had an error earlier, but its errno was lost.  If the
         error was not temporary, we can get the same errno by writing and
         flushing one more byte.  We can do so because at this point the
         stream's contents is garbage anyway.  */
      if (fputc ('\0', fp) == EOF)
        goto close_preserving_errno; /* errno is set here */
      if (fflush (fp))
        goto close_preserving_errno; /* errno is set here */
      /* Give up on errno.  */
      errno = 0;
      goto close_preserving_errno;
    }

  if (ignore_ebadf)
    {
      /* We need an explicit fflush to tell whether some output was already
         done on FP.  */
      if (fflush (fp))
        goto close_preserving_errno; /* errno is set here */
      if (fclose (fp) && errno != EBADF)
        goto got_errno; /* errno is set here */
    }
  else
    {
      if (fclose (fp))
        goto got_errno; /* errno is set here */
    }

  return 0;

 close_preserving_errno:
  /* There's an error.  Nevertheless call fclose(fp), for consistency
     with the other cases.  */
  {
    int saved_errno = errno;
    fclose (fp);
    errno = saved_errno;
  }
 got_errno:
  /* There's an error.  Ignore EPIPE.  */
  if (errno == EPIPE)
    return 0;
  else
    return -1;
}

int
fwriteerror (FILE *fp)
{
  return do_fwriteerror (fp, false);
}

int
fwriteerror_no_ebadf (FILE *fp)
{
  return do_fwriteerror (fp, true);
}


#if TEST

/* Name of a file on which writing fails.  On systems without /dev/full,
   you can choose a filename on a full file system.  */
#define UNWRITABLE_FILE "/dev/full"

int
main ()
{
  static int sizes[] =
    {
       511,  512,  513,
      1023, 1024, 1025,
      2047, 2048, 2049,
      4095, 4096, 4097,
      8191, 8192, 8193
    };
  static char dummy[8193];

  for (unsigned int i = 0; i < sizeof (sizes) / sizeof (sizes[0]); i++)
    {
      size_t size = sizes[i];

      for (unsigned int j = 0; j < 2; j++)
        {
          /* Run a test depending on i and j:
             Write size bytes and then calls fflush if j==1.  */
          FILE *stream = fopen (UNWRITABLE_FILE, "w");

          if (stream == NULL)
            fprintf (stderr, "Test %u:%u: could not open file\n", i, j);
          else
            {
              fwrite (dummy, 347, 1, stream);
              fwrite (dummy, size - 347, 1, stream);
              if (j)
                fflush (stream);

              if (fwriteerror (stream) == -1)
                {
                  if (errno != ENOSPC)
                    fprintf (stderr, "Test %u:%u: fwriteerror ok, errno = %d\n",
                             i, j, errno);
                }
              else
                fprintf (stderr, "Test %u:%u: fwriteerror found no error!\n",
                         i, j);
            }
        }
    }

  return 0;
}

#endif