lib/fwriteerror.c

   1 /* Detect write error on a stream.
   2    Copyright (C) 2003-2006, 2008-2021 Free Software Foundation, Inc.
   3    Written by Bruno Haible <bruno@clisp.org>, 2003.
   4
   5    This program is free software: you can redistribute it and/or modify
   6    it under the terms of the GNU General Public License as published by
   7    the Free Software Foundation; either version 3 of the License, or
   8    (at your option) any later version.
   9
  10    This program is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13    GNU General Public License for more details.
  14
  15    You should have received a copy of the GNU General Public License
  16    along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
  17
  18 #include <config.h>
  19
  20 /* Specification.  */
  21 #include "fwriteerror.h"
  22
  23 #include <errno.h>
  24 #include <stdbool.h>
  25
  26 static int
  27 do_fwriteerror (FILE *fp, bool ignore_ebadf)
  28 {
  29   /* State to allow multiple calls to fwriteerror (stdout).  */
  30   static bool stdout_closed = false;
  31
  32   if (fp == stdout)
  33     {
  34       if (stdout_closed)
  35         return 0;
  36
  37       /* If we are closing stdout, don't attempt to do it later again.  */
  38       stdout_closed = true;
  39     }
  40
  41   /* This function returns an error indication if there was a previous failure
  42      or if fclose failed, with two exceptions:
  43        - Ignore an fclose failure if there was no previous error, no data
  44          remains to be flushed, and fclose failed with EBADF.  That can
  45          happen when a program like cp is invoked like this 'cp a b >&-'
  46          (i.e., with standard output closed) and doesn't generate any
  47          output (hence no previous error and nothing to be flushed).
  48        - Ignore an fclose failure due to EPIPE.  That can happen when a
  49          program blocks or ignores SIGPIPE, and the output pipe or socket
  50          has no readers now.  The EPIPE tells us that we should stop writing
  51          to this output.  That's what we are doing anyway here.
  52
  53      Need to
  54      1. test the error indicator of the stream,
  55      2. flush the buffers both in userland and in the kernel, through fclose,
  56         testing for error again.  */
  57
  58   /* Clear errno, so that on non-POSIX systems the caller doesn't see a
  59      wrong value of errno when we return -1.  */
  60   errno = 0;
  61
  62   if (ferror (fp))
  63     {
  64       if (fflush (fp))
  65         goto close_preserving_errno; /* errno is set here */
  66       /* The stream had an error earlier, but its errno was lost.  If the
  67          error was not temporary, we can get the same errno by writing and
  68          flushing one more byte.  We can do so because at this point the
  69          stream's contents is garbage anyway.  */
  70       if (fputc ('\0', fp) == EOF)
  71         goto close_preserving_errno; /* errno is set here */
  72       if (fflush (fp))
  73         goto close_preserving_errno; /* errno is set here */
  74       /* Give up on errno.  */
  75       errno = 0;
  76       goto close_preserving_errno;
  77     }
  78
  79   if (ignore_ebadf)
  80     {
  81       /* We need an explicit fflush to tell whether some output was already
  82          done on FP.  */
  83       if (fflush (fp))
  84         goto close_preserving_errno; /* errno is set here */
  85       if (fclose (fp) && errno != EBADF)
  86         goto got_errno; /* errno is set here */
  87     }
  88   else
  89     {
  90       if (fclose (fp))
  91         goto got_errno; /* errno is set here */
  92     }
  93
  94   return 0;
  95
  96  close_preserving_errno:
  97   /* There's an error.  Nevertheless call fclose(fp), for consistency
  98      with the other cases.  */
  99   {
 100     int saved_errno = errno;
 101     fclose (fp);
 102     errno = saved_errno;
 103   }
 104  got_errno:
 105   /* There's an error.  Ignore EPIPE.  */
 106   if (errno == EPIPE)
 107     return 0;
 108   else
 109     return -1;
 110 }
 111
 112 int
 113 fwriteerror (FILE *fp)
 114 {
 115   return do_fwriteerror (fp, false);
 116 }
 117
 118 int
 119 fwriteerror_no_ebadf (FILE *fp)
 120 {
 121   return do_fwriteerror (fp, true);
 122 }
 123
 124
 125 #if TEST
 126
 127 /* Name of a file on which writing fails.  On systems without /dev/full,
 128    you can choose a filename on a full file system.  */
 129 #define UNWRITABLE_FILE "/dev/full"
 130
 131 int
 132 main ()
 133 {
 134   static int sizes[] =
 135     {
 136        511,  512,  513,
 137       1023, 1024, 1025,
 138       2047, 2048, 2049,
 139       4095, 4096, 4097,
 140       8191, 8192, 8193
 141     };
 142   static char dummy[8193];
 143   unsigned int i, j;
 144
 145   for (i = 0; i < sizeof (sizes) / sizeof (sizes[0]); i++)
 146     {
 147       size_t size = sizes[i];
 148
 149       for (j = 0; j < 2; j++)
 150         {
 151           /* Run a test depending on i and j:
 152              Write size bytes and then calls fflush if j==1.  */
 153           FILE *stream = fopen (UNWRITABLE_FILE, "w");
 154
 155           if (stream == NULL)
 156             {
 157               fprintf (stderr, "Test %u:%u: could not open file\n", i, j);
 158               continue;
 159             }
 160
 161           fwrite (dummy, 347, 1, stream);
 162           fwrite (dummy, size - 347, 1, stream);
 163           if (j)
 164             fflush (stream);
 165
 166           if (fwriteerror (stream) == -1)
 167             {
 168               if (errno != ENOSPC)
 169                 fprintf (stderr, "Test %u:%u: fwriteerror ok, errno = %d\n",
 170                          i, j, errno);
 171             }
 172           else
 173             fprintf (stderr, "Test %u:%u: fwriteerror found no error!\n",
 174                      i, j);
 175         }
 176     }
 177
 178   return 0;
 179 }
 180
 181 #endif