lib/fwriteerror.c

   1 /* Detect write error on a stream.
   2    Copyright (C) 2003-2006, 2008-2024 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
  25 static int
  26 do_fwriteerror (FILE *fp, bool ignore_ebadf)
  27 {
  28   /* State to allow multiple calls to fwriteerror (stdout).  */
  29   static bool stdout_closed = false;
  30
  31   if (fp == stdout)
  32     {
  33       if (stdout_closed)
  34         return 0;
  35
  36       /* If we are closing stdout, don't attempt to do it later again.  */
  37       stdout_closed = true;
  38     }
  39
  40   /* This function returns an error indication if there was a previous failure
  41      or if fclose failed, with two exceptions:
  42        - Ignore an fclose failure if there was no previous error, no data
  43          remains to be flushed, and fclose failed with EBADF.  That can
  44          happen when a program like cp is invoked like this 'cp a b >&-'
  45          (i.e., with standard output closed) and doesn't generate any
  46          output (hence no previous error and nothing to be flushed).
  47        - Ignore an fclose failure due to EPIPE.  That can happen when a
  48          program blocks or ignores SIGPIPE, and the output pipe or socket
  49          has no readers now.  The EPIPE tells us that we should stop writing
  50          to this output.  That's what we are doing anyway here.
  51
  52      Need to
  53      1. test the error indicator of the stream,
  54      2. flush the buffers both in userland and in the kernel, through fclose,
  55         testing for error again.  */
  56
  57   /* Clear errno, so that on non-POSIX systems the caller doesn't see a
  58      wrong value of errno when we return -1.  */
  59   errno = 0;
  60
  61   if (ferror (fp))
  62     {
  63       if (fflush (fp))
  64         goto close_preserving_errno; /* errno is set here */
  65       /* The stream had an error earlier, but its errno was lost.  If the
  66          error was not temporary, we can get the same errno by writing and
  67          flushing one more byte.  We can do so because at this point the
  68          stream's contents is garbage anyway.  */
  69       if (fputc ('\0', fp) == EOF)
  70         goto close_preserving_errno; /* errno is set here */
  71       if (fflush (fp))
  72         goto close_preserving_errno; /* errno is set here */
  73       /* Give up on errno.  */
  74       errno = 0;
  75       goto close_preserving_errno;
  76     }
  77
  78   if (ignore_ebadf)
  79     {
  80       /* We need an explicit fflush to tell whether some output was already
  81          done on FP.  */
  82       if (fflush (fp))
  83         goto close_preserving_errno; /* errno is set here */
  84       if (fclose (fp) && errno != EBADF)
  85         goto got_errno; /* errno is set here */
  86     }
  87   else
  88     {
  89       if (fclose (fp))
  90         goto got_errno; /* errno is set here */
  91     }
  92
  93   return 0;
  94
  95  close_preserving_errno:
  96   /* There's an error.  Nevertheless call fclose(fp), for consistency
  97      with the other cases.  */
  98   {
  99     int saved_errno = errno;
 100     fclose (fp);
 101     errno = saved_errno;
 102   }
 103  got_errno:
 104   /* There's an error.  Ignore EPIPE.  */
 105   if (errno == EPIPE)
 106     return 0;
 107   else
 108     return -1;
 109 }
 110
 111 int
 112 fwriteerror (FILE *fp)
 113 {
 114   return do_fwriteerror (fp, false);
 115 }
 116
 117 int
 118 fwriteerror_no_ebadf (FILE *fp)
 119 {
 120   return do_fwriteerror (fp, true);
 121 }
 122
 123
 124 #if TEST
 125
 126 /* Name of a file on which writing fails.  On systems without /dev/full,
 127    you can choose a filename on a full file system.  */
 128 #define UNWRITABLE_FILE "/dev/full"
 129
 130 int
 131 main ()
 132 {
 133   static int sizes[] =
 134     {
 135        511,  512,  513,
 136       1023, 1024, 1025,
 137       2047, 2048, 2049,
 138       4095, 4096, 4097,
 139       8191, 8192, 8193
 140     };
 141   static char dummy[8193];
 142   unsigned int i, j;
 143
 144   for (i = 0; i < sizeof (sizes) / sizeof (sizes[0]); i++)
 145     {
 146       size_t size = sizes[i];
 147
 148       for (j = 0; j < 2; j++)
 149         {
 150           /* Run a test depending on i and j:
 151              Write size bytes and then calls fflush if j==1.  */
 152           FILE *stream = fopen (UNWRITABLE_FILE, "w");
 153
 154           if (stream == NULL)
 155             {
 156               fprintf (stderr, "Test %u:%u: could not open file\n", i, j);
 157               continue;
 158             }
 159
 160           fwrite (dummy, 347, 1, stream);
 161           fwrite (dummy, size - 347, 1, stream);
 162           if (j)
 163             fflush (stream);
 164
 165           if (fwriteerror (stream) == -1)
 166             {
 167               if (errno != ENOSPC)
 168                 fprintf (stderr, "Test %u:%u: fwriteerror ok, errno = %d\n",
 169                          i, j, errno);
 170             }
 171           else
 172             fprintf (stderr, "Test %u:%u: fwriteerror found no error!\n",
 173                      i, j);
 174         }
 175     }
 176
 177   return 0;
 178 }
 179
 180 #endif