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annotate lib/stdopen.c @ 40186:8964917f9574

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author Karl Berry <karl@freefriends.org>
date Mon, 18 Feb 2019 08:02:49 -0800
parents 30186c67361d
children
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1 /* stdopen.c - ensure that the three standard file descriptors are in use
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2
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3 Copyright (C) 2005-2006, 2019 Free Software Foundation, Inc.
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4
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5 This program is free software: you can redistribute it and/or modify
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6 it under the terms of the GNU General Public License as published by
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7 the Free Software Foundation, either version 3 of the License, or
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8 (at your option) any later version.
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9
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10 This program is distributed in the hope that it will be useful,
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11 but WITHOUT ANY WARRANTY; without even the implied warranty of
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12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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13 GNU General Public License for more details.
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14
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15 You should have received a copy of the GNU General Public License
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16 along with this program. If not, see <http://www.gnu.org/licenses/>. */
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17
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18 /* Written by Paul Eggert and Jim Meyering. */
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19
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20 #include <config.h>
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21
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22 #include "stdopen.h"
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23
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24 #include <sys/stat.h>
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25 #include <fcntl.h>
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26 #include <unistd.h>
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27 #include <errno.h>
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28
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29 /* Try to ensure that all of the standard file numbers (0, 1, 2)
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30 are in use. Without this, each application would have to guard
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31 every call to open, dup, fopen, etc. with tests to ensure they
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32 don't use one of the special file numbers when opening a file.
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33 Return zero if successful, an errno value if at least one of
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34 the file descriptors is initially closed and could not be opened. */
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35
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36 int
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37 stdopen (void)
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38 {
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39 int fd;
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40 for (fd = STDIN_FILENO; fd <= STDERR_FILENO; fd++)
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41 {
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42 if (fcntl (fd, F_GETFD) < 0)
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43 {
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44 /* Open /dev/null with the contrary mode so that the typical
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45 read (stdin) or write (stdout, stderr) operation will fail.
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46 With descriptor 0, we can do even better on systems that
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47 have /dev/full, by opening that write-only instead of
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48 /dev/null. The only drawback is that a write-provoked
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49 failure comes with a misleading errno value, ENOSPC. */
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50 int mode = fd == STDIN_FILENO ? O_WRONLY : O_RDONLY;
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51 int full_fd = fd == STDIN_FILENO ? open ("/dev/full", mode) : -1;
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52 int new_fd = full_fd < 0 ? open ("/dev/null", mode) : full_fd;
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53 if (new_fd < 0)
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54 return errno;
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55 if (STDERR_FILENO < new_fd)
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56 {
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57 /* 0, 1, and 2 are already open somehow.
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58 Our is not to reason why. */
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59 close (new_fd);
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60 return 0;
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61 }
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62 }
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63 }
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64
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65 return 0;
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66 }