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1 | /* stdopen.c - ensure that the three standard file descriptors are in use |
2 | ||
afa743ac | 3 | Copyright 2005-2021 Free Software Foundation, Inc. |
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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 | |
a0fb51e1 | 7 | the Free Software Foundation; either version 3, or (at your option) |
f1f3b524 SP |
8 | 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 | |
cd7bdd40 | 16 | along with this program. If not, see <http://www.gnu.org/licenses/>. */ |
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17 | |
18 | /* Written by Paul Eggert and Jim Meyering. */ | |
19 | ||
20 | #ifdef HAVE_CONFIG_H | |
21 | # include <config.h> | |
22 | #endif | |
23 | ||
24 | #include "stdopen.h" | |
25 | ||
26 | #include <sys/types.h> | |
27 | #include <sys/stat.h> | |
28 | #include <fcntl.h> | |
29 | #include <unistd.h> | |
30 | #include <errno.h> | |
31 | ||
32 | /* Try to ensure that all of the standard file numbers (0, 1, 2) | |
33 | are in use. Without this, each application would have to guard | |
34 | every call to open, dup, fopen, etc. with tests to ensure they | |
35 | don't use one of the special file numbers when opening a file. | |
36 | Return false if at least one of the file descriptors is initially | |
37 | closed and an attempt to reopen it fails. Otherwise, return true. */ | |
38 | bool | |
39 | stdopen (void) | |
40 | { | |
41 | int fd; | |
42 | bool ok = true; | |
43 | ||
44 | for (fd = 0; fd <= 2; fd++) | |
45 | { | |
46 | if (fcntl (fd, F_GETFD) < 0) | |
47 | { | |
48 | if (errno != EBADF) | |
49 | ok = false; | |
50 | else | |
51 | { | |
52 | static const int contrary_mode[] | |
53 | = { O_WRONLY, O_RDONLY, O_RDONLY }; | |
54 | int mode = contrary_mode[fd]; | |
55 | int new_fd; | |
56 | /* Open /dev/null with the contrary mode so that the typical | |
57 | read (stdin) or write (stdout, stderr) operation will fail. | |
58 | With descriptor 0, we can do even better on systems that | |
59 | have /dev/full, by opening that write-only instead of | |
60 | /dev/null. The only drawback is that a write-provoked | |
61 | failure comes with a misleading errno value, ENOSPC. */ | |
62 | if (mode == O_RDONLY | |
63 | || (new_fd = open ("/dev/full", mode) != fd)) | |
64 | new_fd = open ("/dev/null", mode); | |
65 | if (new_fd != fd) | |
66 | { | |
67 | if (0 <= new_fd) | |
68 | close (new_fd); | |
69 | ok = false; | |
70 | } | |
71 | } | |
72 | } | |
73 | } | |
74 | ||
75 | return ok; | |
76 | } |