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1 | .\" Hey Emacs! This file is -*- nroff -*- source. |
2 | .\" | |
3 | .\" This manpage is Copyright (C) 1992 Drew Eckhardt; | |
4 | .\" 1993 Michael Haardt, Ian Jackson. | |
5 | .\" | |
6 | .\" Permission is granted to make and distribute verbatim copies of this | |
7 | .\" manual provided the copyright notice and this permission notice are | |
8 | .\" preserved on all copies. | |
9 | .\" | |
10 | .\" Permission is granted to copy and distribute modified versions of this | |
11 | .\" manual under the conditions for verbatim copying, provided that the | |
12 | .\" entire resulting derived work is distributed under the terms of a | |
13 | .\" permission notice identical to this one. | |
14 | .\" | |
15 | .\" Since the Linux kernel and libraries are constantly changing, this | |
16 | .\" manual page may be incorrect or out-of-date. The author(s) assume no | |
17 | .\" responsibility for errors or omissions, or for damages resulting from | |
18 | .\" the use of the information contained herein. The author(s) may not | |
19 | .\" have taken the same level of care in the production of this manual, | |
20 | .\" which is licensed free of charge, as they might when working | |
21 | .\" professionally. | |
22 | .\" | |
23 | .\" Formatted or processed versions of this manual, if unaccompanied by | |
24 | .\" the source, must acknowledge the copyright and authors of this work. | |
25 | .\" | |
26 | .\" Modified 1993-07-21 Rik Faith (faith@cs.unc.edu) | |
27 | .\" Modified 1994-08-21 by Michael Chastain (mec@shell.portal.com): | |
28 | .\" Removed note about old kernel (pre-1.1.44) using wrong id on path. | |
29 | .\" Modified 1996-03-18 by Martin Schulze (joey@infodrom.north.de): | |
30 | .\" Stated more clearly how it behaves with symbolic links. | |
31 | .\" Added correction due to Nick Duffek (nsd@bbc.com), aeb, 960426 | |
32 | .\" Modified 1996-09-07 by Michael Haardt: | |
33 | .\" Restrictions for NFS | |
34 | .\" Modified 1997-09-09 by Joseph S. Myers <jsm28@cam.ac.uk> | |
35 | .\" Modified 1998-01-13 by Michael Haardt: | |
36 | .\" Using access is often insecure | |
37 | .\" Modified 2001-10-16 by aeb | |
38 | .\" Modified 2002-04-23 by Roger Luethi <rl@hellgate.ch> | |
39 | .\" Modified 2004-06-23 by Michael Kerrisk | |
40 | .\" | |
41 | .TH ACCESS 2 2004-06-23 "Linux" "Linux Programmer's Manual" | |
42 | .SH NAME | |
43 | access \- check user's permissions for a file | |
44 | .SH SYNOPSIS | |
45 | .nf | |
46 | .B #include <unistd.h> | |
47 | .sp | |
48 | .BI "int access(const char *" pathname ", int " mode ); | |
49 | .fi | |
50 | .SH DESCRIPTION | |
e511ffb6 | 51 | .BR access () |
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52 | checks whether the process would be allowed to read, |
53 | write or test for existence of the file (or other file system | |
54 | object) whose name is | |
55 | .IR pathname . | |
56 | If | |
57 | .I pathname | |
58 | is a symbolic link permissions of the file referred to by this | |
59 | symbolic link are tested. | |
60 | ||
61 | .I mode | |
62 | is a mask consisting of one or more of | |
63 | .BR R_OK ", " W_OK ", " X_OK " and " F_OK . | |
64 | ||
65 | .BR R_OK ", " W_OK " and " X_OK | |
66 | request checking whether the file exists and has read, write and | |
67 | execute permissions, respectively. | |
68 | .B F_OK | |
69 | just requests checking for the existence of the file. | |
70 | ||
71 | The tests depend on the permissions of the directories | |
72 | occurring in the path to the file, as given in | |
73 | .IR pathname , | |
74 | and on the permissions of directories and files referred to by symbolic | |
75 | links encountered on the way. | |
76 | ||
77 | The check is done with the process's | |
78 | .I real | |
79 | UID and GID, rather than with the effective IDs as is done when | |
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80 | actually attempting an operation. |
81 | This is to allow set-user-ID programs to | |
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82 | easily determine the invoking user's authority. |
83 | ||
84 | Only access bits are checked, not the file type or contents. Therefore, if | |
85 | a directory is found to be "writable," it probably means that files can be | |
86 | created in the directory, and not that the directory can be written as a | |
87 | file. Similarly, a DOS file may be found to be "executable," but the | |
88 | .BR execve (2) | |
89 | call will still fail. | |
90 | ||
91 | If the process has appropriate privileges, an implementation may | |
92 | indicate success for | |
93 | .B X_OK | |
94 | even if none of the execute file permission bits are set. | |
95 | .SH "RETURN VALUE" | |
96 | On success (all requested permissions granted), zero is returned. | |
97 | On error (at least one bit in | |
98 | .I mode | |
99 | asked for a permission that is denied, or some other error occurred), | |
100 | \-1 is returned, and | |
101 | .I errno | |
102 | is set appropriately. | |
103 | .SH ERRORS | |
e511ffb6 | 104 | .BR access () |
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105 | shall fail if: |
106 | .TP | |
107 | .B EACCES | |
108 | The requested access would be denied to the file or search permission | |
109 | is denied for one of the directories in the path prefix of | |
110 | .IR pathname . | |
111 | (See also | |
112 | .BR path_resolution (2).) | |
113 | .TP | |
114 | .B ELOOP | |
115 | Too many symbolic links were encountered in resolving | |
116 | .IR pathname . | |
117 | .TP | |
118 | .B ENAMETOOLONG | |
119 | .I pathname | |
120 | is too long. | |
121 | .TP | |
122 | .B ENOENT | |
123 | A directory component in | |
124 | .I pathname | |
125 | would have been accessible but does not exist or was a dangling | |
126 | symbolic link. | |
127 | .TP | |
128 | .B ENOTDIR | |
129 | A component used as a directory in | |
130 | .I pathname | |
131 | is not, in fact, a directory. | |
132 | .TP | |
133 | .B EROFS | |
134 | Write permission was requested for a file on a read-only filesystem. | |
135 | .PP | |
e511ffb6 | 136 | .BR access () |
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137 | may fail if: |
138 | .TP | |
139 | .B EFAULT | |
140 | .I pathname | |
141 | points outside your accessible address space. | |
142 | .TP | |
143 | .B EINVAL | |
144 | .I mode | |
145 | was incorrectly specified. | |
146 | .TP | |
147 | .B EIO | |
148 | An I/O error occurred. | |
149 | .TP | |
150 | .B ENOMEM | |
151 | Insufficient kernel memory was available. | |
152 | .TP | |
153 | .B ETXTBSY | |
154 | Write access was requested to an executable which is being | |
155 | executed. | |
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156 | .SH "LINUX NOTES" |
157 | In kernels before 2.6.20, | |
158 | .BR access () | |
159 | ignored the effect of the | |
160 | .B MS_NOEXEC | |
161 | flag if it was used to | |
162 | .BR mount (2) | |
163 | the underlying file system. | |
164 | Since kernel 2.6.20, | |
165 | .BR access () | |
166 | honours this flag. | |
fea681da | 167 | .SH RESTRICTIONS |
e511ffb6 | 168 | .BR access () |
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169 | returns an error if any of the access types in the requested call |
170 | fails, even if other types might be successful. | |
171 | .PP | |
e511ffb6 | 172 | .BR access () |
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173 | may not work correctly on NFS file systems with UID mapping enabled, |
174 | because UID mapping is done on the server and hidden from the client, | |
175 | which checks permissions. | |
176 | .PP | |
177 | Using | |
e511ffb6 | 178 | .BR access () |
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179 | to check if a user is authorized to e.g. open a file before actually |
180 | doing so using | |
181 | .BR open (2) | |
182 | creates a security hole, because the user might exploit the short time | |
183 | interval between checking and opening the file to manipulate it. | |
184 | .SH "CONFORMING TO" | |
75b48e9d | 185 | SVr4, 4.3BSD, POSIX.1-2001. |
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186 | .SH "SEE ALSO" |
187 | .BR chmod (2), | |
188 | .BR chown (2), | |
22e3b8b1 | 189 | .BR faccessat (2), |
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190 | .BR open (2), |
191 | .BR path_resolution (2), | |
192 | .BR setgid (2), | |
193 | .BR setuid (2), | |
194 | .BR stat (2) |