1ACCESS(3P) POSIX Programmer's Manual ACCESS(3P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
12 access, faccessat — determine accessibility of a file descriptor
13
15 #include <unistd.h>
16 int access(const char *path, int amode);
18 #include <fcntl.h>
20 int faccessat(int fd, const char *path, int amode, int flag);
22
24 The access() function shall check the file named by the pathname
25 pointed to by the path argument for accessibility according to the bit
26 pattern contained in amode. The checks for accessibility (including
27 directory permissions checked during pathname resolution) shall be per‐
28 formed using the real user ID in place of the effective user ID and the
29 real group ID in place of the effective group ID.
30 The value of amode is either the bitwise-inclusive OR of the access
32 permissions to be checked (R_OK, W_OK, X_OK) or the existence test
33 (F_OK).
34 If any access permissions are checked, each shall be checked individu‐
36 ally, as described in the Base Definitions volume of POSIX.1‐2017, Sec‐
37 tion 4.5, File Access Permissions, except that where that description
38 refers to execute permission for a process with appropriate privileges,
39 an implementation may indicate success for X_OK even if execute permis‐
40 sion is not granted to any user.
41 The faccessat() function, when called with a flag value of zero, shall
43 be equivalent to the access() function, except in the case where path
44 specifies a relative path. In this case the file whose accessibility is
45 to be determined shall be located relative to the directory associated
46 with the file descriptor fd instead of the current working directory.
47 If the access mode of the open file description associated with the
48 file descriptor is not O_SEARCH, the function shall check whether
49 directory searches are permitted using the current permissions of the
50 directory underlying the file descriptor. If the access mode is
51 O_SEARCH, the function shall not perform the check.
52 If faccessat() is passed the special value AT_FDCWD in the fd parame‐
54 ter, the current working directory shall be used and, if flag is zero,
55 the behavior shall be identical to a call to access().
56 Values for flag are constructed by a bitwise-inclusive OR of flags from
58 the following list, defined in <fcntl.h>:
59 AT_EACCESS The checks for accessibility (including directory permis‐
61 sions checked during pathname resolution) shall be per‐
62 formed using the effective user ID and group ID instead of
63 the real user ID and group ID as required in a call to
64 access().
65
67 Upon successful completion, these functions shall return 0. Otherwise,
68 these functions shall return -1 and set errno to indicate the error.
69
71 These functions shall fail if:
72 EACCES Permission bits of the file mode do not permit the requested
74 access, or search permission is denied on a component of the
75 path prefix.
76 ELOOP A loop exists in symbolic links encountered during resolution of
78 the path argument.
79 ENAMETOOLONG
81 The length of a component of a pathname is longer than
82 {NAME_MAX}.
83 ENOENT A component of path does not name an existing file or path is an
85 empty string.
86 ENOTDIR
88 A component of the path prefix names an existing file that is
89 neither a directory nor a symbolic link to a directory, or the
90 path argument contains at least one non-<slash> character and
91 ends with one or more trailing <slash> characters and the last
92 pathname component names an existing file that is neither a
93 directory nor a symbolic link to a directory.
94 EROFS Write access is requested for a file on a read-only file system.
96 The faccessat() function shall fail if:
98 EACCES The access mode of the open file description associated with fd
100 is not O_SEARCH and the permissions of the directory underlying
101 fd do not permit directory searches.
102 EBADF The path argument does not specify an absolute path and the fd
104 argument is neither AT_FDCWD nor a valid file descriptor open
105 for reading or searching.
106 ENOTDIR
108 The path argument is not an absolute path and fd is a file
109 descriptor associated with a non-directory file.
110 These functions may fail if:
112 EINVAL The value of the amode argument is invalid.
114 ELOOP More than {SYMLOOP_MAX} symbolic links were encountered during
116 resolution of the path argument.
117 ENAMETOOLONG
119 The length of a pathname exceeds {PATH_MAX}, or pathname resolu‐
120 tion of a symbolic link produced an intermediate result with a
121 length that exceeds {PATH_MAX}.
122 ETXTBSY
124 Write access is requested for a pure procedure (shared text)
125 file that is being executed.
126 The faccessat() function may fail if:
128 EINVAL The value of the flag argument is not valid.
130 The following sections are informative.
132
134 Testing for the Existence of a File
135 The following example tests whether a file named myfile exists in the
136 /tmp directory.
137 #include <unistd.h>
140 ...
141 int result;
142 const char *pathname = "/tmp/myfile";
143 result = access (pathname, F_OK);
145
147 Use of these functions is discouraged since by the time the returned
148 information is acted upon, it is out-of-date. (That is, acting upon the
149 information always leads to a time-of-check-to-time-of-use race condi‐
150 tion.) An application should instead attempt the action itself and han‐
151 dle the [EACCES] error that occurs if the file is not accessible (with
152 a change of effective user and group IDs beforehand, and perhaps a
153 change back afterwards, in the case where access() or faccessat() with‐
154 out AT_EACCES would have been used.)
155 Historically, one of the uses of access() was in set-user-ID root pro‐
157 grams to check whether the user running the program had access to a
158 file. This relied on ``super-user'' privileges which were granted based
159 on the effective user ID being zero, so that when access() used the
160 real user ID to check accessibility those privileges were not taken
161 into account. On newer systems where privileges can be assigned which
162 have no association with user or group IDs, if a program with such
163 privileges calls access(), the change of IDs has no effect on the priv‐
164 ileges and therefore they are taken into account in the accessibility
165 checks. Thus, access() (and faccessat() with flag zero) cannot be used
166 for this historical purpose in such programs. Likewise, if a system
167 provides any additional or alternate file access control mechanisms
168 that are not user ID-based, they will still be taken into account.
169 If a relative pathname is used, no account is taken of whether the cur‐
171 rent directory (or the directory associated with the file descriptor
172 fd) is accessible via any absolute pathname. Applications using
173 access(), or faccessat() without AT_EACCES, may consequently act as if
174 the file would be accessible to a user with the real user ID and group
175 ID of the process when such a user would not in practice be able to
176 access the file because access would be denied at some point above the
177 current directory (or the directory associated with the file descriptor
178 fd) in the file hierarchy.
179 If access() or faccessat() is used with W_OK to check for write access
181 to a directory which has the S_ISVTX bit set, a return value indicating
182 the directory is writable can be misleading since some operations on
183 files in the directory would not be permitted based on the ownership of
184 those files (see the Base Definitions volume of POSIX.1‐2017, Section
185 4.3, Directory Protection).
186 Additional values of amode other than the set defined in the descrip‐
188 tion may be valid; for example, if a system has extended access con‐
189 trols.
190 The use of the AT_EACCESS value for flag enables functionality not
192 available in access().
193
195 In early proposals, some inadequacies in the access() function led to
196 the creation of an eaccess() function because:
197 1. Historical implementations of access() do not test file access cor‐
199 rectly when the process' real user ID is superuser. In particular,
200 they always return zero when testing execute permissions without
201 regard to whether the file is executable.
202 2. The superuser has complete access to all files on a system. As a
204 consequence, programs started by the superuser and switched to the
205 effective user ID with lesser privileges cannot use access() to
206 test their file access permissions.
207 However, the historical model of eaccess() does not resolve problem
209 (1), so this volume of POSIX.1‐2017 now allows access() to behave in
210 the desired way because several implementations have corrected the
211 problem. It was also argued that problem (2) is more easily solved by
212 using open(), chdir(), or one of the exec functions as appropriate and
213 responding to the error, rather than creating a new function that would
214 not be as reliable. Therefore, eaccess() is not included in this volume
215 of POSIX.1‐2017.
216 The sentence concerning appropriate privileges and execute permission
218 bits reflects the two possibilities implemented by historical implemen‐
219 tations when checking superuser access for X_OK.
220 New implementations are discouraged from returning X_OK unless at least
222 one execution permission bit is set.
223 The purpose of the faccessat() function is to enable the checking of
225 the accessibility of files in directories other than the current work‐
226 ing directory without exposure to race conditions. Any part of the path
227 of a file could be changed in parallel to a call to access(), resulting
228 in unspecified behavior. By opening a file descriptor for the target
229 directory and using the faccessat() function it can be guaranteed that
230 the file tested for accessibility is located relative to the desired
231 directory.
232
234 These functions may be formally deprecated (for example, by shading
235 them OB) in a future version of this standard.
236
238 chmod(), fstatat()
239 The Base Definitions volume of POSIX.1‐2017, Section 4.5, File Access
241 Permissions, <fcntl.h>, <unistd.h>
242
244 Portions of this text are reprinted and reproduced in electronic form
245 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
246 table Operating System Interface (POSIX), The Open Group Base Specifi‐
247 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
248 Electrical and Electronics Engineers, Inc and The Open Group. In the
249 event of any discrepancy between this version and the original IEEE and
250 The Open Group Standard, the original IEEE and The Open Group Standard
251 is the referee document. The original Standard can be obtained online
252 at http://www.opengroup.org/unix/online.html .
253 Any typographical or formatting errors that appear in this page are
255 most likely to have been introduced during the conversion of the source
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258IEEE/The Open Group 2017 ACCESS(3P)