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
13 access, faccessat — determine accessibility of a file relative to
14 directory file descriptor
15
17 #include <unistd.h>
18 int access(const char *path, int amode);
20 int faccessat(int fd, const char *path, int amode, int flag);
21
23 The access() function shall check the file named by the pathname
24 pointed to by the path argument for accessibility according to the bit
25 pattern contained in amode, using the real user ID in place of the
26 effective user ID and the real group ID in place of the effective group
27 ID.
28 The value of amode is either the bitwise-inclusive OR of the access
30 permissions to be checked (R_OK, W_OK, X_OK) or the existence test
31 (F_OK).
32 If any access permissions are checked, each shall be checked individu‐
34 ally, as described in the Base Definitions volume of POSIX.1‐2008, Sec‐
35 tion 4.4, File Access Permissions, except that where that description
36 refers to execute permission for a process with appropriate privileges,
37 an implementation may indicate success for X_OK even if execute permis‐
38 sion is not granted to any user.
39 The faccessat() function shall be equivalent to the access() function,
41 except in the case where path specifies a relative path. In this case
42 the file whose accessibility is to be determined shall be located rela‐
43 tive to the directory associated with the file descriptor fd instead of
44 the current working directory. If the file descriptor was opened with‐
45 out O_SEARCH, the function shall check whether directory searches are
46 permitted using the current permissions of the directory underlying the
47 file descriptor. If the file descriptor was opened with O_SEARCH, the
48 function shall not perform the check.
49 If faccessat() is passed the special value AT_FDCWD in the fd parame‐
51 ter, the current working directory shall be used and the behavior shall
52 be identical to a call to access().
53 Values for flag are constructed by a bitwise-inclusive OR of flags from
55 the following list, defined in <fcntl.h>:
56 AT_EACCESS The checks for accessibility are performed using the effec‐
58 tive user and group IDs instead of the real user and group
59 ID as required in a call to access().
60
62 Upon successful completion, these functions shall return 0. Otherwise,
63 these functions shall return −1 and set errno to indicate the error.
64
66 These functions shall fail if:
67 EACCES Permission bits of the file mode do not permit the requested
69 access, or search permission is denied on a component of the
70 path prefix.
71 ELOOP A loop exists in symbolic links encountered during resolution of
73 the path argument.
74 ENAMETOOLONG
76 The length of a component of a pathname is longer than
77 {NAME_MAX}.
78 ENOENT A component of path does not name an existing file or path is an
80 empty string.
81 ENOTDIR
83 A component of the path prefix names an existing file that is
84 neither a directory nor a symbolic link to a directory, or the
85 path argument contains at least one non-<slash> character and
86 ends with one or more trailing <slash> characters and the last
87 pathname component names an existing file that is neither a
88 directory nor a symbolic link to a directory.
89 EROFS Write access is requested for a file on a read-only file system.
91 The faccessat() function shall fail if:
93 EACCES fd was not opened with O_SEARCH and the permissions of the
95 directory underlying fd do not permit directory searches.
96 EBADF The path argument does not specify an absolute path and the fd
98 argument is neither AT_FDCWD nor a valid file descriptor open
99 for reading or searching.
100 ENOTDIR
102 The path argument is not an absolute path and fd is a file
103 descriptor associated with a non-directory file.
104 These functions may fail if:
106 EINVAL The value of the amode argument is invalid.
108 ELOOP More than {SYMLOOP_MAX} symbolic links were encountered during
110 resolution of the path argument.
111 ENAMETOOLONG
113 The length of a pathname exceeds {PATH_MAX}, or pathname resolu‐
114 tion of a symbolic link produced an intermediate result with a
115 length that exceeds {PATH_MAX}.
116 ETXTBSY
118 Write access is requested for a pure procedure (shared text)
119 file that is being executed.
120 The faccessat() function may fail if:
122 EINVAL The value of the flag argument is not valid.
124 The following sections are informative.
126
128 Testing for the Existence of a File
129 The following example tests whether a file named myfile exists in the
130 /tmp directory.
131 #include <unistd.h>
133 ...
134 int result;
135 const char *pathname = "/tmp/myfile";
136 result = access (pathname, F_OK);
138
140 Additional values of amode other than the set defined in the descrip‐
141 tion may be valid; for example, if a system has extended access con‐
142 trols.
143 The use of the AT_EACCESS value for flag enables functionality not
145 available in access().
146
148 In early proposals, some inadequacies in the access() function led to
149 the creation of an eaccess() function because:
150 1. Historical implementations of access() do not test file access cor‐
152 rectly when the process' real user ID is superuser. In particular,
153 they always return zero when testing execute permissions without
154 regard to whether the file is executable.
155 2. The superuser has complete access to all files on a system. As a
157 consequence, programs started by the superuser and switched to the
158 effective user ID with lesser privileges cannot use access() to
159 test their file access permissions.
160 However, the historical model of eaccess() does not resolve problem
162 (1), so this volume of POSIX.1‐2008 now allows access() to behave in
163 the desired way because several implementations have corrected the
164 problem. It was also argued that problem (2) is more easily solved by
165 using open(), chdir(), or one of the exec functions as appropriate and
166 responding to the error, rather than creating a new function that would
167 not be as reliable. Therefore, eaccess() is not included in this volume
168 of POSIX.1‐2008.
169 The sentence concerning appropriate privileges and execute permission
171 bits reflects the two possibilities implemented by historical implemen‐
172 tations when checking superuser access for X_OK.
173 New implementations are discouraged from returning X_OK unless at least
175 one execution permission bit is set.
176 The purpose of the faccessat() function is to enable the checking of
178 the accessibility of files in directories other than the current work‐
179 ing directory without exposure to race conditions. Any part of the path
180 of a file could be changed in parallel to a call to access(), resulting
181 in unspecified behavior. By opening a file descriptor for the target
182 directory and using the faccessat() function it can be guaranteed that
183 the file tested for accessibility is located relative to the desired
184 directory.
185
187 None.
188
190 chmod(), fstatat()
191 The Base Definitions volume of POSIX.1‐2008, Section 4.4, File Access
193 Permissions, <fcntl.h>, <unistd.h>
194
196 Portions of this text are reprinted and reproduced in electronic form
197 from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
198 -- Portable Operating System Interface (POSIX), The Open Group Base
199 Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
200 cal and Electronics Engineers, Inc and The Open Group. (This is
201 POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
202 event of any discrepancy between this version and the original IEEE and
203 The Open Group Standard, the original IEEE and The Open Group Standard
204 is the referee document. The original Standard can be obtained online
205 at http://www.unix.org/online.html .
206 Any typographical or formatting errors that appear in this page are
208 most likely to have been introduced during the conversion of the source
209 files to man page format. To report such errors, see https://www.ker‐
210 nel.org/doc/man-pages/reporting_bugs.html .
211IEEE/The Open Group 2013 ACCESS(3P)