1ACCESS(P) POSIX Programmer's Manual ACCESS(P)
2
6 access - determine accessibility of a file
7
9 #include <unistd.h>
10 int access(const char *path, int amode);
12
15 The access() function shall check the file named by the pathname
16 pointed to by the path argument for accessibility according to the bit
17 pattern contained in amode, using the real user ID in place of the
18 effective user ID and the real group ID in place of the effective group
19 ID.
20 The value of amode is either the bitwise-inclusive OR of the access
22 permissions to be checked (R_OK, W_OK, X_OK) or the existence test
23 (F_OK).
24 If any access permissions are checked, each shall be checked individu‐
26 ally, as described in the Base Definitions volume of
27 IEEE Std 1003.1-2001, Chapter 3, Definitions. If the process has appro‐
28 priate privileges, an implementation may indicate success for X_OK even
29 if none of the execute file permission bits are set.
30
32 If the requested access is permitted, access() succeeds and shall
33 return 0; otherwise, -1 shall be returned and errno shall be set to
34 indicate the error.
35
37 The access() function shall fail if:
38 EACCES Permission bits of the file mode do not permit the requested
40 access, or search permission is denied on a component of the
41 path prefix.
42 ELOOP A loop exists in symbolic links encountered during resolution of
44 the path argument.
45 ENAMETOOLONG
47 The length of the path argument exceeds {PATH_MAX} or a pathname
48 component is longer than {NAME_MAX}.
49 ENOENT A component of path does not name an existing file or path is an
51 empty string.
52 ENOTDIR
54 A component of the path prefix is not a directory.
55 EROFS Write access is requested for a file on a read-only file system.
57 The access() function may fail if:
60 EINVAL The value of the amode argument is invalid.
62 ELOOP More than {SYMLOOP_MAX} symbolic links were encountered during
64 resolution of the path argument.
65 ENAMETOOLONG
67 As a result of encountering a symbolic link in resolution of the
68 path argument, the length of the substituted pathname string
69 exceeded {PATH_MAX}.
70 ETXTBSY
72 Write access is requested for a pure procedure (shared text)
73 file that is being executed.
74 The following sections are informative.
77
79 Testing for the Existence of a File
80 The following example tests whether a file named myfile exists in the
81 /tmp directory.
82 #include <unistd.h>
85 ...
86 int result;
87 const char *filename = "/tmp/myfile";
88 result = access (filename, F_OK);
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93 Additional values of amode other than the set defined in the descrip‐
94 tion may be valid; for example, if a system has extended access con‐
95 trols.
96
98 In early proposals, some inadequacies in the access() function led to
99 the creation of an eaccess() function because:
100 1. Historical implementations of access() do not test file access cor‐
102 rectly when the process' real user ID is superuser. In particular,
103 they always return zero when testing execute permissions without
104 regard to whether the file is executable.
105 2. The superuser has complete access to all files on a system. As a
107 consequence, programs started by the superuser and switched to the
108 effective user ID with lesser privileges cannot use access() to
109 test their file access permissions.
110 However, the historical model of eaccess() does not resolve problem
112 (1), so this volume of IEEE Std 1003.1-2001 now allows access() to
113 behave in the desired way because several implementations have cor‐
114 rected the problem. It was also argued that problem (2) is more easily
115 solved by using open(), chdir(), or one of the exec functions as appro‐
116 priate and responding to the error, rather than creating a new function
117 that would not be as reliable. Therefore, eaccess() is not included in
118 this volume of IEEE Std 1003.1-2001.
119 The sentence concerning appropriate privileges and execute permission
121 bits reflects the two possibilities implemented by historical implemen‐
122 tations when checking superuser access for X_OK.
123 New implementations are discouraged from returning X_OK unless at least
125 one execution permission bit is set.
126
128 None.
129
131 chmod() , stat() , the Base Definitions volume of IEEE Std 1003.1-2001,
132 <unistd.h>
133
135 Portions of this text are reprinted and reproduced in electronic form
136 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
137 -- Portable Operating System Interface (POSIX), The Open Group Base
138 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
139 Electrical and Electronics Engineers, Inc and The Open Group. In the
140 event of any discrepancy between this version and the original IEEE and
141 The Open Group Standard, the original IEEE and The Open Group Standard
142 is the referee document. The original Standard can be obtained online
143 at http://www.opengroup.org/unix/online.html .
144IEEE/The Open Group 2003 ACCESS(P)