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 - determine accessibility of a file
13
15 #include <unistd.h>
16 int access(const char *path, int amode);
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21 The access() function shall check the file named by the pathname
22 pointed to by the path argument for accessibility according to the bit
23 pattern contained in amode, using the real user ID in place of the
24 effective user ID and the real group ID in place of the effective group
25 ID.
26 The value of amode is either the bitwise-inclusive OR of the access
28 permissions to be checked (R_OK, W_OK, X_OK) or the existence test
29 (F_OK).
30 If any access permissions are checked, each shall be checked individu‐
32 ally, as described in the Base Definitions volume of
33 IEEE Std 1003.1-2001, Chapter 3, Definitions. If the process has appro‐
34 priate privileges, an implementation may indicate success for X_OK even
35 if none of the execute file permission bits are set.
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38 If the requested access is permitted, access() succeeds and shall
39 return 0; otherwise, -1 shall be returned and errno shall be set to
40 indicate the error.
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43 The access() function shall fail if:
44 EACCES Permission bits of the file mode do not permit the requested
46 access, or search permission is denied on a component of the
47 path prefix.
48 ELOOP A loop exists in symbolic links encountered during resolution of
50 the path argument.
51 ENAMETOOLONG
53 The length of the path argument exceeds {PATH_MAX} or a pathname
54 component is longer than {NAME_MAX}.
55 ENOENT A component of path does not name an existing file or path is an
57 empty string.
58 ENOTDIR
60 A component of the path prefix is not a directory.
61 EROFS Write access is requested for a file on a read-only file system.
63 The access() function may fail if:
66 EINVAL The value of the amode argument is invalid.
68 ELOOP More than {SYMLOOP_MAX} symbolic links were encountered during
70 resolution of the path argument.
71 ENAMETOOLONG
73 As a result of encountering a symbolic link in resolution of the
74 path argument, the length of the substituted pathname string
75 exceeded {PATH_MAX}.
76 ETXTBSY
78 Write access is requested for a pure procedure (shared text)
79 file that is being executed.
80 The following sections are informative.
83
85 Testing for the Existence of a File
86 The following example tests whether a file named myfile exists in the
87 /tmp directory.
88 #include <unistd.h>
91 ...
92 int result;
93 const char *filename = "/tmp/myfile";
94 result = access (filename, F_OK);
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99 Additional values of amode other than the set defined in the descrip‐
100 tion may be valid; for example, if a system has extended access con‐
101 trols.
102
104 In early proposals, some inadequacies in the access() function led to
105 the creation of an eaccess() function because:
106 1. Historical implementations of access() do not test file access cor‐
108 rectly when the process' real user ID is superuser. In particular,
109 they always return zero when testing execute permissions without
110 regard to whether the file is executable.
111 2. The superuser has complete access to all files on a system. As a
113 consequence, programs started by the superuser and switched to the
114 effective user ID with lesser privileges cannot use access() to
115 test their file access permissions.
116 However, the historical model of eaccess() does not resolve problem
118 (1), so this volume of IEEE Std 1003.1-2001 now allows access() to
119 behave in the desired way because several implementations have cor‐
120 rected the problem. It was also argued that problem (2) is more easily
121 solved by using open(), chdir(), or one of the exec functions as appro‐
122 priate and responding to the error, rather than creating a new function
123 that would not be as reliable. Therefore, eaccess() is not included in
124 this volume of IEEE Std 1003.1-2001.
125 The sentence concerning appropriate privileges and execute permission
127 bits reflects the two possibilities implemented by historical implemen‐
128 tations when checking superuser access for X_OK.
129 New implementations are discouraged from returning X_OK unless at least
131 one execution permission bit is set.
132
134 None.
135
137 chmod(), stat(), the Base Definitions volume of IEEE Std 1003.1-2001,
138 <unistd.h>
139
141 Portions of this text are reprinted and reproduced in electronic form
142 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
143 -- Portable Operating System Interface (POSIX), The Open Group Base
144 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
145 Electrical and Electronics Engineers, Inc and The Open Group. In the
146 event of any discrepancy between this version and the original IEEE and
147 The Open Group Standard, the original IEEE and The Open Group Standard
148 is the referee document. The original Standard can be obtained online
149 at http://www.opengroup.org/unix/online.html .
150IEEE/The Open Group 2003 ACCESS(3P)