1CHMOD(P) POSIX Programmer's Manual CHMOD(P)
2
6 chmod - change mode of a file
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9 #include <sys/stat.h>
10 int chmod(const char *path, mode_t mode);
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15 The chmod() function shall change S_ISUID, S_ISGID, S_ISVTX, and the
16 file permission bits of the file named by the pathname pointed to by
17 the path argument to the corresponding bits in the mode argument. The
18 application shall ensure that the effective user ID of the process
19 matches the owner of the file or the process has appropriate privileges
20 in order to do this.
21 S_ISUID, S_ISGID, S_ISVTX, and the file permission bits are
23 described in <sys/stat.h>.
24 If the calling process does not have appropriate privileges, and if the
26 group ID of the file does not match the effective group ID or one of
27 the supplementary group IDs and if the file is a regular file, bit
28 S_ISGID (set-group-ID on execution) in the file's mode shall be cleared
29 upon successful return from chmod().
30 Additional implementation-defined restrictions may cause the S_ISUID
32 and S_ISGID bits in mode to be ignored.
33 The effect on file descriptors for files open at the time of a call to
35 chmod() is implementation-defined.
36 Upon successful completion, chmod() shall mark for update the st_ctime
38 field of the file.
39
41 Upon successful completion, 0 shall be returned; otherwise, -1 shall be
42 returned and errno set to indicate the error. If -1 is returned, no
43 change to the file mode occurs.
44
46 The chmod() function shall fail if:
47 EACCES Search permission is denied on a component of the path prefix.
49 ELOOP A loop exists in symbolic links encountered during resolution of
51 the path argument.
52 ENAMETOOLONG
54 The length of the path argument exceeds {PATH_MAX} or a pathname
55 component is longer than {NAME_MAX}.
56 ENOTDIR
58 A component of the path prefix is not a directory.
59 ENOENT A component of path does not name an existing file or path is an
61 empty string.
62 EPERM The effective user ID does not match the owner of the file and
64 the process does not have appropriate privileges.
65 EROFS The named file resides on a read-only file system.
67 The chmod() function may fail if:
70 EINTR A signal was caught during execution of the function.
72 EINVAL The value of the mode argument is invalid.
74 ELOOP More than {SYMLOOP_MAX} symbolic links were encountered during
76 resolution of the path argument.
77 ENAMETOOLONG
79 As a result of encountering a symbolic link in resolution of the
80 path argument, the length of the substituted pathname strings
81 exceeded {PATH_MAX}.
82 The following sections are informative.
85
87 Setting Read Permissions for User, Group, and Others
88 The following example sets read permissions for the owner, group, and
89 others.
90 #include <sys/stat.h>
93 const char *path;
96 ...
97 chmod(path, S_IRUSR|S_IRGRP|S_IROTH);
98 Setting Read, Write, and Execute Permissions for the Owner Only
100 The following example sets read, write, and execute permissions for the
101 owner, and no permissions for group and others.
102 #include <sys/stat.h>
105 const char *path;
108 ...
109 chmod(path, S_IRWXU);
110 Setting Different Permissions for Owner, Group, and Other
112 The following example sets owner permissions for CHANGEFILE to read,
113 write, and execute, group permissions to read and execute, and other
114 permissions to read.
115 #include <sys/stat.h>
118 #define CHANGEFILE "/etc/myfile"
121 ...
122 chmod(CHANGEFILE, S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH);
123 Setting and Checking File Permissions
125 The following example sets the file permission bits for a file named
126 /home/cnd/mod1, then calls the stat() function to verify the permis‐
127 sions.
128 #include <sys/types.h>
131 #include <sys/stat.h>
132 int status;
135 struct stat buffer
136 ...
137 chmod("home/cnd/mod1", S_IRWXU|S_IRWXG|S_IROTH|S_IWOTH);
138 status = stat("home/cnd/mod1", &buffer;);
139
141 In order to ensure that the S_ISUID and S_ISGID bits are set, an appli‐
142 cation requiring this should use stat() after a successful chmod() to
143 verify this.
144 Any file descriptors currently open by any process on the file could
146 possibly become invalid if the mode of the file is changed to a value
147 which would deny access to that process. One situation where this could
148 occur is on a stateless file system. This behavior will not occur in a
149 conforming environment.
150
152 This volume of IEEE Std 1003.1-2001 specifies that the S_ISGID bit is
153 cleared by chmod() on a regular file under certain conditions. This is
154 specified on the assumption that regular files may be executed, and the
155 system should prevent users from making executable setgid() files per‐
156 form with privileges that the caller does not have. On implementations
157 that support execution of other file types, the S_ISGID bit should be
158 cleared for those file types under the same circumstances.
159 Implementations that use the S_ISUID bit to indicate some other func‐
161 tion (for example, mandatory record locking) on non-executable files
162 need not clear this bit on writing. They should clear the bit for exe‐
163 cutable files and any other cases where the bit grants special powers
164 to processes that change the file contents. Similar comments apply to
165 the S_ISGID bit.
166
168 None.
169
171 chown() , mkdir() , mkfifo() , open() , stat() , statvfs() , the Base
172 Definitions volume of IEEE Std 1003.1-2001, <sys/stat.h>, <sys/types.h>
173
175 Portions of this text are reprinted and reproduced in electronic form
176 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
177 -- Portable Operating System Interface (POSIX), The Open Group Base
178 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
179 Electrical and Electronics Engineers, Inc and The Open Group. In the
180 event of any discrepancy between this version and the original IEEE and
181 The Open Group Standard, the original IEEE and The Open Group Standard
182 is the referee document. The original Standard can be obtained online
183 at http://www.opengroup.org/unix/online.html .
184IEEE/The Open Group 2003 CHMOD(P)