1CHMOD(2) Linux Programmer's Manual CHMOD(2)
2
6 chmod, fchmod, fchmodat - change permissions of a file
7
9 #include <sys/stat.h>
10 int chmod(const char *pathname, mode_t mode);
12 int fchmod(int fd, mode_t mode);
13 #include <fcntl.h> /* Definition of AT_* constants */
15 #include <sys/stat.h>
16 int fchmodat(int dirfd, const char *pathname, mode_t mode, int flags);
18 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
20 fchmod():
22 Since glibc 2.24:
23 _POSIX_C_SOURCE >= 199309L
24 Glibc 2.19 to 2.23
25 _POSIX_C_SOURCE
26 Glibc 2.16 to 2.19:
27 _BSD_SOURCE || _POSIX_C_SOURCE
28 Glibc 2.12 to 2.16:
29 _BSD_SOURCE || _XOPEN_SOURCE >= 500
30 || _POSIX_C_SOURCE >= 200809L
31 Glibc 2.11 and earlier:
32 _BSD_SOURCE || _XOPEN_SOURCE >= 500
33 fchmodat():
35 Since glibc 2.10:
36 _POSIX_C_SOURCE >= 200809L
37 Before glibc 2.10:
38 _ATFILE_SOURCE
39
41 The chmod() and fchmod() system calls change a file's mode bits. (The
42 file mode consists of the file permission bits plus the set-user-ID,
43 set-group-ID, and sticky bits.) These system calls differ only in how
44 the file is specified:
45 * chmod() changes the mode of the file specified whose pathname is
47 given in pathname, which is dereferenced if it is a symbolic link.
48 * fchmod() changes the mode of the file referred to by the open file
50 descriptor fd.
51 The new file mode is specified in mode, which is a bit mask created by
53 ORing together zero or more of the following:
54 S_ISUID [22m(04000) set-user-ID (set process effective user ID on ex‐
56 ecve(2))
57 S_ISGID [22m(02000) set-group-ID (set process effective group ID on ex‐
59 ecve(2); mandatory locking, as described in fcntl(2);
60 take a new file's group from parent directory, as de‐
61 scribed in chown(2) and mkdir(2))
62 S_ISVTX [22m(01000) sticky bit (restricted deletion flag, as described in
64 unlink(2))
65 S_IRUSR [22m(00400) read by owner
67 S_IWUSR [22m(00200) write by owner
69 S_IXUSR [22m(00100) execute/search by owner ("search" applies for direc‐
71 tories, and means that entries within the directory
72 can be accessed)
73 S_IRGRP [22m(00040) read by group
75 S_IWGRP [22m(00020) write by group
77 S_IXGRP [22m(00010) execute/search by group
79 S_IROTH [22m(00004) read by others
81 S_IWOTH [22m(00002) write by others
83 S_IXOTH [22m(00001) execute/search by others
85 The effective UID of the calling process must match the owner of the
87 file, or the process must be privileged (Linux: it must have the
88 CAP_FOWNER capability).
89 If the calling process is not privileged (Linux: does not have the
91 CAP_FSETID capability), and the group of the file does not match the
92 effective group ID of the process or one of its supplementary group
93 IDs, the S_ISGID bit will be turned off, but this will not cause an er‐
94 ror to be returned.
95 As a security measure, depending on the filesystem, the set-user-ID and
97 set-group-ID execution bits may be turned off if a file is written.
98 (On Linux, this occurs if the writing process does not have the
99 CAP_FSETID capability.) On some filesystems, only the superuser can
100 set the sticky bit, which may have a special meaning. For the sticky
101 bit, and for set-user-ID and set-group-ID bits on directories, see in‐
102 ode(7).
103 On NFS filesystems, restricting the permissions will immediately influ‐
105 ence already open files, because the access control is done on the
106 server, but open files are maintained by the client. Widening the per‐
107 missions may be delayed for other clients if attribute caching is en‐
108 abled on them.
109 fchmodat()
111 The fchmodat() system call operates in exactly the same way as chmod(),
112 except for the differences described here.
113 If the pathname given in pathname is relative, then it is interpreted
115 relative to the directory referred to by the file descriptor dirfd
116 (rather than relative to the current working directory of the calling
117 process, as is done by chmod() for a relative pathname).
118 If pathname is relative and dirfd is the special value AT_FDCWD, then
120 pathname is interpreted relative to the current working directory of
121 the calling process (like chmod()).
122 If pathname is absolute, then dirfd is ignored.
124 flags can either be 0, or include the following flag:
126 AT_SYMLINK_NOFOLLOW
128 If pathname is a symbolic link, do not dereference it: instead
129 operate on the link itself. This flag is not currently imple‐
130 mented.
131 See openat(2) for an explanation of the need for fchmodat().
133
135 On success, zero is returned. On error, -1 is returned, and errno is
136 set to indicate the error.
137
139 Depending on the filesystem, errors other than those listed below can
140 be returned.
141 The more general errors for chmod() are listed below:
143 EACCES Search permission is denied on a component of the path prefix.
145 (See also path_resolution(7).)
146 EBADF (fchmod()) The file descriptor fd is not valid.
148 EBADF (fchmodat()) pathname is relative but dirfd is neither AT_FDCWD
150 nor a valid file descriptor.
151 EFAULT pathname points outside your accessible address space.
153 EINVAL (fchmodat()) Invalid flag specified in flags.
155 EIO An I/O error occurred.
157 ELOOP Too many symbolic links were encountered in resolving pathname.
159 ENAMETOOLONG
161 pathname is too long.
162 ENOENT The file does not exist.
164 ENOMEM Insufficient kernel memory was available.
166 ENOTDIR
168 A component of the path prefix is not a directory.
169 ENOTDIR
171 (fchmodat()) pathname is relative and dirfd is a file descriptor
172 referring to a file other than a directory.
173 ENOTSUP
175 (fchmodat()) flags specified AT_SYMLINK_NOFOLLOW, which is not
176 supported.
177 EPERM The effective UID does not match the owner of the file, and the
179 process is not privileged (Linux: it does not have the
180 CAP_FOWNER capability).
181 EPERM The file is marked immutable or append-only. (See
183 ioctl_iflags(2).)
184 EROFS The named file resides on a read-only filesystem.
186
188 fchmodat() was added to Linux in kernel 2.6.16; library support was
189 added to glibc in version 2.4.
190
192 chmod(), fchmod(): 4.4BSD, SVr4, POSIX.1-2001i, POSIX.1-2008.
193 fchmodat(): POSIX.1-2008.
195
197 C library/kernel differences
198 The GNU C library fchmodat() wrapper function implements the POSIX-
199 specified interface described in this page. This interface differs
200 from the underlying Linux system call, which does not have a flags ar‐
201 gument.
202 Glibc notes
204 On older kernels where fchmodat() is unavailable, the glibc wrapper
205 function falls back to the use of chmod(). When pathname is a relative
206 pathname, glibc constructs a pathname based on the symbolic link in
207 /proc/self/fd that corresponds to the dirfd argument.
208
210 chmod(1), chown(2), execve(2), open(2), stat(2), inode(7), path_resolu‐
211 tion(7), symlink(7)
212
214 This page is part of release 5.13 of the Linux man-pages project. A
215 description of the project, information about reporting bugs, and the
216 latest version of this page, can be found at
217 https://www.kernel.org/doc/man-pages/.
218Linux 2021-08-27 CHMOD(2)