CHMOD(2) |
System Calls |
CHMOD(2) |
NAME
chmod, fchmod, fchmodat - change access permission mode of file
SYNOPSIS
#include <sys/types.h>
#include <sys/stat.h>
int chmod(
const char *path,
mode_t mode);
int fchmod(
int fildes,
mode_t mode);
int fchmodat(
int fildes,
const char *path,
mode_t mode,
int flag);
DESCRIPTION
The
chmod(),
fchmod(), and
fchmodat() functions set the access permission portion of the mode of the file whose name is given by
path or referenced by the open file descriptor
fildes to the bit pattern contained in
mode. Access permission bits are interpreted as follows:
S_ISUID |
04000 |
Set user ID on execution. |
S_ISGID |
020#0 |
Set group ID on execution if # is 7, 5, 3, or 1. Enable mandatory file/record locking if # is 6, 4, 2, or 0. |
S_ISVTX |
01000 |
Sticky bit. |
S_IRWXU |
00700 |
Read, write, execute by owner. |
S_IRUSR |
00400 |
Read by owner. |
S_IWUSR |
00200 |
Write by owner. |
S_IXUSR |
00100 |
Execute (search if a directory) by owner. |
S_IRWXG |
00070 |
Read, write, execute by group. |
S_IRGRP |
00040 |
Read by group. |
S_IWGRP |
00020 |
Write by group. |
S_IXGRP |
00010 |
Execute by group. |
S_IRWXO |
00007 |
Read, write, execute (search) by others. |
S_IROTH |
00004 |
Read by others. |
S_IWOTH |
00002 |
Write by others. |
S_IXOTH |
00001 |
Execute by others. |
Modes are constructed by the bitwise OR operation of the access permission bits.
The effective user ID of the process must match the owner of the file or the process must have the appropriate privilege to change the mode of a file.
If the process is not a privileged process and the file is not a directory, mode bit 01000 (save text image on execution) is cleared.
If neither the process is privileged nor the file's group is a member of the process's supplementary group list, and the effective group ID of the process does not match the group ID of the file, mode bit 02000 (set group ID on execution) is cleared.
If a directory is writable and has
S_ISVTX (the sticky bit) set, files within that directory can be removed or renamed only if one or more of the following is true (see
unlink(2) and
rename(2)):
-
o
-
the user owns the directory
-
o
-
the file is writable by the user
-
o
-
the user is a privileged user
If a regular file is not executable and has
S_ISVTX set, the file is assumed to be a swap file. In this case, the system's page cache will not be used to hold the file's data. If the
S_ISVTX bit is set on any other file, the results are unspecified.
If a directory has the set group ID bit set, a given file created within that directory will have the same group ID as the directory. Otherwise, the newly created file's group ID will be set to the effective group ID of the creating process.
If the mode bit 02000 (set group ID on execution) is set and the mode bit 00010 (execute or search by group) is not set, mandatory file/record locking will exist on a regular file, possibly affecting future calls to
open(2),
creat(2),
read(2), and
write(2) on this file.
If
fildes references a shared memory object,
fchmod() need only affect the
S_IRUSR,
S_IRGRP,
S_IROTH,
S_IWUSR,
S_IWGRP,
S_IWOTH,
S_IXUSR,
S_IXGRP, and
S_IXOTH file permission bits.
If
fildes refers to a socket,
fchmod() does not fail but no action is taken.
If
fildes refers to a stream that is attached to an object in the file system name space with
fattach(3C), the
fchmod() call performs no action and returns successfully.
The
fchmodat() function behaves similarly to
chmod(), except when
path is a relative path, it is resolved relative to the directory specified by
filedes. If
filedes has the value
AT_FDCWD, then
path will be resolved relative to the current working directory. The argument
flag should be zero, but may include the value
AT_SYMLINK_NOFOLLOW, which indicates that if
path refers to a symbolic link, then permissions should be changed on the symbolic link itself. However, changing permissions of symbolic links is not supported on illumos, and will result in an error.
Upon successful completion,
chmod(),
fchmod(),
fchmodat() mark for update the
st_ctime field of the file.
RETURN VALUES
Upon successful completion, 0 is returned. Otherwise, −1 is returned, the file mode is unchanged, and errno is set to indicate the error.
ERRORS
The
chmod(),
fchmod(), and
fchmodat() functions will fail if:
EIO
An I/O error occurred while reading from or writing to the file system.
EPERM
The effective user ID does not match the owner of the file and the process does not have appropriate privilege.
The {
PRIV_FILE_OWNER} privilege overrides constraints on ownership when changing permissions on a file.
The {
PRIV_FILE_SETID} privilege overrides constraints on ownership when adding the setuid or setgid bits to an executable file or a directory. When adding the setuid bit to a root owned executable, additional restrictions apply. See
privileges(5).
The
chmod() and
fchmodat() functions will fail if:
EACCES
Search permission is denied on a component of the path prefix of path and for fchmodat(), filedes was not opened with O_SEARCH requested. The privilege { FILE_DAC_SEARCH} overrides file permissions restrictions in that case.
EFAULT
The path argument points to an illegal address.
ELOOP
A loop exists in symbolic links encountered during the resolution of the path argument.
ENAMETOOLONG
The length of the path argument exceeds PATH_MAX, or the length of a path component exceeds NAME_MAX while _POSIX_NO_TRUNC is in effect.
ENOENT
Either a component of the path prefix or the file referred to by path does not exist or is a null pathname.
ENOLINK
The fildes argument points to a remote machine and the link to that machine is no longer active.
ENOTDIR
A component of the prefix of path is not a directory.
EROFS
The file referred to by path resides on a read-only file system.
The
fchmod() function will fail if:
EBADF
The fildes argument is not an open file descriptor
ENOLINK
The path argument points to a remote machine and the link to that machine is no longer active.
EROFS
The file referred to by fildes resides on a read-only file system.
The
chmod() and
fchmod() functions may fail if:
EINTR
A signal was caught during execution of the function.
EINVAL
The value of the mode argument is invalid.
The
fchmodat() will fail if:
EBADF
The argument path is a relative path and filedes is not an open file descriptor or the value AT_FDCWD.
EINVAL
The argument flags has a non-zero value other than AT_SYMLINK_NOFOLLOW.
ENOTDIR
The argument path is a relative path and filedes is a valid file descriptor which does not refer to a file.
EOPNOTSUPP
The AT_SYMLINK_NOFOLLOW bit is set in the flags argument.
The
chmod() and
fchmodat() functions may fail if:
ELOOP
More than {SYMLOOP_MAX} symbolic links were encountered during the resolution of the path argument.
ENAMETOOLONG
As a result of encountering a symbolic link in resolution of the path argument, the length of the substituted pathname strings exceeds { PATH_MAX}.
The
fchmod() function may fail if:
EINVAL
The fildes argument refers to a pipe and the system disallows execution of this function on a pipe.
EXAMPLES
Example 1 Set Read Permissions for User, Group, and Others
The following example sets read permissions for the owner, group, and others.
#include <sys/stat.h>
const char *path;
...
chmod(path, S_IRUSR|S_IRGRP|S_IROTH);
Example 2 Set Read, Write, and Execute Permissions for the Owner Only
The following example sets read, write, and execute permissions for the owner, and no permissions for group and others.
#include <sys/stat.h>
const char *path;
...
chmod(path, S_IRWXU);
Example 3 Set Different Permissions for Owner, Group, and Other
The following example sets owner permissions for CHANGEFILE to read, write, and execute, group permissions to read and execute, and other permissions to read.
#include <sys/stat.h>
#define CHANGEFILE "/etc/myfile"
...
chmod(CHANGEFILE, S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH);
Example 4 Set and Checking File Permissions
The following example sets the file permission bits for a file named
/home/cnd/mod1, then calls the
stat(2) function to verify the permissions.
#include <sys/types.h>
#include <sys/stat.h>
int status;
struct stat buffer
...
chmod("home/cnd/mod1", S_IRWXU|S_IRWXG|S_IROTH|S_IWOTH);
status = stat("home/cnd/mod1", &buffer;);
USAGE
If chmod() or fchmod() is used to change the file group owner permissions on a file with non-trivial ACL entries, only the ACL mask is set to the new permissions and the group owner permission bits in the file's mode field (defined in mknod(2)) are unchanged. A non-trivial ACL entry is one whose meaning cannot be represented in the file's mode field alone. The new ACL mask permissions might change the effective permissions for additional users and groups that have ACL entries on the file.
ATTRIBUTES
See
attributes(5) for descriptions of the following attributes:
ATTRIBUTE TYPE |
ATTRIBUTE VALUE |
|
Interface Stability |
Standard |
|
MT-Level |
Async-Signal-Safe |
SEE ALSO
chmod(1),
chown(2),
creat(2),
fcntl(2),
mknod(2),
open(2),
read(2),
rename(2),
stat(2),
write(2),
fattach(3C),
mkfifo(3C),
stat.h(3HEAD),
attributes(5),
privileges(5),
standards(5)
Programming Interfaces Guide