Tribblix: manual page: chmod.2

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 file

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(7).

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 and path refers to a symbolic link.

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