chmod(2)
NAME
chmod, fchmod - 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);
DESCRIPTION
The chmod() and fchmod() 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 inter-
preted 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 Save text image after execution.
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 oth-
ers.
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 execu-
tion) 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 execu-
tion) 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 directory has the set group ID bit set, a given file
created within that directory will have the same group ID
as the directory, if that group ID is part of the group ID
set of the process that created the file. 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.
This may affect future calls to open(2), creat(2), read(2),
and write(2) on this file.
Upon successful completion, chmod() and fchmod() 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() function will fail if:
EACCES
Search permission is denied on a component of the path
prefix of path.
EFAULT
The path argument points to an illegal address.
EINTR A signal was caught during execution of the function.
EIO An I/O error occurred while reading from or writing to
the file system.
ELOOP Too many symbolic links were encountered in translat-
ing path.
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 path-
name.
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.
EPERM The effective user ID does not match the owner of the
file and is not super-user.
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
EIO An I/O error occurred while reading from or writing to
the file system.
EINTR A signal was caught during execution of the fchmod()
function.
ENOLINK
The path argument points to a remote machine and the
link to that machine is no longer active.
EPERM The effective user ID does not match the owner of the
file and the process does not have appropriate
privilege.
EROFS The file referred to by fildes resides on a read-only
file system.
ATTRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
____________________________________________________________
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
|_____________________________|_____________________________|
| MT-Level | chmod() is 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), mkfifo(3C), attri-
butes(5), stat(3HEAD)
Programming Interfaces Guide
NOTES
If you use chmod() to change the file group owner permis-
sions on a file with ACL entries, both the file group owner
permissions and the ACL mask are changed to the new permis-
sions. Be aware that the new ACL mask permissions may
change the effective permissions for additional users and
groups who have ACL entries on the file.
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