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

chmod() and fchmod() 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_ISUID04000Set user ID on execution.
S_ISGID020#0Set group ID on execution
if # is 7, 5, 3, or 1.
Enable mandatory file/record locking
if # is 6, 4, 2, or 0.
S_ISVTX01000Save text image after execution.
S_IRWXU00700Read, write, execute by owner.
S_IRUSR00400Read by owner.
S_IWUSR00200Write by owner.
S_IXUSR00100Execute (search if a directory) by owner.
S_IRWXG00070Read, write, execute by group.
S_IRGRP00040Read by group.
S_IWGRP00020Write by group.
S_IXGRP00010Execute by group.
S_IRWXO00007Read, write, execute (search) by others.
S_IROTH00004Read by others.
S_IWOTH00002Write by others
S_IXOTH00001Execute by others.

Modes are constructed by OR’ing 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 nor a member of the supplementary group list is privileged, 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 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)):

the user owns the file
the user owns the directory
the file is writable by the user
the user is a privileged user

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, a value of 0 is returned. Otherwise, a value of −1 is returned and errno is set to indicate the error.

ERRORS

chmod() will fail and the file mode will be unchanged if one or more of the following are true:

EACCES Search permission is denied on a component of the path prefix of path.

EFAULT path 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 translating path.

EMULTIHOP Components of path require hopping to multiple remote machines and file system type does not allow it.

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 fildes 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.

fchmod() will fail and the file mode will be unchanged if:

EBADF fildes 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 path 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.

Museum