CHMOD

chmod, fchmod ? change permissions of a file

#include <sys/types.h>
#include <sys/stat.h>

int chmod(const char *path, mode_t mode);
int fchmod(int fildes, mode_t mode);

The mode of the file given by path or referenced by fildes is changed.

Modes are specified by or’ing the following:

S_ISUID

04000 set user ID on execution

S_ISGID

02000 set group ID on execution

S_ISVTX

01000 sticky bit

S_IRUSR (S_IREAD)

00400 read by owner

S_IWUSR (S_IWRITE)

00200 write by owner

S_IXUSR (S_IEXEC)

00100 execute/search by owner

S_IRGRP

00040 read by group

S_IWGRP

00020 write by group

S_IXGRP

00010 execute/search by group

S_IROTH

00004 read by others

S_IWOTH

00002 write by others

S_IXOTH

00001 execute/search by others

The effective UID of the process must be zero or must match the owner of the file.

If the effective UID of the process is not zero and the group of the file does not match the effective group ID of the process or one of its supplementary group IDs, the S_ISGID bit will be turned off, but this will not cause an error to be returned.

Depending on the file system, set user ID and set group ID execution bits may be turned off if a file is written. On some file systems, only the super-user can set the sticky bit, which may have a special meaning (e.g., for directories, a file can only be deleted by the owner or the super-user).

On NFS file systems, restricting the permissions will immediately influence already open files, because the access control is done on the server, but open files are maintained by the client. Widening the permissions may be delayed for other clients if attribute caching is enabled on them.

On success, zero is returned. On error, ?1 is returned, and errno is set appropriately.

Depending on the file system, other errors can be returned. The more general errors for chmod are listed below:

EPERM

The effective UID does not match the owner of the file, and is not zero.

EROFS

The named file resides on a read-only file system.

EFAULT

path points outside your accessible address space.

ENAMETOOLONG

path is too long.

ENOENT

The file does not exist.

ENOMEM

Insufficient kernel memory was available.

ENOTDIR

A component of the path prefix is not a directory.

EACCES

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

ELOOP

Too many symbolic links were encountered in resolving path.

EIO

An I/O error occurred.

The general errors for fchmod are listed below:

EBADF

The file descriptor fildes is not valid.

EROFS

See above.

EPERM

See above.

EIO

See above.

The chmod call conforms to SVr4, SVID, POSIX, X/OPEN, 4.4BSD. SVr4 documents EINTR, ENOLINK and EMULTIHOP returns, but no ENOMEM. POSIX.1 does not document EFAULT, ENOMEM, ELOOP or EIO error conditions, or the macros S_IREAD, S_IWRITE and S_IEXEC.

The fchmod call conforms to 4.4BSD and SVr4. SVr4 documents additional EINTR and ENOLINK error conditions. POSIX requires the fchmod function if at least one of _POSIX_MAPPED_FILES and _POSIX_SHARED_MEMORY_OBJECTS is defined, and documents additional ENOSYS and EINVAL error conditions, but does not document EIO.

POSIX and X/OPEN do not document the sticky bit.

open(2), chown(2), execve(2), stat(2)