NAME

chmod, fchmod - change permissions of a 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 mode of the file given by path or referenced by fildes is changed.

Modes are specified by or'ing the following:

The effective UID of the calling process must match the owner of the file, or the process must be privileged (Linux: it must have the CAP_FOWNER capability).

If the calling process is not privileged (Linux: does not have the CAP_FSETID capability), 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.

As a security measure, depending on the file system, the set-user-ID and set-group-ID execution bits may be turned off if a file is written. (On Linux this occurs if the writing process does not have the CAP_FSETID capability.) On some file systems, only the superuser can set the sticky bit, which may have a special meaning. For the sticky bit, and for set-user-ID and set-group-ID bits on directories, see stat(2).

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.

RETURN VALUE

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

ERRORS

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

EACCES

Search permission is denied on a component of the path prefix. (See also path_resolution(2).)

EFAULT

path points outside your accessible address space.

EIO

An I/O error occurred.

ELOOP

Too many symbolic links were encountered in resolving path.

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.

EPERM

The effective UID does not match the owner of the file, and the process is not privileged (Linux: it does not have the CAP_FOWNER capability).

EROFS

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

The general errors for fchmod() are listed below:

EBADF

The file descriptor fildes is not valid.

EIO

See above.

EPERM

See above.

EROFS

See above.

CONFORMING TO

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.

SEE ALSO

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