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 interpreted as follows:

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)):

• the user owns the file
• the user owns the directory
• the file is writable by the user
• 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.

Upon successful completion, 0 is returned. Otherwise, -1 is returned, the file mode is unchanged, and errno is set to indicate the error.

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 translating 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 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.

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.

See attributes(5) for descriptions of the following attributes:

chmod(1), chown(2), creat(2), fcntl(2), mknod(2), open(2), read(2), rename(2), stat(2), write(2), mkfifo(3C), attributes(5), stat(3HEAD)

Programming Interfaces Guide

If you use chmod() to change the file group owner permissions on a file with ACL entries, both the file group owner permissions and the ACL mask are changed to the new permissions. 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.