CHMOD(2)

NAME
chmod, fchmod - change mode of file

SYNOPSIS
#include <sys/stat.h>
int
chmod(const char *path, mode_t mode);
int
fchmod(int fd, mode_t mode);

DESCRIPTION
The function chmod() sets the file permission bits of the file specified by the pathname path to mode. Fchmod() sets the permission bits of the specified file descriptor fd. Chmod() verifies that the process owner (user) either owns the file specified by path (or fd), or is the super-user. A mode is created from or'd permission bit masks defined in <sys/stat.h>:
    #define S_IRWXU 0000700    /* RWX mask for owner */
    #define S_IRUSR 0000400    /* R for owner */
    #define S_IWUSR 0000200    /* W for owner */
    #define S_IXUSR 0000100    /* X for owner */
    #define S_IRWXG 0000070    /* RWX mask for group */
    #define S_IRGRP 0000040    /* R for group */
    #define S_IWGRP 0000020    /* W for group */
    #define S_IXGRP 0000010    /* X for group */
    #define S_IRWXO 0000007    /* RWX mask for other */
    #define S_IROTH 0000004    /* R for other */
    #define S_IWOTH 0000002    /* W for other */
    #define S_IXOTH 0000001    /* X for other */
    #define S_ISUID 0004000    /* set user id on execution */
    #define S_ISGID 0002000    /* set group id on execution */
    #define S_ISVTX 0001000    /* save swapped text even after use */

The ISVTX (the sticky bit) indicates to the system which executable files are shareable (the default) and the system maintains the program text of the files in the swap area. The sticky bit may only be set by the super user on shareable executable files.

If mode ISVTX (the `sticky bit') is set on a directory, an unprivileged user may not delete or rename files of other users in that directory. The sticky bit may be set by any user on a directory which the user owns or has appropriate permissions. For more details of the properties of the sticky bit, see sticky(8).

Writing or changing the owner of a file turns off the set-user-id and set-group-id bits unless the user is the super-user. This makes the system somewhat more secure by protecting set-user-id (set-group-id) files from remaining set-user-id (set-group-id) if they are modified, at the expense of a degree of compatibility.

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:

[ENOTDIR]
    A component of the path prefix is not a directory.

[EINVAL]
    The pathname contains a character with the high-order bit set.

[ENAMETOOLONG]
    A component of a pathname exceeded 255 characters, or an entire path name exceeded 1023 characters.

[ENOENT]
    The named file does not exist.

[EACCES]
    Search permission is denied for a component of the path prefix.

[ELOOP]
    Too many symbolic links were encountered in translating the pathname.

[EPERM]
    The effective user ID does not match the owner of the file and the effective user ID is not the super-user.

[EROFS]
    The named file resides on a read-only file system.

[EFAULT]
    Path points outside the process's allocated address space.

[EIO]
    An I/O error occurred while reading from or writing to the file system.

Fchmod() will fail if:

[EBADF]
    The descriptor is not valid.

[EINVAL]
    Fd refers to a socket, not to a file.

[EROFS]
    The file resides on a read-only file system.

[EIO]
    An I/O error occurred while reading from or writing to the file system.

SEE ALSO
chmod(1), open(2), chown(2), stat(2), sticky(8)

STANDARDS
Chmod() is expected to conform to IEEE Std 1003.1-1988 (“POSIX”).

HISTORY
The fchmod() function call appeared in 4.2BSD.