Manual Pages  — CHMOD

The lchmod() system call is similar to chmod() but does not follow symbolic links.

The fchmodat() is equivalent to either chmod() or lchmod() depending on the flag except in the case where path specifies a relative path. In this case the file to be changed is determined relative to the directory associated with the file descriptor fd instead of the current working directory. The values for the flag are constructed by a bitwise-inclusive OR of flags from the following list, defined in <fcntl.h>:

AT_SYMLINK_NOFOLLOW
	If path names a symbolic link, then the mode of the symbolic link is changed.
AT_RESOLVE_BENEATH
	Only walk paths below the directory specified by the fd descriptor. See the description of the O_RESOLVE_BENEATH flag in the open(2) manual page.

If fchmodat() is passed the special value AT_FDCWD in the fd parameter, the current working directory is used. If also flag is zero, the behavior is identical to a call to chmod().

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    /* sticky bit */

The non-standard S_ISTXT is a synonym for S_ISVTX.

The FreeBSD VM system totally ignores the sticky bit ( S_ISVTX) for executables. On UFS-based file systems (FFS, LFS) the sticky bit may only be set upon directories.