DIR(5)

NAME
dir - format of directories

SYNOPSIS
#include <sys/types.h>
#include <sys/dir.h>

DESCRIPTION
A directory behaves exactly like an ordinary file, save that no user may write into a directory. The fact that a file is a directory is indicated by a bit in the flag word of its i-node entry; see fs(5). The structure of a directory entry as given in the include file is:

    /*
     * A directory consists of some number of blocks of DIRBLKSIZ
     * bytes, where DIRBLKSIZ is chosen such that it can be transferred
     * to disk in a single atomic operation (e.g. 512 bytes on most machines).
     *
     * Each DIRBLKSIZ byte block contains some number of directory entry
     * structures, which are of variable length.  Each directory entry has
     * a struct direct at the front of it, containing its inode number,
     * the length of the entry, and the length of the name contained in
     * the entry.  These are followed by the name padded to a 4 byte boundary
     * with null bytes.  All names are guaranteed null terminated.
     * The maximum length of a name in a directory is MAXNAMLEN.
     *
     * The macro DIRSIZ(dp) gives the amount of space required to represent
     * a directory entry.  Free space in a directory is represented by
     * entries which have dp->d_reclen > DIRSIZ(dp).  All DIRBLKSIZ bytes
     * in a directory block are claimed by the directory entries.  This
     * usually results in the last entry in a directory having a large
     * dp->d_reclen.  When entries are deleted from a directory, the
     * space is returned to the previous entry in the same directory
     * block by increasing its dp->d_reclen.  If the first entry of
     * a directory block is free, then its dp->d_ino is set to 0.
     * Entries other than the first in a directory do not normally have
     * dp->d_ino set to 0.
     */
    #ifdef KERNEL
    #define DIRBLKSIZ DEV_BSIZE
    #else
    #define	DIRBLKSIZ 512
    #endif
    #define MAXNAMLEN 255
    /*
     * The DIRSIZ macro gives the minimum record length which will hold
     * the directory entry.  This requires the amount of space in struct direct
     * without the d_name field, plus enough space for the name with a terminating
     * null byte (dp->d_namlen+1), rounded up to a 4 byte boundary.
     */
    #undef DIRSIZ
    #define DIRSIZ(dp) \
        ((sizeof (struct direct) - (MAXNAMLEN+1)) + (((dp)->d_namlen+1 + 3) &~ 3))
    struct	direct {
    	u_long	d_ino;
    	short	d_reclen;
    	short	d_namlen;
    	char	d_name[MAXNAMLEN + 1];
    	/* typically shorter */
    };
    struct _dirdesc {
    	int	dd_fd;
    	long	dd_loc;
    	long	dd_size;
    	char	dd_buf[DIRBLKSIZ];
    };

By convention, the first two entries in each directory are for `.' and `..'. The first is an entry for the directory itself. The second is for the parent directory. The meaning of `..' is modified for the root directory of the master file system (“/”), where `..' has the same meaning as `.'.

SEE ALSO
fs(5)