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struct dirent is padded to an 8 byte boundary according to sys/dirent.h.

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.\"     @(#)dir.5	8.3 (Berkeley) 4/19/94
.\"
.Dd June 22, 2007
.Dt DIRENT 5
.Os
.Sh NAME
.Nm dirent
.Nd directory format
.Sh SYNOPSIS
.In sys/types.h
.In sys/dirent.h
.Sh DESCRIPTION
Directories provide a convenient hierarchical method of grouping
files while obscuring the underlying details of the storage medium.
A directory file is differentiated from a plain file
by a flag in its
.Xr inode 5
entry.
It consists of records (directory entries) each of which contains
information about a file and a pointer to the file itself.
Directory entries may contain other directories
as well as plain files; such nested directories are referred to as
subdirectories.
A hierarchy of directories and files is formed in this manner
and is called a file system (or referred to as a file system tree).
.\" An entry in this tree,
.\" nested or not nested,
.\" is a pathname.
.Pp
Each directory file contains two special directory entries; one is a pointer
to the directory itself
called dot
.Ql \&.
and the other a pointer to its parent directory called dot-dot
.Ql \&.. .
Dot and dot-dot
are valid pathnames, however,
the system root directory
.Ql / ,
has no parent and dot-dot points to itself like dot.
.Pp
File system nodes are ordinary directory files on which has
been grafted a file system object, such as a physical disk or a
partitioned area of such a disk.
(See
.Xr mount 8 . )
.Pp
The directory entry format is defined in the file
.Aq Pa sys/dirent.h :
.Bd -literal
/*
 * A directory entry has a struct dirent 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 an
 * 8 byte boundary with null bytes.  All names are guaranteed null
 * terminated.  The maximum length of a name in a directory is MAXNAMLEN.
 */

struct dirent {
	ino_t     d_fileno;	/* file number of entry */
	uint16_t d_reclen;	/* length of this record */
	uint8_t  d_type;	/* file type, see below */
	uint8_t  d_namlen;	/* length of string in d_name */
#define MAXNAMLEN       511
	char d_name[MAXNAMLEN + 1]; /* name must be no longer than this */
};

/*
 * File types
 */
#define DT_UNKNOWN	0
#define DT_FIFO		1
#define DT_CHR		2
#define DT_DIR		4
#define DT_BLK		6
#define DT_REG		8
#define DT_LNK		10
#define DT_SOCK		12
#define DT_WHT		14
.Ed
.Sh SEE ALSO
.Xr getdents 2 ,
.Xr fs 5 ,
.Xr inode 5
.Sh HISTORY
A
dir structure appeared in
.At v7 .
The
.Nm
structure appeared in
.Nx 1.3 .