2.9BSD/usr/man/man5/dir.5

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.TH DIR 5 
.SH NAME
dir \- format of directories
.SH SYNOPSIS
.B #include <sys/dir.h>
.SH 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,
.IR filsys (5).
The structure of a directory entry as given in the
include file is:
.RS
.ta 8n +6n
.PP
.nf
#ifndef	DIRSIZ
#define	DIRSIZ	14
#endif
struct	direct
{
	ino_t	d_ino;
	char	d_name[DIRSIZ];
};
.fi
.RE
.PP
By convention, the first two entries in each directory
are for `\fB.\fP' and `\fB..\fP'.  The first is an entry for the
directory itself.  The second is for the parent
directory.
The meaning of `\fB..\fP' is modified for the root directory
of the master file system and for the root directories of removable
file systems.
In the first case, there is no parent, and in the second,
the parent will be relative to the mounted-on file.
Therefore in both cases `\fB..\fP' has the
same inode number as `\fB.\fP'.
.SH "SEE ALSO"
filsys(5)