usr/man/man5/dir.5

.TH DIR 5  "15 January 1983"
.SH NAME
dir \- format of directories
.SH SYNOPSIS
.B #include <sys/types.h>
.br
.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 fs (5).
The structure of a directory entry as given in the include file is:
.RS
.ta 8n +10n +10n
.PP
.nf
/*
 * This sets the "page size" for directories.
 * Requirements are DEV_BSIZE <= DIRBLKSIZ <= MINBSIZE with
 * DIRBLKSIZ a power of two.
 * Dennis Ritchie feels that directory pages should be atomic
 * operations to the disk, so we use DEV_BSIZE.
 */
#ifdef KERNEL
#define DIRBLKSIZ DEV_BSIZE
#else
#define DIRBLKSIZ 512
#endif

/*
 * This limits the directory name length. Its main constraint
 * is that it appears twice in the user structure. (u. area)
 */
#define MAXNAMLEN 255

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];
};
.fi
.RE
.PP
By convention, the first two entries in each directory
are for `\fB.\fR' and `\fB..\fR'.  The first is an entry for the
directory itself.  The second is for the parent directory.
The meaning of `\fB..\fR' is modified for the root directory
of the master file system
.RB (\*(lq / \*(rq),
where `\fB..\fR' has the same meaning as `\fB.\fR'.
.SH "SEE ALSO"
fs(5)