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 and directories must be read using the getdirentries(2) system call or the directory(3V) library routines.  The fact that a file is a directory is indicated by a bit in the flag word of its inode entry; see fs(5). 

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 (512 bytes on most machines):

#ifdef KERNEL
#define DIRBLKSIZ DEV_BSIZE
#else
#define DIRBLKSIZ 512
#endif
#define MAXNAMLEN 255

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 that 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. 

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))
structdirect {
u_longd_ino;
shortd_reclen;
shortd_namlen;
chard_name[MAXNAMLEN + 1];
/∗ typically shorter ∗/
};

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 (“/”), for which ‘..’ has the same meaning as ‘.’. 

SEE ALSO

getdirentries(2), directory(3V), fs(5)

Solbourne Computer, Inc.  —  19 October 1987