dir.5

来自「<B>Digital的Unix操作系统VAX 4.2源码</B>」· 5 代码 · 共 88 行

.\" SCCSID: @(#)dir.5	8.1	9/11/90
.TH dir 5 
.SH Name
dir \- format of directories
.SH Syntax
.B #include <sys/types.h>
.br
.B #include <sys/dir.h>
.SH Description
.NXR "dir keyword" 
.NXR "directory" "file format"
A directory behaves exactly like an ordinary file, except 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.  For further
information, see 
.MS fs 5 .
The structure of a directory entry is given in the include file.
.PP
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 (for example, 512 bytes on most machines).
.PP
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.
.PP
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
action usually results in the last entry in a directory having a large
.br
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
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.
.EX
#ifdef KERNEL
#define DIRBLKSIZ DEV_BSIZE
#else
#define	DIRBLKSIZ 512
#endif

#define MAXNAMLEN 255
.EE
.PP
The DIRSIZ macro gives the minimum record length that 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.
.EX
#undef DIRSIZ
#define DIRSIZ(dp) \e
    ((sizeof (struct direct) - (MAXNAMLEN+1)) + \e
    (((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];
};
.EE
.PP
By convention, the first two entries in each directory
are for dot (.) and dot dot (..).  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 (\*(lq/\*(rq),
where dot dot has the same meaning as dot.
.SH See Also
fs(5)