+/*
+ * 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 (e.g. 512 bytes on most machines).
+ *
+ * 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 which 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.
+ */
+#ifdef KERNEL
+#define DIRBLKSIZ DEV_BSIZE
+#else
+#define DIRBLKSIZ 512
+#endif
+
+#define MAXNAMLEN 255
+
+/*
+ * 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) \e
+ ((sizeof (struct direct) - (MAXNAMLEN+1)) + (((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];
+};