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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).
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 entries 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.
#define DIRBLKSIZ DEV_BSIZE
#define MAXNAMLEN 256
struct direct {
u_long d_ino; /* inode number of entry */
u_short d_reclen; /* length of this record */
u_short d_namlen; /* length of string in d_name */
char d_name[MAXNAMLEN + 1]; /* name must be no longer than this */
};