K&R chapter 8, readdir function - c

Question

I'm stuck at this function (found in fsize() example in K&R chapter 8):
#include <sys/dir.h>
/* local directory structure */
/* readdir: read directory entries in sequence */
Dirent *readdir(DIR *dp)
{
struct direct dirbuf; /* local directory structure */
static Dirent d; /* return: portable structure */
while (read(dp->fd, (char *) &dirbuf, sizeof(dirbuf)) == sizeof(dirbuf)) {
if (dirbuf.d_ino == 0) /* slot not in use */
continue;
d.ino = dirbuf.d_ino;
strncpy(d.name, dirbuf.d_name, DIRSIZ);
d.name[DIRSIZ] = '\0'; /* ensure termination */
return &d;
}
return NULL;
}
In this function Dirent and DIR are custom structs written by K&R (not the one found in dirent.h):
typedef struct { /* portable directory entry */
long ino; /* inode number */
char name[NAME_MAX+1]; /* name + '\0' terminator */
} Dirent;
typedef struct {
int fd;
Dirent d;
} DIR;
When I use the code in the book, it runs fine but there were two problems (questions):
The file listing process does not happen recursively. It only applies once with the current directory.
I can't understand the line with read() function as above.
1) If dp->fd is a directory's, read() returns with errno 21 (directory error),
2) How could a read() like that fill in the memory structure dirbuf, doesn't it suppose to read only character/byte of some sorts?
Thanks.

Answer 1

Think for a moment about the costs of a recursive structure.
You would need a list of sub directories, for each dirent. That increases your memory requirements drastically, as well as complexities your memory allocation (can't use stack-allocated structs anymore, you must use malloc/free) code.
For this reason, I say that #1 Is invalid.
Not entirely sure if this is homework, but I cannot reproduce #2, so for now I'll leave it alone.

Answer 2

Calling the function once returns the "next" directory entry. It is intended to be called repeatedly - once for each directory entry.
The read syscall (declared in unistd.h) cannot be given a directory file descriptor. This is most likely a different "read" function. dirbuf is declared in the function, so it isn't read only.