cannot read `struct direct` as shown in K&R2 - c

Question

An example of implementing own readdir as shown in K&R2 here:
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <string.h>
#define NAME_MAX 14 /*longest filenames component; system-dependent */
#ifndef DIRSIZ
#define DIRSIZ 14
#endif
typedef struct {
long ino; /*inode number */
char name[NAME_MAX+1]; /*name + '\0' terminator */
} my_dirent;
typedef struct {
int fd; /* file descriptor for directory */
my_dirent d; /*the directory entry */
} MY_DIR;
/*
* opendir: open a directory for readdir calls
*/
MY_DIR *my_opendir(char *dirname)
{
int fd;
struct stat stbuf;
MY_DIR *dp;
if((fd = open(dirname, O_RDONLY, 0)) == -1
|| fstat(fd, &stbuf) == -1
|| (stbuf.st_mode & S_IFMT) != S_IFDIR
|| (dp = malloc(sizeof(MY_DIR))) == NULL)
return NULL;
dp->fd = fd;
return dp;
}
/*
* closedir: close directory opened by opendir
*/
void my_closedir(MY_DIR *dp)
{
if(dp) {
close(dp->fd);
free(dp);
}
}
#include <sys/dir.h>
/*
* readdir: read directory entries in sequence
*/
my_dirent *my_readdir(MY_DIR *dp)
{
struct direct dirbuf; /* local directory structure */
static my_dirent d; /* portable structure */
// HERE BELOW: the body of while loop never executes (I have no idea why) so NULL is returned and causes segfault when dereferencing in printf
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';
return &d;
}
return NULL;
}
int main()
{
MY_DIR *dp = my_opendir(".");
my_dirent *dent = my_readdir(dp);
printf("directory info:\nname: %s; fd: %d; ino: %ld\n", dent->name, dp->fd, dent->ino);
}
I made debugging so I know why. As in comments, the while header
while(read(dp->fd, (char*) &dirbuf, sizeof(dirbuf)) == sizeof(dirbuf)) {
...
}
Is fails so the function returns NULL, which is dereferenced in printf. So the question is How to read that struct. I have found from dir.h
#define direct dirent
So that structure is in effect dirent, which has following defintion dirent.h:
struct dirent
{
#ifndef __USE_FILE_OFFSET64
__ino_t d_ino;
__off_t d_off;
#else
__ino64_t d_ino;
__off64_t d_off;
#endif
unsigned short int d_reclen;
unsigned char d_type;
char d_name[256]; /* We must not include limits.h! */
};
But that should not matter since, in read(2), I am using sizeof which will get the proper size. So why does the while header fails?

Answer 1

Remember that K&R 2 was written almost 35 years ago. Besides discussing ANSI C aka C89, of which most (but not quite all) is still applicable to modern C, K&R also discuss many features that are not standardized but are specific to UNIX, or rather, to the UNIX of its day.
In former times, one would access directory entries as they do: by open()ing the directory like a file, and reading data from it in some specified format. K&R themselves say that the format they use is specific to Version 7 and System V UNIX, so there is no reason at all to expect it to work with other versions of UNIX, much less with Linux which evolved completely independently. In fact, the whole idea of using read() to get directory entries is now obsolete and generally not available. The business of getting directory entries off the disk is done within the kernel, and it provides this data to user-space through more standardized APIs like readdir or getdents.
K&R is a classic and there is much to be learned from it, but its age does show and you cannot be surprised when some of what they say is not applicable to the present day.
Note when a system call fails, your first step in determining the reason should be to check the value of errno, perhaps using perror(). If so, you would have seen that it was EISDIR "Is a directory". Referring to the read(2) man page indicates that this error occurs when "fd refers to a directory", implying that read() from a directory is generally not allowed. That would at least have helped you shift your focus from "How to read that struct" to "since I can't read the struct, by what method should I be getting directory entries instead?"