Porting to OS/2: Case #12 readdir_r() and struct dirent

readdir() and readdir_r() are used to read directory entries. And readdir_r() is a re-entrant version of readdir(). Because of this, readdir_r() requires additional arguments. Let's see prototypes of them.

• struct dirent *readdir(DIR *dirp); 
• int readdir_r(DIR *dirp, struct dirent *entry, struct dirent **result);

As you see, readdir_r() requires [struct dirent *entry]. And entry should be large enough for a dirent. For this, the recommended portable codes are like this.

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name_max = pathconf(dirpath, _PC_NAME_MAX); if (name_max == -1)         /* Limit not defined, or error */     name_max = 255;         /* Take a guess */ len = offsetof(struct dirent, d_name) + name_max + 1; entryp = malloc(len);

By the way, this code does not work on kLIBC. Why ? First, let's see [struct dirent]. In general, It is defined like this,

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struct dirent {     ino_t          d_ino;       /* inode number */     off_t          d_off;       /* offset to the next dirent */     unsigned short d_reclen;    /* length of this record */     unsigned char  d_type;      /* type of file; not supported                                    by all file system types */     char           d_name[256]; /* filename */ };

Whereas, on kLIBC it is defined like this.

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struct dirent {     __uint32_t d_fileno;        /* file number of entry */     __uint16_t d_reclen;        /* length of this record */     __uint8_t  d_type;         /* file type, see below */     __uint8_t  d_namlen;        /* length of string in d_name */ #if __BSD_VISIBLE #ifndef MAXNAMLEN                   /* bird */ #ifdef NAME_MAX                     /* bird */ #define    MAXNAMLEN    NAME_MAX    /* bird */ #else                               /* bird */ #define    MAXNAMLEN    256 #endif                              /* bird */ #endif                              /* bird */     char    d_name[MAXNAMLEN + 1];    /* name must be no longer than this */ #else     char    d_name[255 + 1];    /* name must be no longer than this */ #endif /* bird: Extra EMX fields - start */ /** @todo move these up before the name! LIBC07 */         __uint8_t  d_attr;              /* OS file attributes        */         __uint16_t d_time;              /* OS file modification time */         __uint16_t d_date;              /* OS file modification date */         __off_t    d_size;              /* File size (bytes)         */ /* bird: Extra EMX fields - end */ };

What's the difference ? That is, the position of d_name. The portable code assumes that d_name is the last member of struct dirent. Unfortunately, however, it is not true for kLIBC. kLIBC has extra members after d_name. So the portable code causes a memory corruption when it is used with readdir_r() on kLIBC actually. To avoid this, you should allocate the entire size of struct dirent.

Here are the codes used in VLC.

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/**  * Reads the next file name from an open directory.  *  * @param dir The directory that is being read  *  * @return a UTF-8 string of the directory entry. Use free() to release it.  * If there are no more entries in the directory, NULL is returned.  * If an error occurs, errno is set and NULL is returned.  */ char *vlc_readdir( DIR *dir ) {     /* Beware that readdir_r() assumes <buf> is large enough to hold the result      * dirent including the file name. A buffer overflow could occur otherwise.      * In particular, pathconf() and _POSIX_NAME_MAX cannot be used here. */     struct dirent *ent;     char *path = NULL;     /* In the implementation of Innotek LIBC, aka kLIBC on OS/2,      * fpathconf (_PC_NAME_MAX) is broken, and errno is set to EBADF.      * Moreover, d_name is not the last member of struct dirent.      * So just allocate as many as the size of struct dirent. */ #if 1     long len = sizeof (struct dirent); #else     long len = fpathconf (dirfd (dir), _PC_NAME_MAX);     len += offsetof (struct dirent, d_name) + 1; #endif     struct dirent *buf = malloc (len);     if (unlikely(buf == NULL))         return NULL;     int val = readdir_r (dir, buf, &ent);     if (val != 0)         errno = val;     else if (ent != NULL)         path = FromLocaleDup (ent->d_name);     free (buf);     return path; }

And as you read at line 18 to line 21, fpathconf(), which is a dirfd version of pathconf(), for _PC_NAME_MAX on kLIBC was broken.

Consequently, if you use readdir_r(), allocate the whole size of struct dirent for a second argument, entry, of readdir_r().