why shift 12 bit for d_type in fusexmp

Question

In fusexmp.c http://www.gtoal.com/fusestuff/fuse-2.7.0/example/fusexmp.c.html

there is a line <<12

st.st_mode = de->d_type << 12;

Please kindly instruct me why left shift 12 bit

d_type the type of the file, i.e:DT_UNKNOWN I didn't find the bits define in /usr/include/bits/dirent.h

thanks

Answer 1

The bit shift converts from dir-entry d_type bitfield to the values used int struct stat.st_mode:

# define IFTODT(mode)   (((mode) & 0170000) >> 12)
# define DTTOIF(dirtype)    ((dirtype) << 12)

The documentation would simply be found in man stat, as the value is being assigned to st_mode of the stat struct:

The following flags are defined for the st_mode field:

S_IFMT     0170000   bit mask for the file type bit fields
S_IFSOCK   0140000   socket
S_IFLNK    0120000   symbolic link
S_IFREG    0100000   regular file
S_IFBLK    0060000   block device
S_IFDIR    0040000   directory
S_IFCHR    0020000   character device
S_IFIFO    0010000   FIFO
S_ISUID    0004000   set UID bit
S_ISGID    0002000   set-group-ID bit (see below)
S_ISVTX    0001000   sticky bit (see below)
S_IRWXU    00700     mask for file owner permissions
S_IRUSR    00400     owner has read permission
S_IWUSR    00200     owner has write permission
S_IXUSR    00100     owner has execute permission
S_IRWXG    00070     mask for group permissions
S_IRGRP    00040     group has read permission
S_IWGRP    00020     group has write permission
S_IXGRP    00010     group has execute permission
S_IRWXO    00007     mask for permissions for others (not in group)
S_IROTH    00004     others have read permission
S_IWOTH    00002     others have write permission
S_IXOTH    00001     others have execute permission

Apparently, the de->d_type is equivalent to the upper bits of st_mode when left-shifted 12 bits (for info, 1<<12 == 0x1000). man readdir to the rescue:

Now the full conversion and bitmasks are in sys/dirent.h:

/* File types for `d_type'.  */
enum
{
    DT_UNKNOWN = 0, // The file type is unknown
    DT_FIFO = 1,    // This is a named pipe (FIFO).
    DT_CHR = 2,     // This is a character device.
    DT_DIR = 4,     // This is directory.
    DT_BLK = 6,     // This is a block device.
    DT_REG = 8,     // This is a regular file.
    DT_LNK = 10,    // This is a symbolic link.
    DT_SOCK = 12,   // This is a Unix domain socket.
    DT_WHT = 14     // DT_CHR | DT_DIR | DT_REG
};

/* Convert between stat structure types and directory types.  */
# define IFTODT(mode)   (((mode) & 0170000) >> 12)
# define DTTOIF(dirtype)    ((dirtype) << 12)
#endif