Ch04 Files and Directories

struct stat

• Linux: /usr/include/x86_64-linux-gnu/bits/struct_stat.h:26
• macOS: /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/sys/stat.h:158

#define __DARWIN_STRUCT_STAT64 { \
	dev_t		st_dev;                 /* [XSI] ID of device containing file */ \
	mode_t		st_mode;                /* [XSI] Mode of file (see below) */ \
	nlink_t		st_nlink;               /* [XSI] Number of hard links */ \
	__darwin_ino64_t st_ino;                /* [XSI] File serial number */ \
	uid_t		st_uid;                 /* [XSI] User ID of the file */ \
	gid_t		st_gid;                 /* [XSI] Group ID of the file */ \
	dev_t		st_rdev;                /* [XSI] Device ID */ \
	__DARWIN_STRUCT_STAT64_TIMES \
	off_t		st_size;                /* [XSI] file size, in bytes */ \
	blkcnt_t	st_blocks;              /* [XSI] blocks allocated for file */ \
	blksize_t	st_blksize;             /* [XSI] optimal blocksize for I/O */ \
	__uint32_t	st_flags;               /* user defined flags for file */ \
	__uint32_t	st_gen;                 /* file generation number */ \
	__int32_t	st_lspare;              /* RESERVED: DO NOT USE! */ \
	__int64_t	st_qspare[2];           /* RESERVED: DO NOT USE! */ \
}

File Types

• Regular file

• Directory file

• Block special file

• Character special file

• FIFO (a.k.a named pipe)

• Socket

• Symbolic link

Holes in a File

• Create a real 1GB file filling with zeros

dd if=/dev/zero of=real1G bs=1G count=1

• Create a sparse1G file with a hole

dd if=/dev/zero of=sparse1G bs=1 count=0 seek=1G

• Logical size and disk usage

ls -lhs hole1G sparse1G real1G
1.0G -rw-r--r-- 1 gpanda staff 1.0G Aug  7 12:29 real1G
   0 -rw-r--r-- 1 gpanda staff 1.0G Aug  7 12:25 sparse1G
8.0K -rw-r--r-- 1 gpanda staff 1.1G Jun 27 07:39 hole1G

ls -l: shows logical size, st_size ls -s: shows disk usage, st_blocks

ls -s use 1KB as block size to report the disk usage, this doesn’t mean the real block size (allocation unit) of the file system is 1KB. To show file system block size: stat -f /

• Use stat file to show file logical size and blocks usage

Exercises

4.1

When using stat instead of lstat, symbolic links will be followed. The struct stat retrieved for a symbolic link like /dev/stdin is that of the target file it points to, not the link itself. Therefore, the file type is not symbolic link but character special. /dev/stdin -> /dev/fd/0 (macOS), /dev/stdin -> /proc/self/fd/0 (Linux)

4.2

When umask is set to 0777, the new created file mode will all turned off, resulting in ---------- (000) permissions.

> umask
022
> umask 0777
> touch x
> ll x
---------- 1 david staff 0 Jun 22 21:05 x
> umask 022
> touch y
> ll y
-rw-r--r-- 1 david staff 0 Jun 22 21:07 y

4.3

> ll a
--w-r--r-- 1 david staff 6 Jun 23 13:36 a
> echo "hello" >> ./a
> echo "world" >> ./a
> cat ./a
cat: ./a: Permission denied
> sudo cat ./a
hello
world

Even if a file is owned by a user, turning off its user-read permission denies the user’s reading access. However, if user-write permission is on, it still allows the user to write to it.

4.4

> cat foo
foo
> cat bar
bar
> ll foo bar
-rw-r--r-- 1 gpanda staff 4 Jun 23 14:02 bar
-rw-r--r-- 1 gpanda staff 4 Jun 23 14:02 foo
> ./umask
> cat foo
> cat bar
> ll foo bar
-rw-r--r-- 1 gpanda staff 0 Jun 23 14:03 bar
-rw-r--r-- 1 gpanda staff 0 Jun 23 14:03 foo
>

If a file already exists, changing umask doesn’t affect file creation mode. However, the new creation will truncate the original file because the umask script uses creat(2), which is equivalent to open(path, O_CREAT | O_TRUNC | O_WRONLY, mode);.

4.5

No, a directory type file’s content has at least two directory entries: dot and dot-dot. A symbolic link’s file content is the path of the target file the link points to. Therefore, the content size of either type of file cannot be zero.

4.6

Ex_4.6c