chapter 39 - interlude: files and directories

Question 1

what is a file

Answer 1

a linear sequence of bytes identified by an inode number

Question 2

What is a directory?

Answer 2

A special file mapping user-readable names to inode numbers

Question 2

What is a directory tree?

Answer 2

A hierarchy starting from root /, containing nested directories and files

Question 3

what does int fd = open(“foo”, O_CREAT | O_WRONLY | O_TRUNC, S_IRUSR | S_IWUSR); do

Answer 3

• O_CREAT → create if not exist
• O_WRONLY → open for writing
• O_TRUNC → empty the file if it exists
• S_IRUSR | S_IWUSR → user can read/write
• Returns a file descriptor, placed in the process’s ofile[NOFILE] array

Question 4

What is a file descriptor?

Answer 4

A per-process integer used to access an open file

Question 5

What does read(fd, buf, size) do?

Answer 5

Reads size bytes into buf from file fd, and updates the offset

Question 6

What does write(fd, buf, size) do?

Answer 6

Writes size bytes from buf to file fd, also updating the offset

Question 7

What does lseek(fd, offset, whence) do?

Answer 7

Moves the file offset (SEEK_SET, SEEK_CUR, or SEEK_END)

Question 8

What is stored in the Open File Table?

Answer 8

Read/write flags, inode pointer, offset, and reference count

Question 9

What happens after fork() to file descriptors?

Answer 9

Child gets copies of parent’s FDs, sharing the same offset and inode

Question 10

What does dup(fd) do?

Answer 10

Duplicates fd, both share offset, inode, and permissions

Question 11

What does fsync(fd) do?

Answer 11

Forces all dirty data for that file to disk immediately

Question 11

What does rename(“old”, “new”) do?

Answer 11

Atomically renames a file; ensures no corruption even during crashes

Question 12

What does stat() or fstat() return?

Answer 12

Metadata like inode, size, permissions, owner, and timestamps

Question 13

What does unlink() do?

Answer 13

Removes a file; deletes it once all links and open FDs are gone