File Descriptors

tags

Systems

To read: https://x.com/7etsuo/status/1840268043982909838 🌟

FAQ

sharing of fd w child, how?

• Each running process has its own fd table. But this is an exception with child processes.
• After fork() or a clone() (wo CLONE_FILES set), a child and a parent have an equal set of fd(s)

What is file offset?

• file offset == location for next read() / write()

How to set? how is this related to ulimit nofile?

• See How to Increase Number of Open Files Limit in Linux 
• set
  • system wide
    • cat /proc/sys/fs/file-max
    • sysctl -w fs.file-max=500000 (/etc/sysctl.conf)
  • ulimit
    • See ulimits

Intro

• FD is an abstract indicator to access a file or other input/output resources.
  • It’s 100% opaque +ve integers
  • Even if its called “file” descriptor it can be indicator to something which is not a file too (but in unix everything is a file makes it fuzzy)
    • Up to V7 UNIX (1973-1979[2,3]), the file description table could literally only reference a file on disk, UNIX domain/TCP/UDP sockets weren’t introduced until 4.2BSD.
• It decouples a file path (more correctly, an inode) from a file object inside a process and the Linux kernel.
• Allows for opening the same file
  • An arbitrary number of times
  • For different purposes
  • With various flags
  • At different offsets.
• Each running program has its own list of file descriptors; they aren’t shared.

std[in,out,err]

• /dev/stdin, /dev/stdout, /dev/stderr are filenames for fd for each process.
• /proc/self/fdinfo contains per file descriptor info.

λ ll /dev/ | rg fd
lrwxrwxrwx      13 root      13 Mar 18:45  fd -> /proc/self/fd
lrwxrwxrwx      15 root      13 Mar 18:45  stderr -> /proc/self/fd/2
lrwxrwxrwx      15 root      13 Mar 18:45  stdin -> /proc/self/fd/0
lrwxrwxrwx      15 root      13 Mar 18:45  stdout -> /proc/self/fd/1
λ ll /proc/self/fd/
lrwx------    64 geekodour 15 Mar 15:44  0 -> /dev/pts/1
lrwx------    64 geekodour 15 Mar 15:44  1 -> /dev/pts/1
lrwx------    64 geekodour 15 Mar 15:44  2 -> /dev/pts/1
λ lsof -d 0 +fg # same fd points to different files
COMMAND      PID      USER   FD   TYPE FILE-FLAG DEVICE SIZE/OFF    NODE NAME
systemd      663 geekodour    0r   CHR        LG    1,3      0t0       4 /dev/null
emacs        676 geekodour    0r   CHR        LG    1,3      0t0       4 /dev/null
alacritty    933 geekodour    0u   CHR  RW,AP,LG    4,1      0t0      20 /dev/tty1
fish         947 geekodour    0u   CHR     RW,ND  136,0      0t0       3 /dev/pts/0
λ cat /proc/self/fdinfo/{0,1,2}
pos:    0
flags:  02002
mnt_id: 29
ino:    7

TODO The system fd and per process fd table and inode

Related syscalls : dup, dup2, dup3, fcntl (also allows us to specify certain fd number)

The tables

Open FD table (OFD table)

It’s an abstract thing, no actual entity in the kernel.

• Each entry stores status and position of the fd.

Per process FD table

This is a tangible thing

• Multiple FDs in the same process referring to the same OFD. (man 2 dup)
• Multiple processes w their own FDs referring to the same OFD. (man 2 fork)
  • If parent and child now start writing to the fd, the kernel will handle the synchronization
• Multiple processes w their own FDs referring to distinct OFD, but OFD points to same inode. (man 2 open by both processes)

TODO Shared and Private

• Some properties are stored in the OFD and some in per process
• If we change property of a FD from one process, and its shared, changes will reflect in other.

FD Internals

• Threads have task_struct, it points to files_struct which points to file struct. file contains flags, position, inode  etc.

Usage of FD

Creating FD

• Using open, openat, create etc. it’ll create the fd in both the tables.
• When we create new fd, kernel grantees to return the lowest positive number not currently opened by the calling process. i.e if we close a fd of a file, the next fd we create will get the fd of the file that we closed.