Duplicating (Whats Going On?)

File Descriptors, Open File, I-Node

File descriptor is a handle to a more complex structure inside the kernel Open File Offset Flags I-Node Type Block list …

File Descriptors and Inheritance

A call to texttt{fork()} inherits file descriptors ⟶ reference counted copy of the same Open File ⟶ Processes share flags and offset! File closed (open file freed) only when last copy is closed

Duplicating File Descriptors

Documentation

• man -s 2 dup

• man -s 2 dup2

#include <unistd.h> int dup(int oldfd); Return: new file descriptor #include <unistd.h> int dup2(int oldfd, int newfd); newfd already open/occupied ⟶ implicit close()

Example: Shell Stdout-Redirection (1)

$ /bin/echo Hello > /dev/null

• Redirection is a shell responsibility /bin/bash

• echo writes “Hello” to standard output

• … and does not want/have to care where it actually goes

Example: Shell Stdout-Redirection (2)

$ strace -f bash -c '/bin/echo Hallo > /dev/null'
[3722] open("/dev/null", O_WRONLY|O_...) = 3
[3722] dup2(3, 1) = 1
[3722] close(3) = 0
[3722] execve("/bin/echo", ...) = 0

(fork(), exec(), wait() omitted for clarity.)

Example: Shell Stdout-Redirection (3)

open("/dev/null")

dup2(3, 1)

close(3)