Linux System Programming

Reference

• System Programming in Perl: The Unix Process Model (slides)

Process

A process contains:

• A unique process ID number ('pid')
• A current working directory ('cwd')
• A user ID and group ID number (actually more than one of each)
• An open file table
• An environment' (just a bunch of string data)
• A signal table
• An alarm clock
• Lots of other equipment

Why fork & exec

• 2 main primitives of kernel to manage processes:
  • fork: create a new process
  • exec: replace a process's object code with the contents of a file
• Launching a new process is done in 2 steps:
  1. Fork the current process
  2. In the child process, exec the new file that will then replace the code of the current child but keep environment.
• Why fork and exec with 2 separate commands?
  Because the child process can alter its environment before it does the exec. For example:
  

  ps > /tmp/procs                # The child shell will first change the output FD, without ps knowing
  

Fork

• The child will get a new copy of the file descriptor (FD) table.
  So closing a file in the child will not interfere with the parents.
• However the system open file table is not copied; the same table is shared between child and parent. Why? Because parent must keep the same Seek pointer as the child, even after child is dead:

cat ./pie
# #!/bin/sh
# echo "I like pie.";                      # Will move seek pointer, also in parent
# echo "Especially blackberry.";           # Will write starting from new position of seek pointer!
./pie > piefile

/proc filesystem

Some links:

• Solving problems with proc
  • phantom progress bar — Showing progress of a process in a file, after process was launched

• For a shell, /dev/fd/###, /proc/self/fd/### and /proc/$$/fd/### refer to the same file

• If a program only takes a filename as argument and you want them to process file from standard input, use the fake file /proc/self/fd/O ([1]):

antiword /proc/self/fd/0 < test.doc > test.txt
somecommand | antiword /proc/self/fd/0 > text.txt   # Not guarantee to work, because pipe does not support random access

Examples

Kill on ALARM signal

Small script to kill a process on ALARM signal, implementing a kindof timeout kill (see Perl page).