Linux System Programming

From miki
Revision as of 11:34, 15 February 2011 by Mip (talk | contribs) (→‎Why fork & exec)
Jump to navigation Jump to search

Reference

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

Examples

Kill on ALARM signal

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