Working with the Unix OS

Processes I
(init process)
- If process ID, process group ID, terminal group ID are all equal then hangup signal “SIGHUP” is sent to
each process with process group ID equal to terminating process
- To prevent a child process from becoming a zombie
signal (SIGCLD, SIG_IGN)
! Process Relationships
For each terminal to be activated, init process forks a copy of itself and each child process execs the getty program
which sets terminal speed, output greeting message and waits for login name.
getty execs the program login which checks your login name and password in /etc/passwd
If the login is successful the login program sets the current working directory, chdir sets the group ID and user
ID, setgid & setuid execs the shell program /bin/sh
PID=
l init init getty login sh
fork exec exec exec
To execute a command the shell forks a copy of itself and waits for child to terminate, the child execs the program,
and when finished, it calls exit which terminates the child.
! Job Control
- consider process groups with/without job-control
BSD4.3 supports job-control - need to check system
main()
{
printf ("lipid = %d, pgrp = %d\n ", getpid(), getpgrp());
exit(0);
}
a.out
Bourne, C & Korn shells
a.out & a.out & twice in background
(a.out & a.out &) from a subshell
e.g.
BSD C shell pid = 2530, pgrp = 2528
pid = 2529, pgrp = 2528
- process group leader
- kill with a pid argument of zero sends a
signal to all processes in the sender's process
group
! File Sharing
There are 3 kernel tables used to access a file:
- every process has a process table entry
- file pointers in the process table point to
entries in the file table (current file position)
- i-node table (every open file has an entry)
Since the i-node table does not keep the file's
current position, an i-node entry for a. file can be
shared by any number of processes.
e.g. When two or more processes are reading the
same file at some point in time - the file position
of one process must be independent of the other
BSD Korn shell pid = 2530, pgrp = 2530
pid = 2529, pgrp = 2529
fd0:
fd1:
fd2:
fdi:
parent process file table i-node table
table entry
child process
table entry
fd0:
fd1:
fd2:
fdi:
other process
table entry
fd0:
fd1:
fd2:
fdi:
current file
position
i-node ptr
current file
position
i-node ptr
…
…
i-node
information
…
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