351 CSCE Operating System Kernels The UNIX/MINIX Operating System Processes & IPC
Steve Goddard
[e-mail]
http://www.cse.unl.edu/~goddard/Courses/CSCE351
1
UNIX History
x
The original UNIX
An experimental operating system developed by Ken Thompson & Dennis Ritchie in the late 60s
x
Main variants (Standards)
System V (1983)
y y
POSIX
y
developed by AT&T Open Software Foundation
IEEE/ISO
4.4 BSD (1993)
x
Commercial products
Ultrix, DEC UNIX DEC SunOS, Solaris Sun HP/UX Hewlett Packard
FreeBSD Linux AIX IBM Xenix Microsoft ...
2
MINIX History
x
The original MINIX was developed by Andrew Tanenbaum after AT&T closed the source code (1978)
An educational operating system Version 1.0 was released circa 1987 Version 2.0 was released circa 1997
x
Version 2.0
POSIX compliant Fully documented Requires only 30MB Micro-kernel design
x
Looks like UNIX from the outside
3
Processes
x
A process is created by the fork() system call
creates a new address space that is a duplicate of the callers
Parent address space
main (argc, argv) int childpid; { switch (childpid = fork()) { case 0: /* child */ child_func(); exit(0); default: /* parent */ parent_func(); while(wait((int *) 0) != childpid); exit(0); case -1: /* oops */ error("fork:%s\n",sys_errlist[errno]); } }
childpid = 1
Child address space
childpid = 0
4
Processes
x
Alternatively, processes can be created by an execve()
replaces the memory image of the caller with a new program
parent address space
main() { exec( prog) }
child address space
prog() { }
x
This is how the shell executes commands
a fork() followed by an exec()
csh
fork()
wait()
csh
exec()
ls
5
Example: How users logs in
init
fork() wait()
init
exec()
getty
login
csh csh ls
x x x x x
There exists a master process in UNIX: init init forks a process for each terminal port each init copy execs getty which prints the login prompt and then reads the login and password getty then execs login which verifies the login login then execs csh which forks processes new for each command
6
(Simple) Interprocess Communication
x x
Like message passing except more general Pipes a shared, in-memory file
a queue of 4K bytes buffered, asynchronous message passing
y y
blocks reader when queue is empty blocks writer when queue is full Pipe
read (fd,buf,len)
write (fd,buf,len)
7
(Simple) Interprocess Communication
write()
Server
Pipe1 Pipe2
read() write()
read()
Client
main() { int pipe1[2], pipe2[2]; if (pipe(pipe1) == -1 || pipe(pipe2) == -1) error(...
switch (childpid = fork()) { case 0: /* child */ close(pipe1[1]); /* write descriptor close(pipe2[0]); /* read descriptor client(pipe1[0],pipe2[1]); /* client program default : /* parent */ close(pipe1[0]); /* read descriptor close(pipe2[1]); /* write descriptor server(pipe2[0],pipe1[1]); /* server program while (wait((int *) 0) != childpid); /* wait } }
for pipe1 */ for pipe2 */ */ for pipe1 */ for pipe2 */ */ for child */
8
Open File Table
x
UNIX maintains an open file table for each process which lists each file in use by the process
the OFT is copied when processes are forked
system-wide file structure table
per process OFT
in-core i-node list
0: stdin 1: stdout 2: stderr : : 63:
file block & offset p...